calc
ltcalc
mfcalc
yyparse
yypush_parse
yypull_parse
yystate_new
yystate_delete
yylex
yyerror
This manual (14 May 2011) is for GNU Bison (version 2.5), the GNU parser generator.
Copyright © 1988-1993, 1995, 1998-2011 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”(a) The FSF's Back-Cover Text is: “You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.”
Tutorial sections:
Reference sections:
--- The Detailed Node Listing ---
The Concepts of Bison
Writing GLR Parsers
Examples
Reverse Polish Notation Calculator
Grammar Rules for rpcalc
Location Tracking Calculator: ltcalc
Multi-Function Calculator: mfcalc
Bison Grammar Files
Outline of a Bison Grammar
Defining Language Semantics
Tracking Locations
Bison Declarations
Parser C-Language Interface
The Lexical Analyzer Function yylex
The Bison Parser Algorithm
Operator Precedence
Tuning LR
Handling Context Dependencies
Debugging Your Parser
Invoking Bison
Parsers Written In Other Languages
C++ Parsers
A Complete C++ Example
Java Parsers
Frequently Asked Questions
Copying This Manual
Bison is a general-purpose parser generator that converts an annotated context-free grammar into a deterministic LR or generalized LR (GLR) parser employing LALR(1) parser tables. As an experimental feature, Bison can also generate IELR(1) or canonical LR(1) parser tables. Once you are proficient with Bison, you can use it to develop a wide range of language parsers, from those used in simple desk calculators to complex programming languages.
Bison is upward compatible with Yacc: all properly-written Yacc grammars ought to work with Bison with no change. Anyone familiar with Yacc should be able to use Bison with little trouble. You need to be fluent in C or C++ programming in order to use Bison or to understand this manual. Java is also supported as an experimental feature.
We begin with tutorial chapters that explain the basic concepts of using Bison and show three explained examples, each building on the last. If you don't know Bison or Yacc, start by reading these chapters. Reference chapters follow, which describe specific aspects of Bison in detail.
Bison was written originally by Robert Corbett. Richard Stallman made it Yacc-compatible. Wilfred Hansen of Carnegie Mellon University added multi-character string literals and other features. Since then, Bison has grown more robust and evolved many other new features thanks to the hard work of a long list of volunteers. For details, see the THANKS and ChangeLog files included in the Bison distribution.
This edition corresponds to version 2.5 of Bison.
The distribution terms for Bison-generated parsers permit using the parsers in nonfree programs. Before Bison version 2.2, these extra permissions applied only when Bison was generating LALR(1) parsers in C. And before Bison version 1.24, Bison-generated parsers could be used only in programs that were free software.
The other GNU programming tools, such as the GNU C compiler, have never had such a requirement. They could always be used for nonfree software. The reason Bison was different was not due to a special policy decision; it resulted from applying the usual General Public License to all of the Bison source code.
The main output of the Bison utility—the Bison parser implementation file—contains a verbatim copy of a sizable piece of Bison, which is the code for the parser's implementation. (The actions from your grammar are inserted into this implementation at one point, but most of the rest of the implementation is not changed.) When we applied the GPL terms to the skeleton code for the parser's implementation, the effect was to restrict the use of Bison output to free software.
We didn't change the terms because of sympathy for people who want to make software proprietary. Software should be free. But we concluded that limiting Bison's use to free software was doing little to encourage people to make other software free. So we decided to make the practical conditions for using Bison match the practical conditions for using the other GNU tools.
This exception applies when Bison is generating code for a parser. You can tell whether the exception applies to a Bison output file by inspecting the file for text beginning with “As a special exception...”. The text spells out the exact terms of the exception.
Copyright © 2007 Free Software Foundation, Inc. http://fsf.org/
Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.
The GNU General Public License is a free, copyleft license for software and other kinds of works.
The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program—to make sure it remains free software for all its users. We, the Free Software Foundation, use the GNU General Public License for most of our software; it applies also to any other work released this way by its authors. You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for them if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs, and that you know you can do these things.
To protect your rights, we need to prevent others from denying you these rights or asking you to surrender the rights. Therefore, you have certain responsibilities if you distribute copies of the software, or if you modify it: responsibilities to respect the freedom of others.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must pass on to the recipients the same freedoms that you received. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
Developers that use the GNU GPL protect your rights with two steps: (1) assert copyright on the software, and (2) offer you this License giving you legal permission to copy, distribute and/or modify it.
For the developers' and authors' protection, the GPL clearly explains that there is no warranty for this free software. For both users' and authors' sake, the GPL requires that modified versions be marked as changed, so that their problems will not be attributed erroneously to authors of previous versions.
Some devices are designed to deny users access to install or run modified versions of the software inside them, although the manufacturer can do so. This is fundamentally incompatible with the aim of protecting users' freedom to change the software. The systematic pattern of such abuse occurs in the area of products for individuals to use, which is precisely where it is most unacceptable. Therefore, we have designed this version of the GPL to prohibit the practice for those products. If such problems arise substantially in other domains, we stand ready to extend this provision to those domains in future versions of the GPL, as needed to protect the freedom of users.
Finally, every program is threatened constantly by software patents. States should not allow patents to restrict development and use of software on general-purpose computers, but in those that do, we wish to avoid the special danger that patents applied to a free program could make it effectively proprietary. To prevent this, the GPL assures that patents cannot be used to render the program non-free.
The precise terms and conditions for copying, distribution and modification follow.
“This License” refers to version 3 of the GNU General Public License.
“Copyright” also means copyright-like laws that apply to other kinds of works, such as semiconductor masks.
“The Program” refers to any copyrightable work licensed under this License. Each licensee is addressed as “you”. “Licensees” and “recipients” may be individuals or organizations.
To “modify” a work means to copy from or adapt all or part of the work in a fashion requiring copyright permission, other than the making of an exact copy. The resulting work is called a “modified version” of the earlier work or a work “based on” the earlier work.
A “covered work” means either the unmodified Program or a work based on the Program.
To “propagate” a work means to do anything with it that, without permission, would make you directly or secondarily liable for infringement under applicable copyright law, except executing it on a computer or modifying a private copy. Propagation includes copying, distribution (with or without modification), making available to the public, and in some countries other activities as well.
To “convey” a work means any kind of propagation that enables other parties to make or receive copies. Mere interaction with a user through a computer network, with no transfer of a copy, is not conveying.
An interactive user interface displays “Appropriate Legal Notices” to the extent that it includes a convenient and prominently visible feature that (1) displays an appropriate copyright notice, and (2) tells the user that there is no warranty for the work (except to the extent that warranties are provided), that licensees may convey the work under this License, and how to view a copy of this License. If the interface presents a list of user commands or options, such as a menu, a prominent item in the list meets this criterion.
The “source code” for a work means the preferred form of the work for making modifications to it. “Object code” means any non-source form of a work.
A “Standard Interface” means an interface that either is an official standard defined by a recognized standards body, or, in the case of interfaces specified for a particular programming language, one that is widely used among developers working in that language.
The “System Libraries” of an executable work include anything, other than the work as a whole, that (a) is included in the normal form of packaging a Major Component, but which is not part of that Major Component, and (b) serves only to enable use of the work with that Major Component, or to implement a Standard Interface for which an implementation is available to the public in source code form. A “Major Component”, in this context, means a major essential component (kernel, window system, and so on) of the specific operating system (if any) on which the executable work runs, or a compiler used to produce the work, or an object code interpreter used to run it.
The “Corresponding Source” for a work in object code form means all the source code needed to generate, install, and (for an executable work) run the object code and to modify the work, including scripts to control those activities. However, it does not include the work's System Libraries, or general-purpose tools or generally available free programs which are used unmodified in performing those activities but which are not part of the work. For example, Corresponding Source includes interface definition files associated with source files for the work, and the source code for shared libraries and dynamically linked subprograms that the work is specifically designed to require, such as by intimate data communication or control flow between those subprograms and other parts of the work.
The Corresponding Source need not include anything that users can regenerate automatically from other parts of the Corresponding Source.
The Corresponding Source for a work in source code form is that same work.
All rights granted under this License are granted for the term of copyright on the Program, and are irrevocable provided the stated conditions are met. This License explicitly affirms your unlimited permission to run the unmodified Program. The output from running a covered work is covered by this License only if the output, given its content, constitutes a covered work. This License acknowledges your rights of fair use or other equivalent, as provided by copyright law.
You may make, run and propagate covered works that you do not convey, without conditions so long as your license otherwise remains in force. You may convey covered works to others for the sole purpose of having them make modifications exclusively for you, or provide you with facilities for running those works, provided that you comply with the terms of this License in conveying all material for which you do not control copyright. Those thus making or running the covered works for you must do so exclusively on your behalf, under your direction and control, on terms that prohibit them from making any copies of your copyrighted material outside their relationship with you.
Conveying under any other circumstances is permitted solely under the conditions stated below. Sublicensing is not allowed; section 10 makes it unnecessary.
No covered work shall be deemed part of an effective technological measure under any applicable law fulfilling obligations under article 11 of the WIPO copyright treaty adopted on 20 December 1996, or similar laws prohibiting or restricting circumvention of such measures.
When you convey a covered work, you waive any legal power to forbid circumvention of technological measures to the extent such circumvention is effected by exercising rights under this License with respect to the covered work, and you disclaim any intention to limit operation or modification of the work as a means of enforcing, against the work's users, your or third parties' legal rights to forbid circumvention of technological measures.
You may convey verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice; keep intact all notices stating that this License and any non-permissive terms added in accord with section 7 apply to the code; keep intact all notices of the absence of any warranty; and give all recipients a copy of this License along with the Program.
You may charge any price or no price for each copy that you convey, and you may offer support or warranty protection for a fee.
You may convey a work based on the Program, or the modifications to produce it from the Program, in the form of source code under the terms of section 4, provided that you also meet all of these conditions:
A compilation of a covered work with other separate and independent works, which are not by their nature extensions of the covered work, and which are not combined with it such as to form a larger program, in or on a volume of a storage or distribution medium, is called an “aggregate” if the compilation and its resulting copyright are not used to limit the access or legal rights of the compilation's users beyond what the individual works permit. Inclusion of a covered work in an aggregate does not cause this License to apply to the other parts of the aggregate.
You may convey a covered work in object code form under the terms of sections 4 and 5, provided that you also convey the machine-readable Corresponding Source under the terms of this License, in one of these ways:
A separable portion of the object code, whose source code is excluded from the Corresponding Source as a System Library, need not be included in conveying the object code work.
A “User Product” is either (1) a “consumer product”, which means any tangible personal property which is normally used for personal, family, or household purposes, or (2) anything designed or sold for incorporation into a dwelling. In determining whether a product is a consumer product, doubtful cases shall be resolved in favor of coverage. For a particular product received by a particular user, “normally used” refers to a typical or common use of that class of product, regardless of the status of the particular user or of the way in which the particular user actually uses, or expects or is expected to use, the product. A product is a consumer product regardless of whether the product has substantial commercial, industrial or non-consumer uses, unless such uses represent the only significant mode of use of the product.
“Installation Information” for a User Product means any methods, procedures, authorization keys, or other information required to install and execute modified versions of a covered work in that User Product from a modified version of its Corresponding Source. The information must suffice to ensure that the continued functioning of the modified object code is in no case prevented or interfered with solely because modification has been made.
If you convey an object code work under this section in, or with, or specifically for use in, a User Product, and the conveying occurs as part of a transaction in which the right of possession and use of the User Product is transferred to the recipient in perpetuity or for a fixed term (regardless of how the transaction is characterized), the Corresponding Source conveyed under this section must be accompanied by the Installation Information. But this requirement does not apply if neither you nor any third party retains the ability to install modified object code on the User Product (for example, the work has been installed in ROM).
The requirement to provide Installation Information does not include a requirement to continue to provide support service, warranty, or updates for a work that has been modified or installed by the recipient, or for the User Product in which it has been modified or installed. Access to a network may be denied when the modification itself materially and adversely affects the operation of the network or violates the rules and protocols for communication across the network.
Corresponding Source conveyed, and Installation Information provided, in accord with this section must be in a format that is publicly documented (and with an implementation available to the public in source code form), and must require no special password or key for unpacking, reading or copying.
“Additional permissions” are terms that supplement the terms of this License by making exceptions from one or more of its conditions. Additional permissions that are applicable to the entire Program shall be treated as though they were included in this License, to the extent that they are valid under applicable law. If additional permissions apply only to part of the Program, that part may be used separately under those permissions, but the entire Program remains governed by this License without regard to the additional permissions.
When you convey a copy of a covered work, you may at your option remove any additional permissions from that copy, or from any part of it. (Additional permissions may be written to require their own removal in certain cases when you modify the work.) You may place additional permissions on material, added by you to a covered work, for which you have or can give appropriate copyright permission.
Notwithstanding any other provision of this License, for material you add to a covered work, you may (if authorized by the copyright holders of that material) supplement the terms of this License with terms:
All other non-permissive additional terms are considered “further restrictions” within the meaning of section 10. If the Program as you received it, or any part of it, contains a notice stating that it is governed by this License along with a term that is a further restriction, you may remove that term. If a license document contains a further restriction but permits relicensing or conveying under this License, you may add to a covered work material governed by the terms of that license document, provided that the further restriction does not survive such relicensing or conveying.
If you add terms to a covered work in accord with this section, you must place, in the relevant source files, a statement of the additional terms that apply to those files, or a notice indicating where to find the applicable terms.
Additional terms, permissive or non-permissive, may be stated in the form of a separately written license, or stated as exceptions; the above requirements apply either way.
You may not propagate or modify a covered work except as expressly provided under this License. Any attempt otherwise to propagate or modify it is void, and will automatically terminate your rights under this License (including any patent licenses granted under the third paragraph of section 11).
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, you do not qualify to receive new licenses for the same material under section 10.
You are not required to accept this License in order to receive or run a copy of the Program. Ancillary propagation of a covered work occurring solely as a consequence of using peer-to-peer transmission to receive a copy likewise does not require acceptance. However, nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so.
Each time you convey a covered work, the recipient automatically receives a license from the original licensors, to run, modify and propagate that work, subject to this License. You are not responsible for enforcing compliance by third parties with this License.
An “entity transaction” is a transaction transferring control of an organization, or substantially all assets of one, or subdividing an organization, or merging organizations. If propagation of a covered work results from an entity transaction, each party to that transaction who receives a copy of the work also receives whatever licenses to the work the party's predecessor in interest had or could give under the previous paragraph, plus a right to possession of the Corresponding Source of the work from the predecessor in interest, if the predecessor has it or can get it with reasonable efforts.
You may not impose any further restrictions on the exercise of the rights granted or affirmed under this License. For example, you may not impose a license fee, royalty, or other charge for exercise of rights granted under this License, and you may not initiate litigation (including a cross-claim or counterclaim in a lawsuit) alleging that any patent claim is infringed by making, using, selling, offering for sale, or importing the Program or any portion of it.
A “contributor” is a copyright holder who authorizes use under this License of the Program or a work on which the Program is based. The work thus licensed is called the contributor's “contributor version”.
A contributor's “essential patent claims” are all patent claims owned or controlled by the contributor, whether already acquired or hereafter acquired, that would be infringed by some manner, permitted by this License, of making, using, or selling its contributor version, but do not include claims that would be infringed only as a consequence of further modification of the contributor version. For purposes of this definition, “control” includes the right to grant patent sublicenses in a manner consistent with the requirements of this License.
Each contributor grants you a non-exclusive, worldwide, royalty-free patent license under the contributor's essential patent claims, to make, use, sell, offer for sale, import and otherwise run, modify and propagate the contents of its contributor version.
In the following three paragraphs, a “patent license” is any express agreement or commitment, however denominated, not to enforce a patent (such as an express permission to practice a patent or covenant not to sue for patent infringement). To “grant” such a patent license to a party means to make such an agreement or commitment not to enforce a patent against the party.
If you convey a covered work, knowingly relying on a patent license, and the Corresponding Source of the work is not available for anyone to copy, free of charge and under the terms of this License, through a publicly available network server or other readily accessible means, then you must either (1) cause the Corresponding Source to be so available, or (2) arrange to deprive yourself of the benefit of the patent license for this particular work, or (3) arrange, in a manner consistent with the requirements of this License, to extend the patent license to downstream recipients. “Knowingly relying” means you have actual knowledge that, but for the patent license, your conveying the covered work in a country, or your recipient's use of the covered work in a country, would infringe one or more identifiable patents in that country that you have reason to believe are valid.
If, pursuant to or in connection with a single transaction or arrangement, you convey, or propagate by procuring conveyance of, a covered work, and grant a patent license to some of the parties receiving the covered work authorizing them to use, propagate, modify or convey a specific copy of the covered work, then the patent license you grant is automatically extended to all recipients of the covered work and works based on it.
A patent license is “discriminatory” if it does not include within the scope of its coverage, prohibits the exercise of, or is conditioned on the non-exercise of one or more of the rights that are specifically granted under this License. You may not convey a covered work if you are a party to an arrangement with a third party that is in the business of distributing software, under which you make payment to the third party based on the extent of your activity of conveying the work, and under which the third party grants, to any of the parties who would receive the covered work from you, a discriminatory patent license (a) in connection with copies of the covered work conveyed by you (or copies made from those copies), or (b) primarily for and in connection with specific products or compilations that contain the covered work, unless you entered into that arrangement, or that patent license was granted, prior to 28 March 2007.
Nothing in this License shall be construed as excluding or limiting any implied license or other defenses to infringement that may otherwise be available to you under applicable patent law.
If conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot convey a covered work so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not convey it at all. For example, if you agree to terms that obligate you to collect a royalty for further conveying from those to whom you convey the Program, the only way you could satisfy both those terms and this License would be to refrain entirely from conveying the Program.
Notwithstanding any other provision of this License, you have permission to link or combine any covered work with a work licensed under version 3 of the GNU Affero General Public License into a single combined work, and to convey the resulting work. The terms of this License will continue to apply to the part which is the covered work, but the special requirements of the GNU Affero General Public License, section 13, concerning interaction through a network will apply to the combination as such.
The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.
Each version is given a distinguishing version number. If the Program specifies that a certain numbered version of the GNU General Public License “or any later version” applies to it, you have the option of following the terms and conditions either of that numbered version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of the GNU General Public License, you may choose any version ever published by the Free Software Foundation.
If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Program.
Later license versions may give you additional or different permissions. However, no additional obligations are imposed on any author or copyright holder as a result of your choosing to follow a later version.
THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in connection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee.
If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does.
Copyright (C) year name of author
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or (at
your option) any later version.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see http://www.gnu.org/licenses/.
Also add information on how to contact you by electronic and paper mail.
If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:
program Copyright (C) year name of author
This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’.
This is free software, and you are welcome to redistribute it
under certain conditions; type ‘show c’ for details.
The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the General Public License. Of course, your program's commands might be different; for a GUI interface, you would use an “about box”.
You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see http://www.gnu.org/licenses/.
The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read http://www.gnu.org/philosophy/why-not-lgpl.html.
This chapter introduces many of the basic concepts without which the details of Bison will not make sense. If you do not already know how to use Bison or Yacc, we suggest you start by reading this chapter carefully.
In order for Bison to parse a language, it must be described by a context-free grammar. This means that you specify one or more syntactic groupings and give rules for constructing them from their parts. For example, in the C language, one kind of grouping is called an `expression'. One rule for making an expression might be, “An expression can be made of a minus sign and another expression”. Another would be, “An expression can be an integer”. As you can see, rules are often recursive, but there must be at least one rule which leads out of the recursion.
The most common formal system for presenting such rules for humans to read is Backus-Naur Form or “BNF”, which was developed in order to specify the language Algol 60. Any grammar expressed in BNF is a context-free grammar. The input to Bison is essentially machine-readable BNF.
There are various important subclasses of context-free grammars. Although it can handle almost all context-free grammars, Bison is optimized for what are called LR(1) grammars. In brief, in these grammars, it must be possible to tell how to parse any portion of an input string with just a single token of lookahead. For historical reasons, Bison by default is limited by the additional restrictions of LALR(1), which is hard to explain simply. See Mysterious Conflicts, for more information on this. As an experimental feature, you can escape these additional restrictions by requesting IELR(1) or canonical LR(1) parser tables. See LR Table Construction, to learn how.
Parsers for LR(1) grammars are deterministic, meaning roughly that the next grammar rule to apply at any point in the input is uniquely determined by the preceding input and a fixed, finite portion (called a lookahead) of the remaining input. A context-free grammar can be ambiguous, meaning that there are multiple ways to apply the grammar rules to get the same inputs. Even unambiguous grammars can be nondeterministic, meaning that no fixed lookahead always suffices to determine the next grammar rule to apply. With the proper declarations, Bison is also able to parse these more general context-free grammars, using a technique known as GLR parsing (for Generalized LR). Bison's GLR parsers are able to handle any context-free grammar for which the number of possible parses of any given string is finite.
In the formal grammatical rules for a language, each kind of syntactic unit or grouping is named by a symbol. Those which are built by grouping smaller constructs according to grammatical rules are called nonterminal symbols; those which can't be subdivided are called terminal symbols or token types. We call a piece of input corresponding to a single terminal symbol a token, and a piece corresponding to a single nonterminal symbol a grouping.
We can use the C language as an example of what symbols, terminal and nonterminal, mean. The tokens of C are identifiers, constants (numeric and string), and the various keywords, arithmetic operators and punctuation marks. So the terminal symbols of a grammar for C include `identifier', `number', `string', plus one symbol for each keyword, operator or punctuation mark: `if', `return', `const', `static', `int', `char', `plus-sign', `open-brace', `close-brace', `comma' and many more. (These tokens can be subdivided into characters, but that is a matter of lexicography, not grammar.)
Here is a simple C function subdivided into tokens:
int /* keyword `int' */ square (int x) /* identifier, open-paren, keyword `int', identifier, close-paren */ { /* open-brace */ return x * x; /* keyword `return', identifier, asterisk, identifier, semicolon */ } /* close-brace */
The syntactic groupings of C include the expression, the statement, the declaration, and the function definition. These are represented in the grammar of C by nonterminal symbols `expression', `statement', `declaration' and `function definition'. The full grammar uses dozens of additional language constructs, each with its own nonterminal symbol, in order to express the meanings of these four. The example above is a function definition; it contains one declaration, and one statement. In the statement, each ‘x’ is an expression and so is ‘x * x’.
Each nonterminal symbol must have grammatical rules showing how it is made
out of simpler constructs. For example, one kind of C statement is the
return statement; this would be described with a grammar rule which
reads informally as follows:
A `statement' can be made of a `return' keyword, an `expression' and a `semicolon'.
There would be many other rules for `statement', one for each kind of statement in C.
One nonterminal symbol must be distinguished as the special one which defines a complete utterance in the language. It is called the start symbol. In a compiler, this means a complete input program. In the C language, the nonterminal symbol `sequence of definitions and declarations' plays this role.
For example, ‘1 + 2’ is a valid C expression—a valid part of a C program—but it is not valid as an entire C program. In the context-free grammar of C, this follows from the fact that `expression' is not the start symbol.
The Bison parser reads a sequence of tokens as its input, and groups the tokens using the grammar rules. If the input is valid, the end result is that the entire token sequence reduces to a single grouping whose symbol is the grammar's start symbol. If we use a grammar for C, the entire input must be a `sequence of definitions and declarations'. If not, the parser reports a syntax error.
A formal grammar is a mathematical construct. To define the language for Bison, you must write a file expressing the grammar in Bison syntax: a Bison grammar file. See Bison Grammar Files.
A nonterminal symbol in the formal grammar is represented in Bison input
as an identifier, like an identifier in C. By convention, it should be
in lower case, such as expr, stmt or declaration.
The Bison representation for a terminal symbol is also called a token
type. Token types as well can be represented as C-like identifiers. By
convention, these identifiers should be upper case to distinguish them from
nonterminals: for example, INTEGER, IDENTIFIER, IF or
RETURN. A terminal symbol that stands for a particular keyword in
the language should be named after that keyword converted to upper case.
The terminal symbol error is reserved for error recovery.
See Symbols.
A terminal symbol can also be represented as a character literal, just like a C character constant. You should do this whenever a token is just a single character (parenthesis, plus-sign, etc.): use that same character in a literal as the terminal symbol for that token.
A third way to represent a terminal symbol is with a C string constant containing several characters. See Symbols, for more information.
The grammar rules also have an expression in Bison syntax. For example,
here is the Bison rule for a C return statement. The semicolon in
quotes is a literal character token, representing part of the C syntax for
the statement; the naked semicolon, and the colon, are Bison punctuation
used in every rule.
stmt: RETURN expr ';'
;
A formal grammar selects tokens only by their classifications: for example, if a rule mentions the terminal symbol `integer constant', it means that any integer constant is grammatically valid in that position. The precise value of the constant is irrelevant to how to parse the input: if ‘x+4’ is grammatical then ‘x+1’ or ‘x+3989’ is equally grammatical.
But the precise value is very important for what the input means once it is parsed. A compiler is useless if it fails to distinguish between 4, 1 and 3989 as constants in the program! Therefore, each token in a Bison grammar has both a token type and a semantic value. See Defining Language Semantics, for details.
The token type is a terminal symbol defined in the grammar, such as
INTEGER, IDENTIFIER or ','. It tells everything
you need to know to decide where the token may validly appear and how to
group it with other tokens. The grammar rules know nothing about tokens
except their types.
The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
identifier. (A token such as ',' which is just punctuation doesn't
need to have any semantic value.)
For example, an input token might be classified as token type
INTEGER and have the semantic value 4. Another input token might
have the same token type INTEGER but value 3989. When a grammar
rule says that INTEGER is allowed, either of these tokens is
acceptable because each is an INTEGER. When the parser accepts the
token, it keeps track of the token's semantic value.
Each grouping can also have a semantic value as well as its nonterminal symbol. For example, in a calculator, an expression typically has a semantic value that is a number. In a compiler for a programming language, an expression typically has a semantic value that is a tree structure describing the meaning of the expression.
In order to be useful, a program must do more than parse input; it must also produce some output based on the input. In a Bison grammar, a grammar rule can have an action made up of C statements. Each time the parser recognizes a match for that rule, the action is executed. See Actions.
Most of the time, the purpose of an action is to compute the semantic value of the whole construct from the semantic values of its parts. For example, suppose we have a rule which says an expression can be the sum of two expressions. When the parser recognizes such a sum, each of the subexpressions has a semantic value which describes how it was built up. The action for this rule should create a similar sort of value for the newly recognized larger expression.
For example, here is a rule that says an expression can be the sum of two subexpressions:
expr: expr '+' expr { $$ = $1 + $3; }
;
The action says how to produce the semantic value of the sum expression from the values of the two subexpressions.
In some grammars, Bison's deterministic LR(1) parsing algorithm cannot decide whether to apply a certain grammar rule at a given point. That is, it may not be able to decide (on the basis of the input read so far) which of two possible reductions (applications of a grammar rule) applies, or whether to apply a reduction or read more of the input and apply a reduction later in the input. These are known respectively as reduce/reduce conflicts (see Reduce/Reduce), and shift/reduce conflicts (see Shift/Reduce).
To use a grammar that is not easily modified to be LR(1), a
more general parsing algorithm is sometimes necessary. If you include
%glr-parser among the Bison declarations in your file
(see Grammar Outline), the result is a Generalized LR
(GLR) parser. These parsers handle Bison grammars that
contain no unresolved conflicts (i.e., after applying precedence
declarations) identically to deterministic parsers. However, when
faced with unresolved shift/reduce and reduce/reduce conflicts,
GLR parsers use the simple expedient of doing both,
effectively cloning the parser to follow both possibilities. Each of
the resulting parsers can again split, so that at any given time, there
can be any number of possible parses being explored. The parsers
proceed in lockstep; that is, all of them consume (shift) a given input
symbol before any of them proceed to the next. Each of the cloned
parsers eventually meets one of two possible fates: either it runs into
a parsing error, in which case it simply vanishes, or it merges with
another parser, because the two of them have reduced the input to an
identical set of symbols.
During the time that there are multiple parsers, semantic actions are recorded, but not performed. When a parser disappears, its recorded semantic actions disappear as well, and are never performed. When a reduction makes two parsers identical, causing them to merge, Bison records both sets of semantic actions. Whenever the last two parsers merge, reverting to the single-parser case, Bison resolves all the outstanding actions either by precedences given to the grammar rules involved, or by performing both actions, and then calling a designated user-defined function on the resulting values to produce an arbitrary merged result.
In the simplest cases, you can use the GLR algorithm to parse grammars that are unambiguous but fail to be LR(1). Such grammars typically require more than one symbol of lookahead.
Consider a problem that arises in the declaration of enumerated and subrange types in the programming language Pascal. Here are some examples:
type subrange = lo .. hi;
type enum = (a, b, c);
The original language standard allows only numeric literals and constant identifiers for the subrange bounds (‘lo’ and ‘hi’), but Extended Pascal (ISO/IEC 10206) and many other Pascal implementations allow arbitrary expressions there. This gives rise to the following situation, containing a superfluous pair of parentheses:
type subrange = (a) .. b;
Compare this to the following declaration of an enumerated type with only one value:
type enum = (a);
(These declarations are contrived, but they are syntactically valid, and more-complicated cases can come up in practical programs.)
These two declarations look identical until the ‘..’ token. With normal LR(1) one-token lookahead it is not possible to decide between the two forms when the identifier ‘a’ is parsed. It is, however, desirable for a parser to decide this, since in the latter case ‘a’ must become a new identifier to represent the enumeration value, while in the former case ‘a’ must be evaluated with its current meaning, which may be a constant or even a function call.
You could parse ‘(a)’ as an “unspecified identifier in parentheses”, to be resolved later, but this typically requires substantial contortions in both semantic actions and large parts of the grammar, where the parentheses are nested in the recursive rules for expressions.
You might think of using the lexer to distinguish between the two forms by returning different tokens for currently defined and undefined identifiers. But if these declarations occur in a local scope, and ‘a’ is defined in an outer scope, then both forms are possible—either locally redefining ‘a’, or using the value of ‘a’ from the outer scope. So this approach cannot work.
A simple solution to this problem is to declare the parser to use the GLR algorithm. When the GLR parser reaches the critical state, it merely splits into two branches and pursues both syntax rules simultaneously. Sooner or later, one of them runs into a parsing error. If there is a ‘..’ token before the next ‘;’, the rule for enumerated types fails since it cannot accept ‘..’ anywhere; otherwise, the subrange type rule fails since it requires a ‘..’ token. So one of the branches fails silently, and the other one continues normally, performing all the intermediate actions that were postponed during the split.
If the input is syntactically incorrect, both branches fail and the parser reports a syntax error as usual.
The effect of all this is that the parser seems to “guess” the correct branch to take, or in other words, it seems to use more lookahead than the underlying LR(1) algorithm actually allows for. In this example, LR(2) would suffice, but also some cases that are not LR(k) for any k can be handled this way.
In general, a GLR parser can take quadratic or cubic worst-case time, and the current Bison parser even takes exponential time and space for some grammars. In practice, this rarely happens, and for many grammars it is possible to prove that it cannot happen. The present example contains only one conflict between two rules, and the type-declaration context containing the conflict cannot be nested. So the number of branches that can exist at any time is limited by the constant 2, and the parsing time is still linear.
Here is a Bison grammar corresponding to the example above. It parses a vastly simplified form of Pascal type declarations.
%token TYPE DOTDOT ID
%left '+' '-'
%left '*' '/'
%%
type_decl : TYPE ID '=' type ';'
;
type : '(' id_list ')'
| expr DOTDOT expr
;
id_list : ID
| id_list ',' ID
;
expr : '(' expr ')'
| expr '+' expr
| expr '-' expr
| expr '*' expr
| expr '/' expr
| ID
;
When used as a normal LR(1) grammar, Bison correctly complains about one reduce/reduce conflict. In the conflicting situation the parser chooses one of the alternatives, arbitrarily the one declared first. Therefore the following correct input is not recognized:
type t = (a) .. b;
The parser can be turned into a GLR parser, while also telling Bison to be silent about the one known reduce/reduce conflict, by adding these two declarations to the Bison grammar file (before the first ‘%%’):
%glr-parser
%expect-rr 1
No change in the grammar itself is required. Now the parser recognizes all valid declarations, according to the limited syntax above, transparently. In fact, the user does not even notice when the parser splits.
So here we have a case where we can use the benefits of GLR, almost without disadvantages. Even in simple cases like this, however, there are at least two potential problems to beware. First, always analyze the conflicts reported by Bison to make sure that GLR splitting is only done where it is intended. A GLR parser splitting inadvertently may cause problems less obvious than an LR parser statically choosing the wrong alternative in a conflict. Second, consider interactions with the lexer (see Semantic Tokens) with great care. Since a split parser consumes tokens without performing any actions during the split, the lexer cannot obtain information via parser actions. Some cases of lexer interactions can be eliminated by using GLR to shift the complications from the lexer to the parser. You must check the remaining cases for correctness.
In our example, it would be safe for the lexer to return tokens based on their current meanings in some symbol table, because no new symbols are defined in the middle of a type declaration. Though it is possible for a parser to define the enumeration constants as they are parsed, before the type declaration is completed, it actually makes no difference since they cannot be used within the same enumerated type declaration.
Let's consider an example, vastly simplified from a C++ grammar.
%{
#include <stdio.h>
#define YYSTYPE char const *
int yylex (void);
void yyerror (char const *);
%}
%token TYPENAME ID
%right '='
%left '+'
%glr-parser
%%
prog :
| prog stmt { printf ("\n"); }
;
stmt : expr ';' %dprec 1
| decl %dprec 2
;
expr : ID { printf ("%s ", $$); }
| TYPENAME '(' expr ')'
{ printf ("%s <cast> ", $1); }
| expr '+' expr { printf ("+ "); }
| expr '=' expr { printf ("= "); }
;
decl : TYPENAME declarator ';'
{ printf ("%s <declare> ", $1); }
| TYPENAME declarator '=' expr ';'
{ printf ("%s <init-declare> ", $1); }
;
declarator : ID { printf ("\"%s\" ", $1); }
| '(' declarator ')'
;
This models a problematic part of the C++ grammar—the ambiguity between certain declarations and statements. For example,
T (x) = y+z;
parses as either an expr or a stmt
(assuming that ‘T’ is recognized as a TYPENAME and
‘x’ as an ID).
Bison detects this as a reduce/reduce conflict between the rules
expr : ID and declarator : ID, which it cannot resolve at the
time it encounters x in the example above. Since this is a
GLR parser, it therefore splits the problem into two parses, one for
each choice of resolving the reduce/reduce conflict.
Unlike the example from the previous section (see Simple GLR Parsers),
however, neither of these parses “dies,” because the grammar as it stands is
ambiguous. One of the parsers eventually reduces stmt : expr ';' and
the other reduces stmt : decl, after which both parsers are in an
identical state: they've seen ‘prog stmt’ and have the same unprocessed
input remaining. We say that these parses have merged.
At this point, the GLR parser requires a specification in the
grammar of how to choose between the competing parses.
In the example above, the two %dprec
declarations specify that Bison is to give precedence
to the parse that interprets the example as a
decl, which implies that x is a declarator.
The parser therefore prints
"x" y z + T <init-declare>
The %dprec declarations only come into play when more than one
parse survives. Consider a different input string for this parser:
T (x) + y;
This is another example of using GLR to parse an unambiguous
construct, as shown in the previous section (see Simple GLR Parsers).
Here, there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters x, it does not
have enough information to resolve the reduce/reduce conflict (again,
between x as an expr or a declarator). In this
case, no precedence declaration is used. Again, the parser splits
into two, one assuming that x is an expr, and the other
assuming x is a declarator. The second of these parsers
then vanishes when it sees +, and the parser prints
x T <cast> y +
Suppose that instead of resolving the ambiguity, you wanted to see all
the possibilities. For this purpose, you must merge the semantic
actions of the two possible parsers, rather than choosing one over the
other. To do so, you could change the declaration of stmt as
follows:
stmt : expr ';' %merge <stmtMerge>
| decl %merge <stmtMerge>
;
and define the stmtMerge function as:
static YYSTYPE
stmtMerge (YYSTYPE x0, YYSTYPE x1)
{
printf ("<OR> ");
return "";
}
with an accompanying forward declaration in the C declarations at the beginning of the file:
%{
#define YYSTYPE char const *
static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
%}
With these declarations, the resulting parser parses the first example
as both an expr and a decl, and prints
"x" y z + T <init-declare> x T <cast> y z + = <OR>
Bison requires that all of the productions that participate in any particular merge have identical ‘%merge’ clauses. Otherwise, the ambiguity would be unresolvable, and the parser will report an error during any parse that results in the offending merge.
By definition, a deferred semantic action is not performed at the same time as the associated reduction. This raises caveats for several Bison features you might use in a semantic action in a GLR parser.
In any semantic action, you can examine yychar to determine the type of
the lookahead token present at the time of the associated reduction.
After checking that yychar is not set to YYEMPTY or YYEOF,
you can then examine yylval and yylloc to determine the
lookahead token's semantic value and location, if any.
In a nondeferred semantic action, you can also modify any of these variables to
influence syntax analysis.
See Lookahead Tokens.
In a deferred semantic action, it's too late to influence syntax analysis.
In this case, yychar, yylval, and yylloc are set to
shallow copies of the values they had at the time of the associated reduction.
For this reason alone, modifying them is dangerous.
Moreover, the result of modifying them is undefined and subject to change with
future versions of Bison.
For example, if a semantic action might be deferred, you should never write it
to invoke yyclearin (see Action Features) or to attempt to free
memory referenced by yylval.
Another Bison feature requiring special consideration is YYERROR
(see Action Features), which you can invoke in a semantic action to
initiate error recovery.
During deterministic GLR operation, the effect of YYERROR is
the same as its effect in a deterministic parser.
In a deferred semantic action, its effect is undefined.
Also, see Default Action for Locations, which
describes a special usage of YYLLOC_DEFAULT in GLR parsers.
The GLR parsers require a compiler for ISO C89 or
later. In addition, they use the inline keyword, which is not
C89, but is C99 and is a common extension in pre-C99 compilers. It is
up to the user of these parsers to handle
portability issues. For instance, if using Autoconf and the Autoconf
macro AC_C_INLINE, a mere
%{
#include <config.h>
%}
will suffice. Otherwise, we suggest
%{
#if __STDC_VERSION__ < 199901 && ! defined __GNUC__ && ! defined inline
#define inline
#endif
%}
Many applications, like interpreters or compilers, have to produce verbose and useful error messages. To achieve this, one must be able to keep track of the textual location, or location, of each syntactic construct. Bison provides a mechanism for handling these locations.
Each token has a semantic value. In a similar fashion, each token has an associated location, but the type of locations is the same for all tokens and groupings. Moreover, the output parser is equipped with a default data structure for storing locations (see Locations, for more details).
Like semantic values, locations can be reached in actions using a dedicated
set of constructs. In the example above, the location of the whole grouping
is @$, while the locations of the subexpressions are @1 and
@3.
When a rule is matched, a default action is used to compute the semantic value
of its left hand side (see Actions). In the same way, another default
action is used for locations. However, the action for locations is general
enough for most cases, meaning there is usually no need to describe for each
rule how @$ should be formed. When building a new location for a given
grouping, the default behavior of the output parser is to take the beginning
of the first symbol, and the end of the last symbol.
When you run Bison, you give it a Bison grammar file as input. The most important output is a C source file that implements a parser for the language described by the grammar. This parser is called a Bison parser, and this file is called a Bison parser implementation file. Keep in mind that the Bison utility and the Bison parser are two distinct programs: the Bison utility is a program whose output is the Bison parser implementation file that becomes part of your program.
The job of the Bison parser is to group tokens into groupings according to the grammar rules—for example, to build identifiers and operators into expressions. As it does this, it runs the actions for the grammar rules it uses.
The tokens come from a function called the lexical analyzer that
you must supply in some fashion (such as by writing it in C). The Bison
parser calls the lexical analyzer each time it wants a new token. It
doesn't know what is “inside” the tokens (though their semantic values
may reflect this). Typically the lexical analyzer makes the tokens by
parsing characters of text, but Bison does not depend on this.
See The Lexical Analyzer Function yylex.
The Bison parser implementation file is C code which defines a
function named yyparse which implements that grammar. This
function does not make a complete C program: you must supply some
additional functions. One is the lexical analyzer. Another is an
error-reporting function which the parser calls to report an error.
In addition, a complete C program must start with a function called
main; you have to provide this, and arrange for it to call
yyparse or the parser will never run. See Parser C-Language Interface.
Aside from the token type names and the symbols in the actions you
write, all symbols defined in the Bison parser implementation file
itself begin with ‘yy’ or ‘YY’. This includes interface
functions such as the lexical analyzer function yylex, the
error reporting function yyerror and the parser function
yyparse itself. This also includes numerous identifiers used
for internal purposes. Therefore, you should avoid using C
identifiers starting with ‘yy’ or ‘YY’ in the Bison grammar
file except for the ones defined in this manual. Also, you should
avoid using the C identifiers ‘malloc’ and ‘free’ for
anything other than their usual meanings.
In some cases the Bison parser implementation file includes system
headers, and in those cases your code should respect the identifiers
reserved by those headers. On some non-GNU hosts, <alloca.h>,
<malloc.h>, <stddef.h>, and <stdlib.h> are
included as needed to declare memory allocators and related types.
<libintl.h> is included if message translation is in use
(see Internationalization). Other system headers may be included
if you define YYDEBUG to a nonzero value (see Tracing Your Parser).
The actual language-design process using Bison, from grammar specification to a working compiler or interpreter, has these parts:
yylex). It could also be produced
using Lex, but the use of Lex is not discussed in this manual.
To turn this source code as written into a runnable program, you must follow these steps:
The input file for the Bison utility is a Bison grammar file. The general form of a Bison grammar file is as follows:
%{
Prologue
%}
Bison declarations
%%
Grammar rules
%%
Epilogue
The ‘%%’, ‘%{’ and ‘%}’ are punctuation that appears in every Bison grammar file to separate the sections.
The prologue may define types and variables used in the actions. You can
also use preprocessor commands to define macros used there, and use
#include to include header files that do any of these things.
You need to declare the lexical analyzer yylex and the error
printer yyerror here, along with any other global identifiers
used by the actions in the grammar rules.
The Bison declarations declare the names of the terminal and nonterminal symbols, and may also describe operator precedence and the data types of semantic values of various symbols.
The grammar rules define how to construct each nonterminal symbol from its parts.
The epilogue can contain any code you want to use. Often the definitions of functions declared in the prologue go here. In a simple program, all the rest of the program can go here.
Now we show and explain three sample programs written using Bison: a reverse polish notation calculator, an algebraic (infix) notation calculator, and a multi-function calculator. All three have been tested under BSD Unix 4.3; each produces a usable, though limited, interactive desk-top calculator.
These examples are simple, but Bison grammars for real programming languages are written the same way. You can copy these examples into a source file to try them.
The first example is that of a simple double-precision reverse polish notation calculator (a calculator using postfix operators). This example provides a good starting point, since operator precedence is not an issue. The second example will illustrate how operator precedence is handled.
The source code for this calculator is named rpcalc.y. The ‘.y’ extension is a convention used for Bison grammar files.
rpcalcHere are the C and Bison declarations for the reverse polish notation calculator. As in C, comments are placed between ‘/*...*/’.
/* Reverse polish notation calculator. */
%{
#define YYSTYPE double
#include <math.h>
int yylex (void);
void yyerror (char const *);
%}
%token NUM
%% /* Grammar rules and actions follow. */
The declarations section (see The prologue) contains two preprocessor directives and two forward declarations.
The #define directive defines the macro YYSTYPE, thus
specifying the C data type for semantic values of both tokens and
groupings (see Data Types of Semantic Values). The
Bison parser will use whatever type YYSTYPE is defined as; if you
don't define it, int is the default. Because we specify
double, each token and each expression has an associated value,
which is a floating point number.
The #include directive is used to declare the exponentiation
function pow.
The forward declarations for yylex and yyerror are
needed because the C language requires that functions be declared
before they are used. These functions will be defined in the
epilogue, but the parser calls them so they must be declared in the
prologue.
The second section, Bison declarations, provides information to Bison
about the token types (see The Bison Declarations Section). Each terminal symbol that is not a
single-character literal must be declared here. (Single-character
literals normally don't need to be declared.) In this example, all the
arithmetic operators are designated by single-character literals, so the
only terminal symbol that needs to be declared is NUM, the token
type for numeric constants.
rpcalcHere are the grammar rules for the reverse polish notation calculator.
input: /* empty */
| input line
;
line: '\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
;
exp: NUM { $$ = $1; }
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
| exp exp '*' { $$ = $1 * $2; }
| exp exp '/' { $$ = $1 / $2; }
/* Exponentiation */
| exp exp '^' { $$ = pow ($1, $2); }
/* Unary minus */
| exp 'n' { $$ = -$1; }
;
%%
The groupings of the rpcalc “language” defined here are the expression
(given the name exp), the line of input (line), and the
complete input transcript (input). Each of these nonterminal
symbols has several alternate rules, joined by the vertical bar ‘|’
which is read as “or”. The following sections explain what these rules
mean.
The semantics of the language is determined by the actions taken when a grouping is recognized. The actions are the C code that appears inside braces. See Actions.
You must specify these actions in C, but Bison provides the means for
passing semantic values between the rules. In each action, the
pseudo-variable $$ stands for the semantic value for the grouping
that the rule is going to construct. Assigning a value to $$ is the
main job of most actions. The semantic values of the components of the
rule are referred to as $1, $2, and so on.
inputConsider the definition of input:
input: /* empty */
| input line
;
This definition reads as follows: “A complete input is either an empty
string, or a complete input followed by an input line”. Notice that
“complete input” is defined in terms of itself. This definition is said
to be left recursive since input appears always as the
leftmost symbol in the sequence. See Recursive Rules.
The first alternative is empty because there are no symbols between the
colon and the first ‘|’; this means that input can match an
empty string of input (no tokens). We write the rules this way because it
is legitimate to type Ctrl-d right after you start the calculator.
It's conventional to put an empty alternative first and write the comment
‘/* empty */’ in it.
The second alternate rule (input line) handles all nontrivial input.
It means, “After reading any number of lines, read one more line if
possible.” The left recursion makes this rule into a loop. Since the
first alternative matches empty input, the loop can be executed zero or
more times.
The parser function yyparse continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end-of-input.
lineNow consider the definition of line:
line: '\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
;
The first alternative is a token which is a newline character; this means
that rpcalc accepts a blank line (and ignores it, since there is no
action). The second alternative is an expression followed by a newline.
This is the alternative that makes rpcalc useful. The semantic value of
the exp grouping is the value of $1 because the exp in
question is the first symbol in the alternative. The action prints this
value, which is the result of the computation the user asked for.
This action is unusual because it does not assign a value to $$. As
a consequence, the semantic value associated with the line is
uninitialized (its value will be unpredictable). This would be a bug if
that value were ever used, but we don't use it: once rpcalc has printed the
value of the user's input line, that value is no longer needed.
exprThe exp grouping has several rules, one for each kind of expression.
The first rule handles the simplest expressions: those that are just numbers.
The second handles an addition-expression, which looks like two expressions
followed by a plus-sign. The third handles subtraction, and so on.
exp: NUM
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
...
;
We have used ‘|’ to join all the rules for exp, but we could
equally well have written them separately:
exp: NUM ;
exp: exp exp '+' { $$ = $1 + $2; } ;
exp: exp exp '-' { $$ = $1 - $2; } ;
...
Most of the rules have actions that compute the value of the expression in
terms of the value of its parts. For example, in the rule for addition,
$1 refers to the first component exp and $2 refers to
the second one. The third component, '+', has no meaningful
associated semantic value, but if it had one you could refer to it as
$3. When yyparse recognizes a sum expression using this
rule, the sum of the two subexpressions' values is produced as the value of
the entire expression. See Actions.
You don't have to give an action for every rule. When a rule has no
action, Bison by default copies the value of $1 into $$.
This is what happens in the first rule (the one that uses NUM).
The formatting shown here is the recommended convention, but Bison does not require it. You can add or change white space as much as you wish. For example, this:
exp : NUM | exp exp '+' {$$ = $1 + $2; } | ... ;
means the same thing as this:
exp: NUM
| exp exp '+' { $$ = $1 + $2; }
| ...
;
The latter, however, is much more readable.
rpcalc Lexical Analyzer
The lexical analyzer's job is low-level parsing: converting characters
or sequences of characters into tokens. The Bison parser gets its
tokens by calling the lexical analyzer. See The Lexical Analyzer Function yylex.
Only a simple lexical analyzer is needed for the RPN
calculator. This
lexical analyzer skips blanks and tabs, then reads in numbers as
double and returns them as NUM tokens. Any other character
that isn't part of a number is a separate token. Note that the token-code
for such a single-character token is the character itself.
The return value of the lexical analyzer function is a numeric code which
represents a token type. The same text used in Bison rules to stand for
this token type is also a C expression for the numeric code for the type.
This works in two ways. If the token type is a character literal, then its
numeric code is that of the character; you can use the same
character literal in the lexical analyzer to express the number. If the
token type is an identifier, that identifier is defined by Bison as a C
macro whose definition is the appropriate number. In this example,
therefore, NUM becomes a macro for yylex to use.
The semantic value of the token (if it has one) is stored into the
global variable yylval, which is where the Bison parser will look
for it. (The C data type of yylval is YYSTYPE, which was
defined at the beginning of the grammar; see Declarations for rpcalc.)
A token type code of zero is returned if the end-of-input is encountered. (Bison recognizes any nonpositive value as indicating end-of-input.)
Here is the code for the lexical analyzer:
/* The lexical analyzer returns a double floating point
number on the stack and the token NUM, or the numeric code
of the character read if not a number. It skips all blanks
and tabs, and returns 0 for end-of-input. */
#include <ctype.h>
int
yylex (void)
{
int c;
/* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
;
/* Process numbers. */
if (c == '.' || isdigit (c))
{
ungetc (c, stdin);
scanf ("%lf", &yylval);
return NUM;
}
/* Return end-of-input. */
if (c == EOF)
return 0;
/* Return a single char. */
return c;
}
In keeping with the spirit of this example, the controlling function is
kept to the bare minimum. The only requirement is that it call
yyparse to start the process of parsing.
int
main (void)
{
return yyparse ();
}
When yyparse detects a syntax error, it calls the error reporting
function yyerror to print an error message (usually but not
always "syntax error"). It is up to the programmer to supply
yyerror (see Parser C-Language Interface), so
here is the definition we will use:
#include <stdio.h>
/* Called by yyparse on error. */
void
yyerror (char const *s)
{
fprintf (stderr, "%s\n", s);
}
After yyerror returns, the Bison parser may recover from the error
and continue parsing if the grammar contains a suitable error rule
(see Error Recovery). Otherwise, yyparse returns nonzero. We
have not written any error rules in this example, so any invalid input will
cause the calculator program to exit. This is not clean behavior for a
real calculator, but it is adequate for the first example.
Before running Bison to produce a parser, we need to decide how to
arrange all the source code in one or more source files. For such a
simple example, the easiest thing is to put everything in one file,
the grammar file. The definitions of yylex, yyerror and
main go at the end, in the epilogue of the grammar file
(see The Overall Layout of a Bison Grammar).
For a large project, you would probably have several source files, and use
make to arrange to recompile them.
With all the source in the grammar file, you use the following command to convert it into a parser implementation file:
bison file.y
In this example, the grammar file is called rpcalc.y (for
“Reverse Polish calculator”). Bison produces a parser
implementation file named file.tab.c, removing the
‘.y’ from the grammar file name. The parser implementation file
contains the source code for yyparse. The additional functions
in the grammar file (yylex, yyerror and main) are
copied verbatim to the parser implementation file.
Here is how to compile and run the parser implementation file:
# List files in current directory.
$ ls
rpcalc.tab.c rpcalc.y
# Compile the Bison parser.
# ‘-lm’ tells compiler to search math library for pow.
$ cc -lm -o rpcalc rpcalc.tab.c
# List files again.
$ ls
rpcalc rpcalc.tab.c rpcalc.y
The file rpcalc now contains the executable code. Here is an
example session using rpcalc.
$ rpcalc
4 9 +
13
3 7 + 3 4 5 *+-
-13
3 7 + 3 4 5 * + - n Note the unary minus, ‘n’
13
5 6 / 4 n +
-3.166666667
3 4 ^ Exponentiation
81
^D End-of-file indicator
$
calcWe now modify rpcalc to handle infix operators instead of postfix. Infix notation involves the concept of operator precedence and the need for parentheses nested to arbitrary depth. Here is the Bison code for calc.y, an infix desk-top calculator.
/* Infix notation calculator. */
%{
#define YYSTYPE double
#include <math.h>
#include <stdio.h>
int yylex (void);
void yyerror (char const *);
%}
/* Bison declarations. */
%token NUM
%left '-' '+'
%left '*' '/'
%left NEG /* negation--unary minus */
%right '^' /* exponentiation */
%% /* The grammar follows. */
input: /* empty */
| input line
;
line: '\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
;
exp: NUM { $$ = $1; }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp { $$ = $1 / $3; }
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
;
%%
The functions yylex, yyerror and main can be the
same as before.
There are two important new features shown in this code.
In the second section (Bison declarations), %left declares token
types and says they are left-associative operators. The declarations
%left and %right (right associativity) take the place of
%token which is used to declare a token type name without
associativity. (These tokens are single-character literals, which
ordinarily don't need to be declared. We declare them here to specify
the associativity.)
Operator precedence is determined by the line ordering of the
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence. Hence, exponentiation
has the highest precedence, unary minus (NEG) is next, followed
by ‘*’ and ‘/’, and so on. See Operator Precedence.
The other important new feature is the %prec in the grammar
section for the unary minus operator. The %prec simply instructs
Bison that the rule ‘| '-' exp’ has the same precedence as
NEG—in this case the next-to-highest. See Context-Dependent Precedence.
Here is a sample run of calc.y:
$ calc
4 + 4.5 - (34/(8*3+-3))
6.880952381
-56 + 2
-54
3 ^ 2
9
Up to this point, this manual has not addressed the issue of error
recovery—how to continue parsing after the parser detects a syntax
error. All we have handled is error reporting with yyerror.
Recall that by default yyparse returns after calling
yyerror. This means that an erroneous input line causes the
calculator program to exit. Now we show how to rectify this deficiency.
The Bison language itself includes the reserved word error, which
may be included in the grammar rules. In the example below it has
been added to one of the alternatives for line:
line: '\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
| error '\n' { yyerrok; }
;
This addition to the grammar allows for simple error recovery in the
event of a syntax error. If an expression that cannot be evaluated is
read, the error will be recognized by the third rule for line,
and parsing will continue. (The yyerror function is still called
upon to print its message as well.) The action executes the statement
yyerrok, a macro defined automatically by Bison; its meaning is
that error recovery is complete (see Error Recovery). Note the
difference between yyerrok and yyerror; neither one is a
misprint.
This form of error recovery deals with syntax errors. There are other
kinds of errors; for example, division by zero, which raises an exception
signal that is normally fatal. A real calculator program must handle this
signal and use longjmp to return to main and resume parsing
input lines; it would also have to discard the rest of the current line of
input. We won't discuss this issue further because it is not specific to
Bison programs.
ltcalcThis example extends the infix notation calculator with location tracking. This feature will be used to improve the error messages. For the sake of clarity, this example is a simple integer calculator, since most of the work needed to use locations will be done in the lexical analyzer.
ltcalcThe C and Bison declarations for the location tracking calculator are the same as the declarations for the infix notation calculator.
/* Location tracking calculator. */
%{
#define YYSTYPE int
#include <math.h>
int yylex (void);
void yyerror (char const *);
%}
/* Bison declarations. */
%token NUM
%left '-' '+'
%left '*' '/'
%left NEG
%right '^'
%% /* The grammar follows. */
Note there are no declarations specific to locations. Defining a data
type for storing locations is not needed: we will use the type provided
by default (see Data Types of Locations), which is a
four member structure with the following integer fields:
first_line, first_column, last_line and
last_column. By conventions, and in accordance with the GNU
Coding Standards and common practice, the line and column count both
start at 1.
ltcalcWhether handling locations or not has no effect on the syntax of your language. Therefore, grammar rules for this example will be very close to those of the previous example: we will only modify them to benefit from the new information.
Here, we will use locations to report divisions by zero, and locate the wrong expressions or subexpressions.
input : /* empty */
| input line
;
line : '\n'
| exp '\n' { printf ("%d\n", $1); }
;
exp : NUM { $$ = $1; }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp
{
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
fprintf (stderr, "%d.%d-%d.%d: division by zero",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
This code shows how to reach locations inside of semantic actions, by
using the pseudo-variables @n for rule components, and the
pseudo-variable @$ for groupings.
We don't need to assign a value to @$: the output parser does it
automatically. By default, before executing the C code of each action,
@$ is set to range from the beginning of @1 to the end
of @n, for a rule with n components. This behavior
can be redefined (see Default Action for Locations), and for very specific rules, @$ can be computed by
hand.
ltcalc Lexical Analyzer.Until now, we relied on Bison's defaults to enable location tracking. The next step is to rewrite the lexical analyzer, and make it able to feed the parser with the token locations, as it already does for semantic values.
To this end, we must take into account every single character of the input text, to avoid the computed locations of being fuzzy or wrong:
int
yylex (void)
{
int c;
/* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
++yylloc.last_column;
/* Step. */
yylloc.first_line = yylloc.last_line;
yylloc.first_column = yylloc.last_column;
/* Process numbers. */
if (isdigit (c))
{
yylval = c - '0';
++yylloc.last_column;
while (isdigit (c = getchar ()))
{
++yylloc.last_column;
yylval = yylval * 10 + c - '0';
}
ungetc (c, stdin);
return NUM;
}
/* Return end-of-input. */
if (c == EOF)
return 0;
/* Return a single char, and update location. */
if (c == '\n')
{
++yylloc.last_line;
yylloc.last_column = 0;
}
else
++yylloc.last_column;
return c;
}
Basically, the lexical analyzer performs the same processing as before:
it skips blanks and tabs, and reads numbers or single-character tokens.
In addition, it updates yylloc, the global variable (of type
YYLTYPE) containing the token's location.
Now, each time this function returns a token, the parser has its number
as well as its semantic value, and its location in the text. The last
needed change is to initialize yylloc, for example in the
controlling function:
int
main (void)
{
yylloc.first_line = yylloc.last_line = 1;
yylloc.first_column = yylloc.last_column = 0;
return yyparse ();
}
Remember that computing locations is not a matter of syntax. Every character must be associated to a location update, whether it is in valid input, in comments, in literal strings, and so on.
mfcalc
Now that the basics of Bison have been discussed, it is time to move on to
a more advanced problem. The above calculators provided only five
functions, ‘+’, ‘-’, ‘*’, ‘/’ and ‘^’. It would
be nice to have a calculator that provides other mathematical functions such
as sin, cos, etc.
It is easy to add new operators to the infix calculator as long as they are
only single-character literals. The lexical analyzer yylex passes
back all nonnumeric characters as tokens, so new grammar rules suffice for
adding a new operator. But we want something more flexible: built-in
functions whose syntax has this form:
function_name (argument)
At the same time, we will add memory to the calculator, by allowing you to create named variables, store values in them, and use them later. Here is a sample session with the multi-function calculator:
$ mfcalc
pi = 3.141592653589
3.1415926536
sin(pi)
0.0000000000
alpha = beta1 = 2.3
2.3000000000
alpha
2.3000000000
ln(alpha)
0.8329091229
exp(ln(beta1))
2.3000000000
$
Note that multiple assignment and nested function calls are permitted.
mfcalcHere are the C and Bison declarations for the multi-function calculator.
%{
#include <math.h> /* For math functions, cos(), sin(), etc. */
#include "calc.h" /* Contains definition of `symrec'. */
int yylex (void);
void yyerror (char const *);
%}
%union {
double val; /* For returning numbers. */
symrec *tptr; /* For returning symbol-table pointers. */
}
%token <val> NUM /* Simple double precision number. */
%token <tptr> VAR FNCT /* Variable and Function. */
%type <val> exp
%right '='
%left '-' '+'
%left '*' '/'
%left NEG /* negation--unary minus */
%right '^' /* exponentiation */
%% /* The grammar follows. */
The above grammar introduces only two new features of the Bison language. These features allow semantic values to have various data types (see More Than One Value Type).
The %union declaration specifies the entire list of possible types;
this is instead of defining YYSTYPE. The allowable types are now
double-floats (for exp and NUM) and pointers to entries in
the symbol table. See The Collection of Value Types.
Since values can now have various types, it is necessary to associate a
type with each grammar symbol whose semantic value is used. These symbols
are NUM, VAR, FNCT, and exp. Their
declarations are augmented with information about their data type (placed
between angle brackets).
The Bison construct %type is used for declaring nonterminal
symbols, just as %token is used for declaring token types. We
have not used %type before because nonterminal symbols are
normally declared implicitly by the rules that define them. But
exp must be declared explicitly so we can specify its value type.
See Nonterminal Symbols.
mfcalcHere are the grammar rules for the multi-function calculator.
Most of them are copied directly from calc; three rules,
those which mention VAR or FNCT, are new.
input: /* empty */
| input line
;
line:
'\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
| error '\n' { yyerrok; }
;
exp: NUM { $$ = $1; }
| VAR { $$ = $1->value.var; }
| VAR '=' exp { $$ = $3; $1->value.var = $3; }
| FNCT '(' exp ')' { $$ = (*($1->value.fnctptr))($3); }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp { $$ = $1 / $3; }
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
;
/* End of grammar. */
%%
mfcalc Symbol TableThe multi-function calculator requires a symbol table to keep track of the names and meanings of variables and functions. This doesn't affect the grammar rules (except for the actions) or the Bison declarations, but it requires some additional C functions for support.
The symbol table itself consists of a linked list of records. Its definition, which is kept in the header calc.h, is as follows. It provides for either functions or variables to be placed in the table.
/* Function type. */
typedef double (*func_t) (double);
/* Data type for links in the chain of symbols. */
struct symrec
{
char *name; /* name of symbol */
int type; /* type of symbol: either VAR or FNCT */
union
{
double var; /* value of a VAR */
func_t fnctptr; /* value of a FNCT */
} value;
struct symrec *next; /* link field */
};
typedef struct symrec symrec;
/* The symbol table: a chain of `struct symrec'. */
extern symrec *sym_table;
symrec *putsym (char const *, int);
symrec *getsym (char const *);
The new version of main includes a call to init_table, a
function that initializes the symbol table. Here it is, and
init_table as well:
#include <stdio.h>
/* Called by yyparse on error. */
void
yyerror (char const *s)
{
printf ("%s\n", s);
}
struct init
{
char const *fname;
double (*fnct) (double);
};
struct init const arith_fncts[] =
{
"sin", sin,
"cos", cos,
"atan", atan,
"ln", log,
"exp", exp,
"sqrt", sqrt,
0, 0
};
/* The symbol table: a chain of `struct symrec'. */
symrec *sym_table;
/* Put arithmetic functions in table. */
void
init_table (void)
{
int i;
symrec *ptr;
for (i = 0; arith_fncts[i].fname != 0; i++)
{
ptr = putsym (arith_fncts[i].fname, FNCT);
ptr->value.fnctptr = arith_fncts[i].fnct;
}
}
int
main (void)
{
init_table ();
return yyparse ();
}
By simply editing the initialization list and adding the necessary include files, you can add additional functions to the calculator.
Two important functions allow look-up and installation of symbols in the
symbol table. The function putsym is passed a name and the type
(VAR or FNCT) of the object to be installed. The object is
linked to the front of the list, and a pointer to the object is returned.
The function getsym is passed the name of the symbol to look up. If
found, a pointer to that symbol is returned; otherwise zero is returned.
symrec *
putsym (char const *sym_name, int sym_type)
{
symrec *ptr;
ptr = (symrec *) malloc (sizeof (symrec));
ptr->name = (char *) malloc (strlen (sym_name) + 1);
strcpy (ptr->name,sym_name);
ptr->type = sym_type;
ptr->value.var = 0; /* Set value to 0 even if fctn. */
ptr->next = (struct symrec *)sym_table;
sym_table = ptr;
return ptr;
}
symrec *
getsym (char const *sym_name)
{
symrec *ptr;
for (ptr = sym_table; ptr != (symrec *) 0;
ptr = (symrec *)ptr->next)
if (strcmp (ptr->name,sym_name) == 0)
return ptr;
return 0;
}
The function yylex must now recognize variables, numeric values, and
the single-character arithmetic operators. Strings of alphanumeric
characters with a leading letter are recognized as either variables or
functions depending on what the symbol table says about them.
The string is passed to getsym for look up in the symbol table. If
the name appears in the table, a pointer to its location and its type
(VAR or FNCT) is returned to yyparse. If it is not
already in the table, then it is installed as a VAR using
putsym. Again, a pointer and its type (which must be VAR) is
returned to yyparse.
No change is needed in the handling of numeric values and arithmetic
operators in yylex.
#include <ctype.h>
int
yylex (void)
{
int c;
/* Ignore white space, get first nonwhite character. */
while ((c = getchar ()) == ' ' || c == '\t');
if (c == EOF)
return 0;
/* Char starts a number => parse the number. */
if (c == '.' || isdigit (c))
{
ungetc (c, stdin);
scanf ("%lf", &yylval.val);
return NUM;
}
/* Char starts an identifier => read the name. */
if (isalpha (c))
{
symrec *s;
static char *symbuf = 0;
static int length = 0;
int i;
/* Initially make the buffer long enough
for a 40-character symbol name. */
if (length == 0)
length = 40, symbuf = (char *)malloc (length + 1);
i = 0;
do
{
/* If buffer is full, make it bigger. */
if (i == length)
{
length *= 2;
symbuf = (char *) realloc (symbuf, length + 1);
}
/* Add this character to the buffer. */
symbuf[i++] = c;
/* Get another character. */
c = getchar ();
}
while (isalnum (c));
ungetc (c, stdin);
symbuf[i] = '\0';
s = getsym (symbuf);
if (s == 0)
s = putsym (symbuf, VAR);
yylval.tptr = s;
return s->type;
}
/* Any other character is a token by itself. */
return c;
}
This program is both powerful and flexible. You may easily add new
functions, and it is a simple job to modify this code to install
predefined variables such as pi or e as well.
init_table to add these constants to the symbol table.
It will be easiest to give the constants type VAR.
Bison takes as input a context-free grammar specification and produces a C-language function that recognizes correct instances of the grammar.
The Bison grammar file conventionally has a name ending in ‘.y’. See Invoking Bison.
A Bison grammar file has four main sections, shown here with the appropriate delimiters:
%{
Prologue
%}
Bison declarations
%%
Grammar rules
%%
Epilogue
Comments enclosed in ‘/* ... */’ may appear in any of the sections. As a GNU extension, ‘//’ introduces a comment that continues until end of line.
The Prologue section contains macro definitions and declarations
of functions and variables that are used in the actions in the grammar
rules. These are copied to the beginning of the parser implementation
file so that they precede the definition of yyparse. You can
use ‘#include’ to get the declarations from a header file. If
you don't need any C declarations, you may omit the ‘%{’ and
‘%}’ delimiters that bracket this section.
The Prologue section is terminated by the first occurrence of ‘%}’ that is outside a comment, a string literal, or a character constant.
You may have more than one Prologue section, intermixed with the
Bison declarations. This allows you to have C and Bison
declarations that refer to each other. For example, the %union
declaration may use types defined in a header file, and you may wish to
prototype functions that take arguments of type YYSTYPE. This
can be done with two Prologue blocks, one before and one after the
%union declaration.
%{
#define _GNU_SOURCE
#include <stdio.h>
#include "ptypes.h"
%}
%union {
long int n;
tree t; /* tree is defined in ptypes.h. */
}
%{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
%}
...
When in doubt, it is usually safer to put prologue code before all
Bison declarations, rather than after. For example, any definitions
of feature test macros like _GNU_SOURCE or
_POSIX_C_SOURCE should appear before all Bison declarations, as
feature test macros can affect the behavior of Bison-generated
#include directives.
The functionality of Prologue sections can often be subtle and
inflexible. As an alternative, Bison provides a %code
directive with an explicit qualifier field, which identifies the
purpose of the code and thus the location(s) where Bison should
generate it. For C/C++, the qualifier can be omitted for the default
location, or it can be one of requires, provides,
top. See %code Summary.
Look again at the example of the previous section:
%{
#define _GNU_SOURCE
#include <stdio.h>
#include "ptypes.h"
%}
%union {
long int n;
tree t; /* tree is defined in ptypes.h. */
}
%{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
%}
...
Notice that there are two Prologue sections here, but there's a
subtle distinction between their functionality. For example, if you
decide to override Bison's default definition for YYLTYPE, in
which Prologue section should you write your new definition?
You should write it in the first since Bison will insert that code
into the parser implementation file before the default
YYLTYPE definition. In which Prologue section should you
prototype an internal function, trace_token, that accepts
YYLTYPE and yytokentype as arguments? You should
prototype it in the second since Bison will insert that code
after the YYLTYPE and yytokentype definitions.
This distinction in functionality between the two Prologue sections is
established by the appearance of the %union between them.
This behavior raises a few questions.
First, why should the position of a %union affect definitions related to
YYLTYPE and yytokentype?
Second, what if there is no %union?
In that case, the second kind of Prologue section is not available.
This behavior is not intuitive.
To avoid this subtle %union dependency, rewrite the example using a
%code top and an unqualified %code.
Let's go ahead and add the new YYLTYPE definition and the
trace_token prototype at the same time:
%code top {
#define _GNU_SOURCE
#include <stdio.h>
/* WARNING: The following code really belongs
* in a `%code requires'; see below. */
#include "ptypes.h"
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
{
int first_line;
int first_column;
int last_line;
int last_column;
char *filename;
} YYLTYPE;
}
%union {
long int n;
tree t; /* tree is defined in ptypes.h. */
}
%code {
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
static void trace_token (enum yytokentype token, YYLTYPE loc);
}
...
In this way, %code top and the unqualified %code achieve the same
functionality as the two kinds of Prologue sections, but it's always
explicit which kind you intend.
Moreover, both kinds are always available even in the absence of %union.
The %code top block above logically contains two parts. The
first two lines before the warning need to appear near the top of the
parser implementation file. The first line after the warning is
required by YYSTYPE and thus also needs to appear in the parser
implementation file. However, if you've instructed Bison to generate
a parser header file (see %defines), you probably
want that line to appear before the YYSTYPE definition in that
header file as well. The YYLTYPE definition should also appear
in the parser header file to override the default YYLTYPE
definition there.
In other words, in the %code top block above, all but the first two
lines are dependency code required by the YYSTYPE and YYLTYPE
definitions.
Thus, they belong in one or more %code requires:
%code top {
#define _GNU_SOURCE
#include <stdio.h>
}
%code requires {
#include "ptypes.h"
}
%union {
long int n;
tree t; /* tree is defined in ptypes.h. */
}
%code requires {
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
{
int first_line;
int first_column;
int last_line;
int last_column;
char *filename;
} YYLTYPE;
}
%code {
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
static void trace_token (enum yytokentype token, YYLTYPE loc);
}
...
Now Bison will insert #include "ptypes.h" and the new
YYLTYPE definition before the Bison-generated YYSTYPE
and YYLTYPE definitions in both the parser implementation file
and the parser header file. (By the same reasoning, %code
requires would also be the appropriate place to write your own
definition for YYSTYPE.)
When you are writing dependency code for YYSTYPE and
YYLTYPE, you should prefer %code requires over
%code top regardless of whether you instruct Bison to generate
a parser header file. When you are writing code that you need Bison
to insert only into the parser implementation file and that has no
special need to appear at the top of that file, you should prefer the
unqualified %code over %code top. These practices will
make the purpose of each block of your code explicit to Bison and to
other developers reading your grammar file. Following these
practices, we expect the unqualified %code and %code
requires to be the most important of the four Prologue
alternatives.
At some point while developing your parser, you might decide to
provide trace_token to modules that are external to your
parser. Thus, you might wish for Bison to insert the prototype into
both the parser header file and the parser implementation file. Since
this function is not a dependency required by YYSTYPE or
YYLTYPE, it doesn't make sense to move its prototype to a
%code requires. More importantly, since it depends upon
YYLTYPE and yytokentype, %code requires is not
sufficient. Instead, move its prototype from the unqualified
%code to a %code provides:
%code top {
#define _GNU_SOURCE
#include <stdio.h>
}
%code requires {
#include "ptypes.h"
}
%union {
long int n;
tree t; /* tree is defined in ptypes.h. */
}
%code requires {
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
{
int first_line;
int first_column;
int last_line;
int last_column;
char *filename;
} YYLTYPE;
}
%code provides {
void trace_token (enum yytokentype token, YYLTYPE loc);
}
%code {
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
}
...
Bison will insert the trace_token prototype into both the
parser header file and the parser implementation file after the
definitions for yytokentype, YYLTYPE, and
YYSTYPE.
The above examples are careful to write directives in an order that
reflects the layout of the generated parser implementation and header
files: %code top, %code requires, %code provides,
and then %code. While your grammar files may generally be
easier to read if you also follow this order, Bison does not require
it. Instead, Bison lets you choose an organization that makes sense
to you.
You may declare any of these directives multiple times in the grammar file. In that case, Bison concatenates the contained code in declaration order. This is the only way in which the position of one of these directives within the grammar file affects its functionality.
The result of the previous two properties is greater flexibility in how you may organize your grammar file. For example, you may organize semantic-type-related directives by semantic type:
%code requires { #include "type1.h" }
%union { type1 field1; }
%destructor { type1_free ($$); } <field1>
%printer { type1_print ($$); } <field1>
%code requires { #include "type2.h" }
%union { type2 field2; }
%destructor { type2_free ($$); } <field2>
%printer { type2_print ($$); } <field2>
You could even place each of the above directive groups in the rules section of
the grammar file next to the set of rules that uses the associated semantic
type.
(In the rules section, you must terminate each of those directives with a
semicolon.)
And you don't have to worry that some directive (like a %union) in the
definitions section is going to adversely affect their functionality in some
counter-intuitive manner just because it comes first.
Such an organization is not possible using Prologue sections.
This section has been concerned with explaining the advantages of the four
Prologue alternatives over the original Yacc Prologue.
However, in most cases when using these directives, you shouldn't need to
think about all the low-level ordering issues discussed here.
Instead, you should simply use these directives to label each block of your
code according to its purpose and let Bison handle the ordering.
%code is the most generic label.
Move code to %code requires, %code provides, or %code top
as needed.
The Bison declarations section contains declarations that define terminal and nonterminal symbols, specify precedence, and so on. In some simple grammars you may not need any declarations. See Bison Declarations.
The grammar rules section contains one or more Bison grammar rules, and nothing else. See Syntax of Grammar Rules.
There must always be at least one grammar rule, and the first ‘%%’ (which precedes the grammar rules) may never be omitted even if it is the first thing in the file.
The Epilogue is copied verbatim to the end of the parser
implementation file, just as the Prologue is copied to the
beginning. This is the most convenient place to put anything that you
want to have in the parser implementation file but which need not come
before the definition of yyparse. For example, the definitions
of yylex and yyerror often go here. Because C requires
functions to be declared before being used, you often need to declare
functions like yylex and yyerror in the Prologue, even
if you define them in the Epilogue. See Parser C-Language Interface.
If the last section is empty, you may omit the ‘%%’ that separates it from the grammar rules.
The Bison parser itself contains many macros and identifiers whose names start with ‘yy’ or ‘YY’, so it is a good idea to avoid using any such names (except those documented in this manual) in the epilogue of the grammar file.
Symbols in Bison grammars represent the grammatical classifications of the language.
A terminal symbol (also known as a token type) represents a
class of syntactically equivalent tokens. You use the symbol in grammar
rules to mean that a token in that class is allowed. The symbol is
represented in the Bison parser by a numeric code, and the yylex
function returns a token type code to indicate what kind of token has
been read. You don't need to know what the code value is; you can use
the symbol to stand for it.
A nonterminal symbol stands for a class of syntactically equivalent groupings. The symbol name is used in writing grammar rules. By convention, it should be all lower case.
Symbol names can contain letters, underscores, periods, and non-initial digits and dashes. Dashes in symbol names are a GNU extension, incompatible with POSIX Yacc. Periods and dashes make symbol names less convenient to use with named references, which require brackets around such names (see Named References). Terminal symbols that contain periods or dashes make little sense: since they are not valid symbols (in most programming languages) they are not exported as token names.
There are three ways of writing terminal symbols in the grammar:
%token. See Token Type Names.
'+' is a character token type. A
character token type doesn't need to be declared unless you need to
specify its semantic value data type (see Data Types of Semantic Values), associativity, or precedence (see Operator Precedence).
By convention, a character token type is used only to represent a
token that consists of that particular character. Thus, the token
type '+' is used to represent the character ‘+’ as a
token. Nothing enforces this convention, but if you depart from it,
your program will confuse other readers.
All the usual escape sequences used in character literals in C can be
used in Bison as well, but you must not use the null character as a
character literal because its numeric code, zero, signifies
end-of-input (see Calling Convention for yylex). Also, unlike standard C, trigraphs have no
special meaning in Bison character literals, nor is backslash-newline
allowed.
"<=" is a literal string token. A literal string token
doesn't need to be declared unless you need to specify its semantic
value data type (see Value Type), associativity, or precedence
(see Precedence).
You can associate the literal string token with a symbolic name as an
alias, using the %token declaration (see Token Declarations). If you don't do that, the lexical analyzer has to
retrieve the token number for the literal string token from the
yytname table (see Calling Convention).
Warning: literal string tokens do not work in Yacc.
By convention, a literal string token is used only to represent a token
that consists of that particular string. Thus, you should use the token
type "<=" to represent the string ‘<=’ as a token. Bison
does not enforce this convention, but if you depart from it, people who
read your program will be confused.
All the escape sequences used in string literals in C can be used in Bison as well, except that you must not use a null character within a string literal. Also, unlike Standard C, trigraphs have no special meaning in Bison string literals, nor is backslash-newline allowed. A literal string token must contain two or more characters; for a token containing just one character, use a character token (see above).
How you choose to write a terminal symbol has no effect on its grammatical meaning. That depends only on where it appears in rules and on when the parser function returns that symbol.
The value returned by yylex is always one of the terminal
symbols, except that a zero or negative value signifies end-of-input.
Whichever way you write the token type in the grammar rules, you write
it the same way in the definition of yylex. The numeric code
for a character token type is simply the positive numeric code of the
character, so yylex can use the identical value to generate the
requisite code, though you may need to convert it to unsigned
char to avoid sign-extension on hosts where char is signed.
Each named token type becomes a C macro in the parser implementation
file, so yylex can use the name to stand for the code. (This
is why periods don't make sense in terminal symbols.) See Calling Convention for yylex.
If yylex is defined in a separate file, you need to arrange for the
token-type macro definitions to be available there. Use the ‘-d’
option when you run Bison, so that it will write these macro definitions
into a separate header file name.tab.h which you can include
in the other source files that need it. See Invoking Bison.
If you want to write a grammar that is portable to any Standard C host, you must use only nonnull character tokens taken from the basic execution character set of Standard C. This set consists of the ten digits, the 52 lower- and upper-case English letters, and the characters in the following C-language string:
"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_{|}~"
The yylex function and Bison must use a consistent character set
and encoding for character tokens. For example, if you run Bison in an
ASCII environment, but then compile and run the resulting
program in an environment that uses an incompatible character set like
EBCDIC, the resulting program may not work because the tables
generated by Bison will assume ASCII numeric values for
character tokens. It is standard practice for software distributions to
contain C source files that were generated by Bison in an
ASCII environment, so installers on platforms that are
incompatible with ASCII must rebuild those files before
compiling them.
The symbol error is a terminal symbol reserved for error recovery
(see Error Recovery); you shouldn't use it for any other purpose.
In particular, yylex should never return this value. The default
value of the error token is 256, unless you explicitly assigned 256 to
one of your tokens with a %token declaration.
A Bison grammar rule has the following general form:
result: components...
;
where result is the nonterminal symbol that this rule describes, and components are various terminal and nonterminal symbols that are put together by this rule (see Symbols).
For example,
exp: exp '+' exp
;
says that two groupings of type exp, with a ‘+’ token in between,
can be combined into a larger grouping of type exp.
White space in rules is significant only to separate symbols. You can add extra white space as you wish.
Scattered among the components can be actions that determine the semantics of the rule. An action looks like this:
{C statements}
This is an example of braced code, that is, C code surrounded by braces, much like a compound statement in C. Braced code can contain any sequence of C tokens, so long as its braces are balanced. Bison does not check the braced code for correctness directly; it merely copies the code to the parser implementation file, where the C compiler can check it.
Within braced code, the balanced-brace count is not affected by braces within comments, string literals, or character constants, but it is affected by the C digraphs ‘<%’ and ‘%>’ that represent braces. At the top level braced code must be terminated by ‘}’ and not by a digraph. Bison does not look for trigraphs, so if braced code uses trigraphs you should ensure that they do not affect the nesting of braces or the boundaries of comments, string literals, or character constants.
Usually there is only one action and it follows the components. See Actions.
Multiple rules for the same result can be written separately or can be joined with the vertical-bar character ‘|’ as follows:
result: rule1-components...
| rule2-components...
...
;
They are still considered distinct rules even when joined in this way.
If components in a rule is empty, it means that result can
match the empty string. For example, here is how to define a
comma-separated sequence of zero or more exp groupings:
expseq: /* empty */
| expseq1
;
expseq1: exp
| expseq1 ',' exp
;
It is customary to write a comment ‘/* empty */’ in each rule with no components.
A rule is called recursive when its result nonterminal appears also on its right hand side. Nearly all Bison grammars need to use recursion, because that is the only way to define a sequence of any number of a particular thing. Consider this recursive definition of a comma-separated sequence of one or more expressions:
expseq1: exp
| expseq1 ',' exp
;
Since the recursive use of expseq1 is the leftmost symbol in the
right hand side, we call this left recursion. By contrast, here
the same construct is defined using right recursion:
expseq1: exp
| exp ',' expseq1
;
Any kind of sequence can be defined using either left recursion or right recursion, but you should always use left recursion, because it can parse a sequence of any number of elements with bounded stack space. Right recursion uses up space on the Bison stack in proportion to the number of elements in the sequence, because all the elements must be shifted onto the stack before the rule can be applied even once. See The Bison Parser Algorithm, for further explanation of this.
Indirect or mutual recursion occurs when the result of the rule does not appear directly on its right hand side, but does appear in rules for other nonterminals which do appear on its right hand side.
For example:
expr: primary
| primary '+' primary
;
primary: constant
| '(' expr ')'
;
defines two mutually-recursive nonterminals, since each refers to the other.
The grammar rules for a language determine only the syntax. The semantics are determined by the semantic values associated with various tokens and groupings, and by the actions taken when various groupings are recognized.
For example, the calculator calculates properly because the value associated with each expression is the proper number; it adds properly because the action for the grouping ‘x + y’ is to add the numbers associated with x and y.
In a simple program it may be sufficient to use the same data type for the semantic values of all language constructs. This was true in the RPN and infix calculator examples (see Reverse Polish Notation Calculator).
Bison normally uses the type int for semantic values if your
program uses the same data type for all language constructs. To
specify some other type, define YYSTYPE as a macro, like this:
#define YYSTYPE double
YYSTYPE's replacement list should be a type name
that does not contain parentheses or square brackets.
This macro definition must go in the prologue of the grammar file
(see Outline of a Bison Grammar).
In most programs, you will need different data types for different kinds
of tokens and groupings. For example, a numeric constant may need type
int or long int, while a string constant needs type
char *, and an identifier might need a pointer to an entry in the
symbol table.
To use more than one data type for semantic values in one parser, Bison requires you to do two things:
%union Bison declaration (see The Collection of Value Types), or by using a typedef or a #define to
define YYSTYPE to be a union type whose member names are
the type tags.
%token Bison declaration (see Token Type Names)
and for groupings with the %type Bison declaration (see Nonterminal Symbols).
An action accompanies a syntactic rule and contains C code to be executed each time an instance of that rule is recognized. The task of most actions is to compute a semantic value for the grouping built by the rule from the semantic values associated with tokens or smaller groupings.
An action consists of braced code containing C statements, and can be placed at any position in the rule; it is executed at that position. Most rules have just one action at the end of the rule, following all the components. Actions in the middle of a rule are tricky and used only for special purposes (see Actions in Mid-Rule).
The C code in an action can refer to the semantic values of the
components matched by the rule with the construct $n,
which stands for the value of the nth component. The semantic
value for the grouping being constructed is $$. In addition,
the semantic values of symbols can be accessed with the named
references construct $name or $[name].
Bison translates both of these constructs into expressions of the
appropriate type when it copies the actions into the parser
implementation file. $$ (or $name, when it stands
for the current grouping) is translated to a modifiable lvalue, so it
can be assigned to.
Here is a typical example:
exp: ...
| exp '+' exp
{ $$ = $1 + $3; }
Or, in terms of named references:
exp[result]: ...
| exp[left] '+' exp[right]
{ $result = $left + $right; }
This rule constructs an exp from two smaller exp groupings
connected by a plus-sign token. In the action, $1 and $3
($left and $right)
refer to the semantic values of the two component exp groupings,
which are the first and third symbols on the right hand side of the rule.
The sum is stored into $$ ($result) so that it becomes the
semantic value of
the addition-expression just recognized by the rule. If there were a
useful semantic value associated with the ‘+’ token, it could be
referred to as $2.
See Using Named References, for more information about using the named references construct.
Note that the vertical-bar character ‘|’ is really a rule separator, and actions are attached to a single rule. This is a difference with tools like Flex, for which ‘|’ stands for either “or”, or “the same action as that of the next rule”. In the following example, the action is triggered only when ‘b’ is found:
a-or-b: 'a'|'b' { a_or_b_found = 1; };
If you don't specify an action for a rule, Bison supplies a default:
$$ = $1. Thus, the value of the first symbol in the rule
becomes the value of the whole rule. Of course, the default action is
valid only if the two data types match. There is no meaningful default
action for an empty rule; every empty rule must have an explicit action
unless the rule's value does not matter.
$n with n zero or negative is allowed for reference
to tokens and groupings on the stack before those that match the
current rule. This is a very risky practice, and to use it reliably
you must be certain of the context in which the rule is applied. Here
is a case in which you can use this reliably:
foo: expr bar '+' expr { ... }
| expr bar '-' expr { ... }
;
bar: /* empty */
{ previous_expr = $0; }
;
As long as bar is used only in the fashion shown here, $0
always refers to the expr which precedes bar in the
definition of foo.
It is also possible to access the semantic value of the lookahead token, if
any, from a semantic action.
This semantic value is stored in yylval.
See Special Features for Use in Actions.
If you have chosen a single data type for semantic values, the $$
and $n constructs always have that data type.
If you have used %union to specify a variety of data types, then you
must declare a choice among these types for each terminal or nonterminal
symbol that can have a semantic value. Then each time you use $$ or
$n, its data type is determined by which symbol it refers to
in the rule. In this example,
exp: ...
| exp '+' exp
{ $$ = $1 + $3; }
$1 and $3 refer to instances of exp, so they all
have the data type declared for the nonterminal symbol exp. If
$2 were used, it would have the data type declared for the
terminal symbol '+', whatever that might be.
Alternatively, you can specify the data type when you refer to the value, by inserting ‘<type>’ after the ‘$’ at the beginning of the reference. For example, if you have defined types as shown here:
%union {
int itype;
double dtype;
}
then you can write $<itype>1 to refer to the first subunit of the
rule as an integer, or $<dtype>1 to refer to it as a double.
Occasionally it is useful to put an action in the middle of a rule. These actions are written just like usual end-of-rule actions, but they are executed before the parser even recognizes the following components.
A mid-rule action may refer to the components preceding it using
$n, but it may not refer to subsequent components because
it is run before they are parsed.
The mid-rule action itself counts as one of the components of the rule.
This makes a difference when there is another action later in the same rule
(and usually there is another at the end): you have to count the actions
along with the symbols when working out which number n to use in
$n.
The mid-rule action can also have a semantic value. The action can set
its value with an assignment to $$, and actions later in the rule
can refer to the value using $n. Since there is no symbol
to name the action, there is no way to declare a data type for the value
in advance, so you must use the ‘$<...>n’ construct to
specify a data type each time you refer to this value.
There is no way to set the value of the entire rule with a mid-rule
action, because assignments to $$ do not have that effect. The
only way to set the value for the entire rule is with an ordinary action
at the end of the rule.
Here is an example from a hypothetical compiler, handling a let
statement that looks like ‘let (variable) statement’ and
serves to create a variable named variable temporarily for the
duration of statement. To parse this construct, we must put
variable into the symbol table while statement is parsed, then
remove it afterward. Here is how it is done:
stmt: LET '(' var ')'
{ $<context>$ = push_context ();
declare_variable ($3); }
stmt { $$ = $6;
pop_context ($<context>5); }
As soon as ‘let (variable)’ has been recognized, the first
action is run. It saves a copy of the current semantic context (the
list of accessible variables) as its semantic value, using alternative
context in the data-type union. Then it calls
declare_variable to add the new variable to that list. Once the
first action is finished, the embedded statement stmt can be
parsed. Note that the mid-rule action is component number 5, so the
‘stmt’ is component number 6.
After the embedded statement is parsed, its semantic value becomes the
value of the entire let-statement. Then the semantic value from the
earlier action is used to restore the prior list of variables. This
removes the temporary let-variable from the list so that it won't
appear to exist while the rest of the program is parsed.
In the above example, if the parser initiates error recovery (see Error Recovery) while parsing the tokens in the embedded statement stmt,
it might discard the previous semantic context $<context>5 without
restoring it.
Thus, $<context>5 needs a destructor (see Freeing Discarded Symbols).
However, Bison currently provides no means to declare a destructor specific to
a particular mid-rule action's semantic value.
One solution is to bury the mid-rule action inside a nonterminal symbol and to declare a destructor for that symbol:
%type <context> let
%destructor { pop_context ($$); } let
%%
stmt: let stmt
{ $$ = $2;
pop_context ($1); }
;
let: LET '(' var ')'
{ $$ = push_context ();
declare_variable ($3); }
;
Note that the action is now at the end of its rule. Any mid-rule action can be converted to an end-of-rule action in this way, and this is what Bison actually does to implement mid-rule actions.
Taking action before a rule is completely recognized often leads to conflicts since the parser must commit to a parse in order to execute the action. For example, the following two rules, without mid-rule actions, can coexist in a working parser because the parser can shift the open-brace token and look at what follows before deciding whether there is a declaration or not:
compound: '{' declarations statements '}'
| '{' statements '}'
;
But when we add a mid-rule action as follows, the rules become nonfunctional:
compound: { prepare_for_local_variables (); }
'{' declarations statements '}'
| '{' statements '}'
;
Now the parser is forced to decide whether to run the mid-rule action when it has read no farther than the open-brace. In other words, it must commit to using one rule or the other, without sufficient information to do it correctly. (The open-brace token is what is called the lookahead token at this time, since the parser is still deciding what to do about it. See Lookahead Tokens.)
You might think that you could correct the problem by putting identical actions into the two rules, like this:
compound: { prepare_for_local_variables (); }
'{' declarations statements '}'
| { prepare_for_local_variables (); }
'{' statements '}'
;
But this does not help, because Bison does not realize that the two actions are identical. (Bison never tries to understand the C code in an action.)
If the grammar is such that a declaration can be distinguished from a statement by the first token (which is true in C), then one solution which does work is to put the action after the open-brace, like this:
compound: '{' { prepare_for_local_variables (); }
declarations statements '}'
| '{' statements '}'
;
Now the first token of the following declaration or statement, which would in any case tell Bison which rule to use, can still do so.
Another solution is to bury the action inside a nonterminal symbol which serves as a subroutine:
subroutine: /* empty */
{ prepare_for_local_variables (); }
;
compound: subroutine
'{' declarations statements '}'
| subroutine
'{' statements '}'
;
Now Bison can execute the action in the rule for subroutine without
deciding which rule for compound it will eventually use.
While every semantic value can be accessed with positional references
$n and $$, it's often much more convenient to refer to
them by name. First of all, original symbol names may be used as named
references. For example:
invocation: op '(' args ')'
{ $invocation = new_invocation ($op, $args, @invocation); }
The positional $$, @$, $n, and @n can be
mixed with $name and @name arbitrarily. For example:
invocation: op '(' args ')'
{ $$ = new_invocation ($op, $args, @$); }
However, sometimes regular symbol names are not sufficient due to ambiguities:
exp: exp '/' exp
{ $exp = $exp / $exp; } // $exp is ambiguous.
exp: exp '/' exp
{ $$ = $1 / $exp; } // One usage is ambiguous.
exp: exp '/' exp
{ $$ = $1 / $3; } // No error.
When ambiguity occurs, explicitly declared names may be used for values and locations. Explicit names are declared as a bracketed name after a symbol appearance in rule definitions. For example:
exp[result]: exp[left] '/' exp[right]
{ $result = $left / $right; }
Explicit names may be declared for RHS and for LHS symbols as well. In order to access a semantic value generated by a mid-rule action, an explicit name may also be declared by putting a bracketed name after the closing brace of the mid-rule action code:
exp[res]: exp[x] '+' {$left = $x;}[left] exp[right]
{ $res = $left + $right; }
In references, in order to specify names containing dots and dashes, an explicit
bracketed syntax $[name] and @[name] must be used:
if-stmt: IF '(' expr ')' THEN then.stmt ';'
{ $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); }
It often happens that named references are followed by a dot, dash or other
C punctuation marks and operators. By default, Bison will read
$name.suffix as a reference to symbol value $name followed by
‘.suffix’, i.e., an access to the ‘suffix’ field of the semantic
value. In order to force Bison to recognize name.suffix in its entirety
as the name of a semantic value, bracketed syntax $[name.suffix]
must be used.
Though grammar rules and semantic actions are enough to write a fully functional parser, it can be useful to process some additional information, especially symbol locations.
The way locations are handled is defined by providing a data type, and actions to take when rules are matched.
Defining a data type for locations is much simpler than for semantic values, since all tokens and groupings always use the same type.
You can specify the type of locations by defining a macro called
YYLTYPE, just as you can specify the semantic value type by
defining a YYSTYPE macro (see Value Type).
When YYLTYPE is not defined, Bison uses a default structure type with
four members:
typedef struct YYLTYPE
{
int first_line;
int first_column;
int last_line;
int last_column;
} YYLTYPE;
When YYLTYPE is not defined, at the beginning of the parsing, Bison
initializes all these fields to 1 for yylloc. To initialize
yylloc with a custom location type (or to chose a different
initialization), use the %initial-action directive. See Performing Actions before Parsing.
Actions are not only useful for defining language semantics, but also for describing the behavior of the output parser with locations.
The most obvious way for building locations of syntactic groupings is very
similar to the way semantic values are computed. In a given rule, several
constructs can be used to access the locations of the elements being matched.
The location of the nth component of the right hand side is
@n, while the location of the left hand side grouping is
@$.
In addition, the named references construct @name and
@[name] may also be used to address the symbol locations.
See Using Named References, for more information
about using the named references construct.
Here is a basic example using the default data type for locations:
exp: ...
| exp '/' exp
{
@$.first_column = @1.first_column;
@$.first_line = @1.first_line;
@$.last_column = @3.last_column;
@$.last_line = @3.last_line;
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
fprintf (stderr,
"Division by zero, l%d,c%d-l%d,c%d",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
As for semantic values, there is a default action for locations that is
run each time a rule is matched. It sets the beginning of @$ to the
beginning of the first symbol, and the end of @$ to the end of the
last symbol.
With this default action, the location tracking can be fully automatic. The example above simply rewrites this way:
exp: ...
| exp '/' exp
{
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
fprintf (stderr,
"Division by zero, l%d,c%d-l%d,c%d",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
It is also possible to access the location of the lookahead token, if any,
from a semantic action.
This location is stored in yylloc.
See Special Features for Use in Actions.
Actually, actions are not the best place to compute locations. Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each
rule. The YYLLOC_DEFAULT macro is invoked each time a rule is
matched, before the associated action is run. It is also invoked
while processing a syntax error, to compute the error's location.
Before reporting an unresolvable syntactic ambiguity, a GLR
parser invokes YYLLOC_DEFAULT recursively to compute the location
of that ambiguity.
Most of the time, this macro is general enough to suppress location dedicated code from semantic actions.
The YYLLOC_DEFAULT macro takes three parameters. The first one is
the location of the grouping (the result of the computation). When a
rule is matched, the second parameter identifies locations of
all right hand side elements of the rule being matched, and the third
parameter is the size of the rule's right hand side.
When a GLR parser reports an ambiguity, which of multiple candidate
right hand sides it passes to YYLLOC_DEFAULT is undefined.
When processing a syntax error, the second parameter identifies locations
of the symbols that were discarded during error processing, and the third
parameter is the number of discarded symbols.
By default, YYLLOC_DEFAULT is defined this way:
# define YYLLOC_DEFAULT(Current, Rhs, N) \
do \
if (N) \
{ \
(Current).first_line = YYRHSLOC(Rhs, 1).first_line; \
(Current).first_column = YYRHSLOC(Rhs, 1).first_column; \
(Current).last_line = YYRHSLOC(Rhs, N).last_line; \
(Current).last_column = YYRHSLOC(Rhs, N).last_column; \
} \
else \
{ \
(Current).first_line = (Current).last_line = \
YYRHSLOC(Rhs, 0).last_line; \
(Current).first_column = (Current).last_column = \
YYRHSLOC(Rhs, 0).last_column; \
} \
while (0)
where YYRHSLOC (rhs, k) is the location of the kth symbol
in rhs when k is positive, and the location of the symbol
just before the reduction when k and n are both zero.
When defining YYLLOC_DEFAULT, you should consider that:
YYLLOC_DEFAULT.
The Bison declarations section of a Bison grammar defines the symbols used in formulating the grammar and the data types of semantic values. See Symbols.
All token type names (but not single-character literal tokens such as
'+' and '*') must be declared. Nonterminal symbols must be
declared if you need to specify which data type to use for the semantic
value (see More Than One Value Type).
The first rule in the grammar file also specifies the start symbol, by default. If you want some other symbol to be the start symbol, you must declare it explicitly (see Languages and Context-Free Grammars).
You may require the minimum version of Bison to process the grammar. If the requirement is not met, bison exits with an error (exit status 63).
%require "version"
The basic way to declare a token type name (terminal symbol) is as follows:
%token name
Bison will convert this into a #define directive in
the parser, so that the function yylex (if it is in this file)
can use the name name to stand for this token type's code.
Alternatively, you can use %left, %right, or
%nonassoc instead of %token, if you wish to specify
associativity and precedence. See Operator Precedence.
You can explicitly specify the numeric code for a token type by appending a nonnegative decimal or hexadecimal integer value in the field immediately following the token name:
%token NUM 300
%token XNUM 0x12d // a GNU extension
It is generally best, however, to let Bison choose the numeric codes for all token types. Bison will automatically select codes that don't conflict with each other or with normal characters.
In the event that the stack type is a union, you must augment the
%token or other token declaration to include the data type
alternative delimited by angle-brackets (see More Than One Value Type).
For example:
%union { /* define stack type */
double val;
symrec *tptr;
}
%token <val> NUM /* define token NUM and its type */
You can associate a literal string token with a token type name by
writing the literal string at the end of a %token
declaration which declares the name. For example:
%token arrow "=>"
For example, a grammar for the C language might specify these names with equivalent literal string tokens:
%token <operator> OR "||"
%token <operator> LE 134 "<="
%left OR "<="
Once you equate the literal string and the token name, you can use them
interchangeably in further declarations or the grammar rules. The
yylex function can use the token name or the literal string to
obtain the token type code number (see Calling Convention).
Syntax error messages passed to yyerror from the parser will reference
the literal string instead of the token name.
The token numbered as 0 corresponds to end of file; the following line allows for nicer error messages referring to “end of file” instead of “$end”:
%token END 0 "end of file"
Use the %left, %right or %nonassoc declaration to
declare a token and specify its precedence and associativity, all at
once. These are called precedence declarations.
See Operator Precedence, for general information on
operator precedence.
The syntax of a precedence declaration is nearly the same as that of
%token: either
%left symbols...
or
%left <type> symbols...
And indeed any of these declarations serves the purposes of %token.
But in addition, they specify the associativity and relative precedence for
all the symbols:
%left specifies
left-associativity (grouping x with y first) and
%right specifies right-associativity (grouping y with
z first). %nonassoc specifies no associativity, which
means that ‘x op y op z’ is
considered a syntax error.
For backward compatibility, there is a confusing difference between the
argument lists of %token and precedence declarations.
Only a %token can associate a literal string with a token type name.
A precedence declaration always interprets a literal string as a reference to a
separate token.
For example:
%left OR "<=" // Does not declare an alias.
%left OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=".
The %union declaration specifies the entire collection of
possible data types for semantic values. The keyword %union is
followed by braced code containing the same thing that goes inside a
union in C.
For example:
%union {
double val;
symrec *tptr;
}
This says that the two alternative types are double and symrec
*. They are given names val and tptr; these names are used
in the %token and %type declarations to pick one of the types
for a terminal or nonterminal symbol (see Nonterminal Symbols).
As an extension to POSIX, a tag is allowed after the
union. For example:
%union value {
double val;
symrec *tptr;
}
specifies the union tag value, so the corresponding C type is
union value. If you do not specify a tag, it defaults to
YYSTYPE.
As another extension to POSIX, you may specify multiple
%union declarations; their contents are concatenated. However,
only the first %union declaration can specify a tag.
Note that, unlike making a union declaration in C, you need not write
a semicolon after the closing brace.
Instead of %union, you can define and use your own union type
YYSTYPE if your grammar contains at least one
‘<type>’ tag. For example, you can put the following into
a header file parser.h:
union YYSTYPE {
double val;
symrec *tptr;
};
typedef union YYSTYPE YYSTYPE;
and then your grammar can use the following
instead of %union:
%{
#include "parser.h"
%}
%type <val> expr
%token <tptr> ID
When you use %union to specify multiple value types, you must
declare the value type of each nonterminal symbol for which values are
used. This is done with a %type declaration, like this:
%type <type> nonterminal...
Here nonterminal is the name of a nonterminal symbol, and
type is the name given in the %union to the alternative
that you want (see The Collection of Value Types). You
can give any number of nonterminal symbols in the same %type
declaration, if they have the same value type. Use spaces to separate
the symbol names.
You can also declare the value type of a terminal symbol. To do this,
use the same <type> construction in a declaration for the
terminal symbol. All kinds of token declarations allow
<type>.
Sometimes your parser needs to perform some initializations before
parsing. The %initial-action directive allows for such arbitrary
code.
Declare that the braced code must be invoked before parsing each time
yyparseis called. The code may use$$and@$— initial value and location of the lookahead — and the%parse-param.
For instance, if your locations use a file name, you may use
%parse-param { char const *file_name };
%initial-action
{
@$.initialize (file_name);
};
During error recovery (see Error Recovery), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded
until the parser falls on its feet. If the parser runs out of memory,
or if it returns via YYABORT or YYACCEPT, all the
symbols on the stack must be discarded. Even if the parser succeeds, it
must discard the start symbol.
When discarded symbols convey heap based information, this memory is lost. While this behavior can be tolerable for batch parsers, such as in traditional compilers, it is unacceptable for programs like shells or protocol implementations that may parse and execute indefinitely.
The %destructor directive defines code that is called when a
symbol is automatically discarded.
Invoke the braced code whenever the parser discards one of the symbols. Within code,
$$designates the semantic value associated with the discarded symbol, and@$designates its location. The additional parser parameters are also available (see The Parser Functionyyparse).When a symbol is listed among symbols, its
%destructoris called a per-symbol%destructor. You may also define a per-type%destructorby listing a semantic type tag among symbols. In that case, the parser will invoke this code whenever it discards any grammar symbol that has that semantic type tag unless that symbol has its own per-symbol%destructor.Finally, you can define two different kinds of default
%destructors. (These default forms are experimental. More user feedback will help to determine whether they should become permanent features.) You can place each of<*>and<>in the symbols list of exactly one%destructordeclaration in your grammar file. The parser will invoke the code associated with one of these whenever it discards any user-defined grammar symbol that has no per-symbol and no per-type%destructor. The parser uses the code for<*>in the case of such a grammar symbol for which you have formally declared a semantic type tag (%typecounts as such a declaration, but$<tag>$does not). The parser uses the code for<>in the case of such a grammar symbol that has no declared semantic type tag.
For example:
%union { char *string; }
%token <string> STRING1
%token <string> STRING2
%type <string> string1
%type <string> string2
%union { char character; }
%token <character> CHR
%type <character> chr
%token TAGLESS
%destructor { } <character>
%destructor { free ($$); } <*>
%destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
%destructor { printf ("Discarding tagless symbol.\n"); } <>
guarantees that, when the parser discards any user-defined symbol that has a
semantic type tag other than <character>, it passes its semantic value
to free by default.
However, when the parser discards a STRING1 or a string1, it also
prints its line number to stdout.
It performs only the second %destructor in this case, so it invokes
free only once.
Finally, the parser merely prints a message whenever it discards any symbol,
such as TAGLESS, that has no semantic type tag.
A Bison-generated parser invokes the default %destructors only for
user-defined as opposed to Bison-defined symbols.
For example, the parser will not invoke either kind of default
%destructor for the special Bison-defined symbols $accept,
$undefined, or $end (see Bison Symbols),
none of which you can reference in your grammar.
It also will not invoke either for the error token (see error), which is always defined by Bison regardless of whether you
reference it in your grammar.
However, it may invoke one of them for the end token (token 0) if you
redefine it from $end to, for example, END:
%token END 0
Finally, Bison will never invoke a %destructor for an unreferenced
mid-rule semantic value (see Actions in Mid-Rule).
That is, Bison does not consider a mid-rule to have a semantic value if you do
not reference $$ in the mid-rule's action or $n (where
n is the RHS symbol position of the mid-rule) in any later action in that
rule.
However, if you do reference either, the Bison-generated parser will invoke the
<> %destructor whenever it discards the mid-rule symbol.
Discarded symbols are the following:
The parser can return immediately because of an explicit call to
YYABORT or YYACCEPT, or failed error recovery, or memory
exhaustion.
Right-hand side symbols of a rule that explicitly triggers a syntax
error via YYERROR are not discarded automatically. As a rule
of thumb, destructors are invoked only when user actions cannot manage
the memory.
Bison normally warns if there are any conflicts in the grammar
(see Shift/Reduce Conflicts), but most real grammars
have harmless shift/reduce conflicts which are resolved in a predictable
way and would be difficult to eliminate. It is desirable to suppress
the warning about these conflicts unless the number of conflicts
changes. You can do this with the %expect declaration.
The declaration looks like this:
%expect n
Here n is a decimal integer. The declaration says there should be n shift/reduce conflicts and no reduce/reduce conflicts. Bison reports an error if the number of shift/reduce conflicts differs from n, or if there are any reduce/reduce conflicts.
For deterministic parsers, reduce/reduce conflicts are more serious, and should be eliminated entirely. Bison will always report reduce/reduce conflicts for these parsers. With GLR parsers, however, both kinds of conflicts are routine; otherwise, there would be no need to use GLR parsing. Therefore, it is also possible to specify an expected number of reduce/reduce conflicts in GLR parsers, using the declaration:
%expect-rr n
In general, using %expect involves these steps:
%expect. Use the ‘-v’ option
to get a verbose list of where the conflicts occur. Bison will also
print the number of conflicts.
%expect declaration, copying the number n from the
number which Bison printed. With GLR parsers, add an
%expect-rr declaration as well.
Now Bison will report an error if you introduce an unexpected conflict, but will keep silent otherwise.
Bison assumes by default that the start symbol for the grammar is the first
nonterminal specified in the grammar specification section. The programmer
may override this restriction with the %start declaration as follows:
%start symbol
A reentrant program is one which does not alter in the course of execution; in other words, it consists entirely of pure (read-only) code. Reentrancy is important whenever asynchronous execution is possible; for example, a nonreentrant program may not be safe to call from a signal handler. In systems with multiple threads of control, a nonreentrant program must be called only within interlocks.
Normally, Bison generates a parser which is not reentrant. This is
suitable for most uses, and it permits compatibility with Yacc. (The
standard Yacc interfaces are inherently nonreentrant, because they use
statically allocated variables for communication with yylex,
including yylval and yylloc.)
Alternatively, you can generate a pure, reentrant parser. The Bison
declaration %define api.pure says that you want the parser to be
reentrant. It looks like this:
%define api.pure
The result is that the communication variables yylval and
yylloc become local variables in yyparse, and a different
calling convention is used for the lexical analyzer function
yylex. See Calling Conventions for Pure Parsers, for the details of this. The variable yynerrs
becomes local in yyparse in pull mode but it becomes a member
of yypstate in push mode. (see The Error Reporting Function yyerror). The convention for calling
yyparse itself is unchanged.
Whether the parser is pure has nothing to do with the grammar rules. You can generate either a pure parser or a nonreentrant parser from any valid grammar.
(The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
A pull parser is called once and it takes control until all its input is completely parsed. A push parser, on the other hand, is called each time a new token is made available.
A push parser is typically useful when the parser is part of a main event loop in the client's application. This is typically a requirement of a GUI, when the main event loop needs to be triggered within a certain time period.
Normally, Bison generates a pull parser. The following Bison declaration says that you want the parser to be a push parser (see api.push-pull):
%define api.push-pull push
In almost all cases, you want to ensure that your push parser is also a pure parser (see A Pure (Reentrant) Parser). The only time you should create an impure push parser is to have backwards compatibility with the impure Yacc pull mode interface. Unless you know what you are doing, your declarations should look like this:
%define api.pure
%define api.push-pull push
There is a major notable functional difference between the pure push parser and the impure push parser. It is acceptable for a pure push parser to have many parser instances, of the same type of parser, in memory at the same time. An impure push parser should only use one parser at a time.
When a push parser is selected, Bison will generate some new symbols in
the generated parser. yypstate is a structure that the generated
parser uses to store the parser's state. yypstate_new is the
function that will create a new parser instance. yypstate_delete
will free the resources associated with the corresponding parser instance.
Finally, yypush_parse is the function that should be called whenever a
token is available to provide the parser. A trivial example
of using a pure push parser would look like this:
int status;
yypstate *ps = yypstate_new ();
do {
status = yypush_parse (ps, yylex (), NULL);
} while (status == YYPUSH_MORE);
yypstate_delete (ps);
If the user decided to use an impure push parser, a few things about
the generated parser will change. The yychar variable becomes
a global variable instead of a variable in the yypush_parse function.
For this reason, the signature of the yypush_parse function is
changed to remove the token as a parameter. A nonreentrant push parser
example would thus look like this:
extern int yychar;
int status;
yypstate *ps = yypstate_new ();
do {
yychar = yylex ();
status = yypush_parse (ps);
} while (status == YYPUSH_MORE);
yypstate_delete (ps);
That's it. Notice the next token is put into the global variable yychar
for use by the next invocation of the yypush_parse function.
Bison also supports both the push parser interface along with the pull parser
interface in the same generated parser. In order to get this functionality,
you should replace the %define api.push-pull push declaration with the
%define api.push-pull both declaration. Doing this will create all of
the symbols mentioned earlier along with the two extra symbols, yyparse
and yypull_parse. yyparse can be used exactly as it normally
would be used. However, the user should note that it is implemented in the
generated parser by calling yypull_parse.
This makes the yyparse function that is generated with the
%define api.push-pull both declaration slower than the normal
yyparse function. If the user
calls the yypull_parse function it will parse the rest of the input
stream. It is possible to yypush_parse tokens to select a subgrammar
and then yypull_parse the rest of the input stream. If you would like
to switch back and forth between between parsing styles, you would have to
write your own yypull_parse function that knows when to quit looking
for input. An example of using the yypull_parse function would look
like this:
yypstate *ps = yypstate_new ();
yypull_parse (ps); /* Will call the lexer */
yypstate_delete (ps);
Adding the %define api.pure declaration does exactly the same thing to
the generated parser with %define api.push-pull both as it did for
%define api.push-pull push.
Here is a summary of the declarations used to define a grammar:
Declare the collection of data types that semantic values may have (see The Collection of Value Types).
Declare a terminal symbol (token type name) with no precedence or associativity specified (see Token Type Names).
Declare a terminal symbol (token type name) that is right-associative (see Operator Precedence).
Declare a terminal symbol (token type name) that is left-associative (see Operator Precedence).
Declare a terminal symbol (token type name) that is nonassociative (see Operator Precedence). Using it in a way that would be associative is a syntax error.
Declare the type of semantic values for a nonterminal symbol (see Nonterminal Symbols).
Declare the expected number of shift-reduce conflicts (see Suppressing Conflict Warnings).
In order to change the behavior of bison, use the following directives:
Insert code verbatim into the output parser source at the default location or at the location specified by qualifier. See %code Summary.
In the parser implementation file, define the macro
YYDEBUGto 1 if it is not already defined, so that the debugging facilities are compiled. See Tracing Your Parser.
Define a variable to adjust Bison's behavior. See %define Summary.
Write a parser header file containing macro definitions for the token type names defined in the grammar as well as a few other declarations. If the parser implementation file is named name.c then the parser header file is named name.h.
For C parsers, the parser header file declares
YYSTYPEunlessYYSTYPEis already defined as a macro or you have used a<type>tag without using%union. Therefore, if you are using a%union(see More Than One Value Type) with components that require other definitions, or if you have defined aYYSTYPEmacro or type definition (see Data Types of Semantic Values), you need to arrange for these definitions to be propagated to all modules, e.g., by putting them in a prerequisite header that is included both by your parser and by any other module that needsYYSTYPE.Unless your parser is pure, the parser header file declares
yylvalas an external variable. See A Pure (Reentrant) Parser.If you have also used locations, the parser header file declares
YYLTYPEandyyllocusing a protocol similar to that of theYYSTYPEmacro andyylval. See Tracking Locations.This parser header file is normally essential if you wish to put the definition of
yylexin a separate source file, becauseyylextypically needs to be able to refer to the above-mentioned declarations and to the token type codes. See Semantic Values of Tokens.If you have declared
%code requiresor%code provides, the output header also contains their code. See %code Summary.
Specify how the parser should reclaim the memory associated to discarded symbols. See Freeing Discarded Symbols.
Specify a prefix to use for all Bison output file names. The names are chosen as if the grammar file were named prefix.y.
Specify the programming language for the generated parser. Currently supported languages include C, C++, and Java. language is case-insensitive.
This directive is experimental and its effect may be modified in future releases.
Generate the code processing the locations (see Special Features for Use in Actions). This mode is enabled as soon as the grammar uses the special ‘@n’ tokens, but if your grammar does not use it, using ‘%locations’ allows for more accurate syntax error messages.
Rename the external symbols used in the parser so that they start with prefix instead of ‘yy’. The precise list of symbols renamed in C parsers is
yyparse,yylex,yyerror,yynerrs,yylval,yychar,yydebug, and (if locations are used)yylloc. If you use a push parser,yypush_parse,yypull_parse,yypstate,yypstate_newandyypstate_deletewill also be renamed. For example, if you use ‘%name-prefix "c_"’, the names becomec_parse,c_lex, and so on. For C++ parsers, see the%define namespacedocumentation in this section. See Multiple Parsers in the Same Program.
Don't generate any
#linepreprocessor commands in the parser implementation file. Ordinarily Bison writes these commands in the parser implementation file so that the C compiler and debuggers will associate errors and object code with your source file (the grammar file). This directive causes them to associate errors with the parser implementation file, treating it as an independent source file in its own right.
Deprecated version of
%define api.pure(see api.pure), for which Bison is more careful to warn about unreasonable usage.
Require version version or higher of Bison. See Require a Version of Bison.
Specify the skeleton to use.
If file does not contain a
/, file is the name of a skeleton file in the Bison installation directory. If it does, file is an absolute file name or a file name relative to the directory of the grammar file. This is similar to how most shells resolve commands.
Generate an array of token names in the parser implementation file. The name of the array is
yytname;yytname[i]is the name of the token whose internal Bison token code number is i. The first three elements ofyytnamecorrespond to the predefined tokens"$end","error", and"$undefined"; after these come the symbols defined in the grammar file.The name in the table includes all the characters needed to represent the token in Bison. For single-character literals and literal strings, this includes the surrounding quoting characters and any escape sequences. For example, the Bison single-character literal
'+'corresponds to a three-character name, represented in C as"'+'"; and the Bison two-character literal string"\\/"corresponds to a five-character name, represented in C as"\"\\\\/\"".When you specify
%token-table, Bison also generates macro definitions for macrosYYNTOKENS,YYNNTS, andYYNRULES, andYYNSTATES:
YYNTOKENS- The highest token number, plus one.
YYNNTS- The number of nonterminal symbols.
YYNRULES- The number of grammar rules,
YYNSTATES- The number of parser states (see Parser States).
Write an extra output file containing verbose descriptions of the parser states and what is done for each type of lookahead token in that state. See Understanding Your Parser, for more information.
Pretend the option --yacc was given, i.e., imitate Yacc, including its naming conventions. See Bison Options, for more.
There are many features of Bison's behavior that can be controlled by
assigning the feature a single value. For historical reasons, some
such features are assigned values by dedicated directives, such as
%start, which assigns the start symbol. However, newer such
features are associated with variables, which are assigned by the
%define directive:
Define variable to value.
value must be placed in quotation marks if it contains any character other than a letter, underscore, period, or non-initial dash or digit. Omitting
"value"entirely is always equivalent to specifying"".It is an error if a variable is defined by
%definemultiple times, but see -D name[=value].
The rest of this section summarizes variables and values that
%define accepts.
Some variables take Boolean values. In this case, Bison will complain if the variable definition does not meet one of the following four conditions:
true
"" is specified).
This is equivalent to true.
false.
What variables are accepted, as well as their meanings and default values, depend on the selected target language and/or the parser skeleton (see %language, see %skeleton). Unaccepted variables produce an error. Some of the accepted variables are:
false
pull, push, both
pull
most, consistent, accepting
accepting if lr.type is canonical-lr.
most otherwise.
false
lalr, ielr, canonical-lr
lalr
%define namespace "foo::bar"
Bison uses foo::bar verbatim in references such as:
foo::bar::parser::semantic_type
However, to open a namespace, Bison removes any leading :: and then
splits on any remaining occurrences:
namespace foo { namespace bar {
class position;
class location;
} }
"::".
For example, "foo" or "::foo::bar".
%name-prefix, which defaults
to yy.
This usage of %name-prefix is for backward compatibility and can be
confusing since %name-prefix also specifies the textual prefix for the
lexical analyzer function.
Thus, if you specify %name-prefix, it is best to also specify
%define namespace so that %name-prefix only affects the
lexical analyzer function.
For example, if you specify:
%define namespace "foo"
%name-prefix "bar::"
The parser namespace is foo and yylex is referenced as
bar::lex.
none, full
none
The %code directive inserts code verbatim into the output
parser source at any of a predefined set of locations. It thus serves
as a flexible and user-friendly alternative to the traditional Yacc
prologue, %{code%}. This section summarizes the
functionality of %code for the various target languages
supported by Bison. For a detailed discussion of how to use
%code in place of %{code%} for C/C++ and why it
is advantageous to do so, see Prologue Alternatives.
This is the unqualified form of the
%codedirective. It inserts code verbatim at a language-dependent default location in the parser implementation.For C/C++, the default location is the parser implementation file after the usual contents of the parser header file. Thus, the unqualified form replaces
%{code%}for most purposes.For Java, the default location is inside the parser class.
This is the qualified form of the
%codedirective. qualifier identifies the purpose of code and thus the location(s) where Bison should insert it. That is, if you need to specify location-sensitive code that does not belong at the default location selected by the unqualified%codeform, use this form instead.
For any particular qualifier or for the unqualified form, if there are
multiple occurrences of the %code directive, Bison concatenates
the specified code in the order in which it appears in the grammar
file.
Not all qualifiers are accepted for all target languages. Unaccepted qualifiers produce an error. Some of the accepted qualifiers are:
YYSTYPE and YYLTYPE.
In other words, it's the best place to define types referenced in %union
directives, and it's the best place to override Bison's default YYSTYPE
and YYLTYPE definitions.
YYSTYPE and YYLTYPE
definitions.
YYSTYPE, YYLTYPE, and
token definitions.
%code or %code requires
should usually be more appropriate than %code top. However,
occasionally it is necessary to insert code much nearer the top of the
parser implementation file. For example:
%code top {
#define _GNU_SOURCE
#include <stdio.h>
}
Though we say the insertion locations are language-dependent, they are technically skeleton-dependent. Writers of non-standard skeletons however should choose their locations consistently with the behavior of the standard Bison skeletons.
Most programs that use Bison parse only one language and therefore contain
only one Bison parser. But what if you want to parse more than one
language with the same program? Then you need to avoid a name conflict
between different definitions of yyparse, yylval, and so on.
The easy way to do this is to use the option ‘-p prefix’ (see Invoking Bison). This renames the interface functions and variables of the Bison parser to start with prefix instead of ‘yy’. You can use this to give each parser distinct names that do not conflict.
The precise list of symbols renamed is yyparse, yylex,
yyerror, yynerrs, yylval, yylloc,
yychar and yydebug. If you use a push parser,
yypush_parse, yypull_parse, yypstate,
yypstate_new and yypstate_delete will also be renamed.
For example, if you use ‘-p c’, the names become cparse,
clex, and so on.
All the other variables and macros associated with Bison are not
renamed. These others are not global; there is no conflict if the same
name is used in different parsers. For example, YYSTYPE is not
renamed, but defining this in different ways in different parsers causes
no trouble (see Data Types of Semantic Values).
The ‘-p’ option works by adding macro definitions to the
beginning of the parser implementation file, defining yyparse
as prefixparse, and so on. This effectively substitutes
one name for the other in the entire parser implementation file.
The Bison parser is actually a C function named yyparse. Here we
describe the interface conventions of yyparse and the other
functions that it needs to use.
Keep in mind that the parser uses many C identifiers starting with ‘yy’ and ‘YY’ for internal purposes. If you use such an identifier (aside from those in this manual) in an action or in epilogue in the grammar file, you are likely to run into trouble.
yyparse
You call the function yyparse to cause parsing to occur. This
function reads tokens, executes actions, and ultimately returns when it
encounters end-of-input or an unrecoverable syntax error. You can also
write an action which directs yyparse to return immediately
without reading further.
The value returned by
yyparseis 0 if parsing was successful (return is due to end-of-input).The value is 1 if parsing failed because of invalid input, i.e., input that contains a syntax error or that causes
YYABORTto be invoked.The value is 2 if parsing failed due to memory exhaustion.
In an action, you can cause immediate return from yyparse by using
these macros:
If you use a reentrant parser, you can optionally pass additional
parameter information to it in a reentrant way. To do so, use the
declaration %parse-param:
Declare that an argument declared by the braced-code argument-declaration is an additional
yyparseargument. The argument-declaration is used when declaring functions or prototypes. The last identifier in argument-declaration must be the argument name.
Here's an example. Write this in the parser:
%parse-param {int *nastiness}
%parse-param {int *randomness}
Then call the parser like this:
{
int nastiness, randomness;
... /* Store proper data in nastiness and randomness. */
value = yyparse (&nastiness, &randomness);
...
}
In the grammar actions, use expressions like this to refer to the data:
exp: ... { ...; *randomness += 1; ... }
yypush_parse(The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
You call the function yypush_parse to parse a single token. This
function is available if either the %define api.push-pull push or
%define api.push-pull both declaration is used.
See A Push Parser.
The value returned by
yypush_parseis the same as for yyparse with the following exception.yypush_parsewill return YYPUSH_MORE if more input is required to finish parsing the grammar.
yypull_parse(The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
You call the function yypull_parse to parse the rest of the input
stream. This function is available if the %define api.push-pull both
declaration is used.
See A Push Parser.
The value returned by
yypull_parseis the same as foryyparse.
yystate_new(The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
You call the function yypstate_new to create a new parser instance.
This function is available if either the %define api.push-pull push or
%define api.push-pull both declaration is used.
See A Push Parser.
The function will return a valid parser instance if there was memory available or 0 if no memory was available. In impure mode, it will also return 0 if a parser instance is currently allocated.
yystate_delete(The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
You call the function yypstate_delete to delete a parser instance.
function is available if either the %define api.push-pull push or
%define api.push-pull both declaration is used.
See A Push Parser.
This function will reclaim the memory associated with a parser instance. After this call, you should no longer attempt to use the parser instance.
yylex
The lexical analyzer function, yylex, recognizes tokens from
the input stream and returns them to the parser. Bison does not create
this function automatically; you must write it so that yyparse can
call it. The function is sometimes referred to as a lexical scanner.
In simple programs, yylex is often defined at the end of the
Bison grammar file. If yylex is defined in a separate source
file, you need to arrange for the token-type macro definitions to be
available there. To do this, use the ‘-d’ option when you run
Bison, so that it will write these macro definitions into the separate
parser header file, name.tab.h, which you can include in
the other source files that need it. See Invoking Bison.
yylexThe value that yylex returns must be the positive numeric code
for the type of token it has just found; a zero or negative value
signifies end-of-input.
When a token is referred to in the grammar rules by a name, that name
in the parser implementation file becomes a C macro whose definition
is the proper numeric code for that token type. So yylex can
use the name to indicate that type. See Symbols.
When a token is referred to in the grammar rules by a character literal,
the numeric code for that character is also the code for the token type.
So yylex can simply return that character code, possibly converted
to unsigned char to avoid sign-extension. The null character
must not be used this way, because its code is zero and that
signifies end-of-input.
Here is an example showing these things:
int
yylex (void)
{
...
if (c == EOF) /* Detect end-of-input. */
return 0;
...
if (c == '+' || c == '-')
return c; /* Assume token type for `+' is '+'. */
...
return INT; /* Return the type of the token. */
...
}
This interface has been designed so that the output from the lex
utility can be used without change as the definition of yylex.
If the grammar uses literal string tokens, there are two ways that
yylex can determine the token type codes for them:
yylex can use these symbolic names like
all others. In this case, the use of the literal string tokens in
the grammar file has no effect on yylex.
yylex can find the multicharacter token in the yytname
table. The index of the token in the table is the token type's code.
The name of a multicharacter token is recorded in yytname with a
double-quote, the token's characters, and another double-quote. The
token's characters are escaped as necessary to be suitable as input
to Bison.
Here's code for looking up a multicharacter token in yytname,
assuming that the characters of the token are stored in
token_buffer, and assuming that the token does not contain any
characters like ‘"’ that require escaping.
for (i = 0; i < YYNTOKENS; i++)
{
if (yytname[i] != 0
&& yytname[i][0] == '"'
&& ! strncmp (yytname[i] + 1, token_buffer,
strlen (token_buffer))
&& yytname[i][strlen (token_buffer) + 1] == '"'
&& yytname[i][strlen (token_buffer) + 2] == 0)
break;
}
The yytname table is generated only if you use the
%token-table declaration. See Decl Summary.
In an ordinary (nonreentrant) parser, the semantic value of the token must
be stored into the global variable yylval. When you are using
just one data type for semantic values, yylval has that type.
Thus, if the type is int (the default), you might write this in
yylex:
...
yylval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
When you are using multiple data types, yylval's type is a union
made from the %union declaration (see The Collection of Value Types). So when you store a token's value, you
must use the proper member of the union. If the %union
declaration looks like this:
%union {
int intval;
double val;
symrec *tptr;
}
then the code in yylex might look like this:
...
yylval.intval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
If you are using the ‘@n’-feature (see Tracking Locations) in actions to keep track of the textual locations
of tokens and groupings, then you must provide this information in
yylex. The function yyparse expects to find the textual
location of a token just parsed in the global variable yylloc.
So yylex must store the proper data in that variable.
By default, the value of yylloc is a structure and you need only
initialize the members that are going to be used by the actions. The
four members are called first_line, first_column,
last_line and last_column. Note that the use of this
feature makes the parser noticeably slower.
The data type of yylloc has the name YYLTYPE.
When you use the Bison declaration %define api.pure to request a
pure, reentrant parser, the global communication variables yylval
and yylloc cannot be used. (See A Pure (Reentrant) Parser.) In such parsers the two global variables are replaced by
pointers passed as arguments to yylex. You must declare them as
shown here, and pass the information back by storing it through those
pointers.
int
yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
{
...
*lvalp = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
}
If the grammar file does not use the ‘@’ constructs to refer to
textual locations, then the type YYLTYPE will not be defined. In
this case, omit the second argument; yylex will be called with
only one argument.
If you wish to pass the additional parameter data to yylex, use
%lex-param just like %parse-param (see Parser Function).
Declare that the braced-code argument-declaration is an additional
yylexargument declaration.
For instance:
%parse-param {int *nastiness}
%lex-param {int *nastiness}
%parse-param {int *randomness}
results in the following signature:
int yylex (int *nastiness);
int yyparse (int *nastiness, int *randomness);
If %define api.pure is added:
int yylex (YYSTYPE *lvalp, int *nastiness);
int yyparse (int *nastiness, int *randomness);
and finally, if both %define api.pure and %locations are used:
int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
int yyparse (int *nastiness, int *randomness);
yyerror
The Bison parser detects a syntax error or parse error
whenever it reads a token which cannot satisfy any syntax rule. An
action in the grammar can also explicitly proclaim an error, using the
macro YYERROR (see Special Features for Use in Actions).
The Bison parser expects to report the error by calling an error
reporting function named yyerror, which you must supply. It is
called by yyparse whenever a syntax error is found, and it
receives one argument. For a syntax error, the string is normally
"syntax error".
If you invoke the directive %error-verbose in the Bison declarations
section (see The Bison Declarations Section), then
Bison provides a more verbose and specific error message string instead of
just plain "syntax error". However, that message sometimes
contains incorrect information if LAC is not enabled (see LAC).
The parser can detect one other kind of error: memory exhaustion. This
can happen when the input contains constructions that are very deeply
nested. It isn't likely you will encounter this, since the Bison
parser normally extends its stack automatically up to a very large limit. But
if memory is exhausted, yyparse calls yyerror in the usual
fashion, except that the argument string is "memory exhausted".
In some cases diagnostics like "syntax error" are
translated automatically from English to some other language before
they are passed to yyerror. See Internationalization.
The following definition suffices in simple programs:
void
yyerror (char const *s)
{
fprintf (stderr, "%s\n", s);
}
After yyerror returns to yyparse, the latter will attempt
error recovery if you have written suitable error recovery grammar rules
(see Error Recovery). If recovery is impossible, yyparse will
immediately return 1.
Obviously, in location tracking pure parsers, yyerror should have
an access to the current location.
This is indeed the case for the GLR
parsers, but not for the Yacc parser, for historical reasons. I.e., if
‘%locations %define api.pure’ is passed then the prototypes for
yyerror are:
void yyerror (char const *msg); /* Yacc parsers. */
void yyerror (YYLTYPE *locp, char const *msg); /* GLR parsers. */
If ‘%parse-param {int *nastiness}’ is used, then:
void yyerror (int *nastiness, char const *msg); /* Yacc parsers. */
void yyerror (int *nastiness, char const *msg); /* GLR parsers. */
Finally, GLR and Yacc parsers share the same yyerror calling
convention for absolutely pure parsers, i.e., when the calling
convention of yylex and the calling convention of
%define api.pure are pure.
I.e.:
/* Location tracking. */
%locations
/* Pure yylex. */
%define api.pure
%lex-param {int *nastiness}
/* Pure yyparse. */
%parse-param {int *nastiness}
%parse-param {int *randomness}
results in the following signatures for all the parser kinds:
int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
int yyparse (int *nastiness, int *randomness);
void yyerror (YYLTYPE *locp,
int *nastiness, int *randomness,
char const *msg);
The prototypes are only indications of how the code produced by Bison
uses yyerror. Bison-generated code always ignores the returned
value, so yyerror can return any type, including void.
Also, yyerror can be a variadic function; that is why the
message is always passed last.
Traditionally yyerror returns an int that is always
ignored, but this is purely for historical reasons, and void is
preferable since it more accurately describes the return type for
yyerror.
The variable yynerrs contains the number of syntax errors
reported so far. Normally this variable is global; but if you
request a pure parser (see A Pure (Reentrant) Parser)
then it is a local variable which only the actions can access.
Here is a table of Bison constructs, variables and macros that are useful in actions.
Acts like a variable that contains the semantic value for the grouping made by the current rule. See Actions.
Acts like a variable that contains the semantic value for the nth component of the current rule. See Actions.
Like
$$but specifies alternative typealt in the union specified by the%uniondeclaration. See Data Types of Values in Actions.
Like
$n but specifies alternative typealt in the union specified by the%uniondeclaration. See Data Types of Values in Actions.
Return immediately from
yyparse, indicating failure. See The Parser Functionyyparse.
Return immediately from
yyparse, indicating success. See The Parser Functionyyparse.
Unshift a token. This macro is allowed only for rules that reduce a single value, and only when there is no lookahead token. It is also disallowed in GLR parsers. It installs a lookahead token with token type token and semantic value value; then it discards the value that was going to be reduced by this rule.
If the macro is used when it is not valid, such as when there is a lookahead token already, then it reports a syntax error with a message ‘cannot back up’ and performs ordinary error recovery.
In either case, the rest of the action is not executed.
Cause an immediate syntax error. This statement initiates error recovery just as if the parser itself had detected an error; however, it does not call
yyerror, and does not print any message. If you want to print an error message, callyyerrorexplicitly before the ‘YYERROR;’ statement. See Error Recovery.
The expression
YYRECOVERING ()yields 1 when the parser is recovering from a syntax error, and 0 otherwise. See Error Recovery.
Variable containing either the lookahead token, or
YYEOFwhen the lookahead is the end of the input stream, orYYEMPTYwhen no lookahead has been performed so the next token is not yet known. Do not modifyyycharin a deferred semantic action (see GLR Semantic Actions). See Lookahead Tokens.
Discard the current lookahead token. This is useful primarily in error rules. Do not invoke
yyclearinin a deferred semantic action (see GLR Semantic Actions). See Error Recovery.
Resume generating error messages immediately for subsequent syntax errors. This is useful primarily in error rules. See Error Recovery.
Variable containing the lookahead token location when
yycharis not set toYYEMPTYorYYEOF. Do not modifyyyllocin a deferred semantic action (see GLR Semantic Actions). See Actions and Locations.
Variable containing the lookahead token semantic value when
yycharis not set toYYEMPTYorYYEOF. Do not modifyyylvalin a deferred semantic action (see GLR Semantic Actions). See Actions.
Acts like a structure variable containing information on the textual location of the grouping made by the current rule. See Tracking Locations.
Acts like a structure variable containing information on the textual location of the nth component of the current rule. See Tracking Locations.
A Bison-generated parser can print diagnostics, including error and tracing messages. By default, they appear in English. However, Bison also supports outputting diagnostics in the user's native language. To make this work, the user should set the usual environment variables. See The User's View. For example, the shell command ‘export LC_ALL=fr_CA.UTF-8’ might set the user's locale to French Canadian using the UTF-8 encoding. The exact set of available locales depends on the user's installation.
The maintainer of a package that uses a Bison-generated parser enables the internationalization of the parser's output through the following steps. Here we assume a package that uses GNU Autoconf and GNU Automake.
cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4
AM_GNU_GETTEXT
invocation, add an invocation of BISON_I18N. This macro is
defined in the file bison-i18n.m4 that you copied earlier. It
causes ‘configure’ to find the value of the
BISON_LOCALEDIR variable, and it defines the source-language
symbol YYENABLE_NLS to enable translations in the
Bison-generated parser.
main function of your program, designate the directory
containing Bison's runtime message catalog, through a call to
‘bindtextdomain’ with domain name ‘bison-runtime’.
For example:
bindtextdomain ("bison-runtime", BISON_LOCALEDIR);
Typically this appears after any other call bindtextdomain
(PACKAGE, LOCALEDIR) that your package already has. Here we rely on
‘BISON_LOCALEDIR’ to be defined as a string through the
Makefile.
main
function, make ‘BISON_LOCALEDIR’ available as a C preprocessor macro,
either in ‘DEFS’ or in ‘AM_CPPFLAGS’. For example:
DEFS = @DEFS@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
or:
AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
As Bison reads tokens, it pushes them onto a stack along with their semantic values. The stack is called the parser stack. Pushing a token is traditionally called shifting.
For example, suppose the infix calculator has read ‘1 + 5 *’, with a ‘3’ to come. The stack will have four elements, one for each token that was shifted.
But the stack does not always have an element for each token read. When the last n tokens and groupings shifted match the components of a grammar rule, they can be combined according to that rule. This is called reduction. Those tokens and groupings are replaced on the stack by a single grouping whose symbol is the result (left hand side) of that rule. Running the rule's action is part of the process of reduction, because this is what computes the semantic value of the resulting grouping.
For example, if the infix calculator's parser stack contains this:
1 + 5 * 3
and the next input token is a newline character, then the last three elements can be reduced to 15 via the rule:
expr: expr '*' expr;
Then the stack contains just these three elements:
1 + 15
At this point, another reduction can be made, resulting in the single value 16. Then the newline token can be shifted.
The parser tries, by shifts and reductions, to reduce the entire input down to a single grouping whose symbol is the grammar's start-symbol (see Languages and Context-Free Grammars).
This kind of parser is known in the literature as a bottom-up parser.
The Bison parser does not always reduce immediately as soon as the last n tokens and groupings match a rule. This is because such a simple strategy is inadequate to handle most languages. Instead, when a reduction is possible, the parser sometimes “looks ahead” at the next token in order to decide what to do.
When a token is read, it is not immediately shifted; first it becomes the lookahead token, which is not on the stack. Now the parser can perform one or more reductions of tokens and groupings on the stack, while the lookahead token remains off to the side. When no more reductions should take place, the lookahead token is shifted onto the stack. This does not mean that all possible reductions have been done; depending on the token type of the lookahead token, some rules may choose to delay their application.
Here is a simple case where lookahead is needed. These three rules define expressions which contain binary addition operators and postfix unary factorial operators (‘!’), and allow parentheses for grouping.
expr: term '+' expr
| term
;
term: '(' expr ')'
| term '!'
| NUMBER
;
Suppose that the tokens ‘1 + 2’ have been read and shifted; what
should be done? If the following token is ‘)’, then the first three
tokens must be reduced to form an expr. This is the only valid
course, because shifting the ‘)’ would produce a sequence of symbols
term ')', and no rule allows this.
If the following token is ‘!’, then it must be shifted immediately so
that ‘2 !’ can be reduced to make a term. If instead the
parser were to reduce before shifting, ‘1 + 2’ would become an
expr. It would then be impossible to shift the ‘!’ because
doing so would produce on the stack the sequence of symbols expr
'!'. No rule allows that sequence.
The lookahead token is stored in the variable yychar.
Its semantic value and location, if any, are stored in the variables
yylval and yylloc.
See Special Features for Use in Actions.
Suppose we are parsing a language which has if-then and if-then-else statements, with a pair of rules like this:
if_stmt:
IF expr THEN stmt
| IF expr THEN stmt ELSE stmt
;
Here we assume that IF, THEN and ELSE are
terminal symbols for specific keyword tokens.
When the ELSE token is read and becomes the lookahead token, the
contents of the stack (assuming the input is valid) are just right for
reduction by the first rule. But it is also legitimate to shift the
ELSE, because that would lead to eventual reduction by the second
rule.
This situation, where either a shift or a reduction would be valid, is called a shift/reduce conflict. Bison is designed to resolve these conflicts by choosing to shift, unless otherwise directed by operator precedence declarations. To see the reason for this, let's contrast it with the other alternative.
Since the parser prefers to shift the ELSE, the result is to attach
the else-clause to the innermost if-statement, making these two inputs
equivalent:
if x then if y then win (); else lose;
if x then do; if y then win (); else lose; end;
But if the parser chose to reduce when possible rather than shift, the result would be to attach the else-clause to the outermost if-statement, making these two inputs equivalent:
if x then if y then win (); else lose;
if x then do; if y then win (); end; else lose;
The conflict exists because the grammar as written is ambiguous: either
parsing of the simple nested if-statement is legitimate. The established
convention is that these ambiguities are resolved by attaching the
else-clause to the innermost if-statement; this is what Bison accomplishes
by choosing to shift rather than reduce. (It would ideally be cleaner to
write an unambiguous grammar, but that is very hard to do in this case.)
This particular ambiguity was first encountered in the specifications of
Algol 60 and is called the “dangling else” ambiguity.
To avoid warnings from Bison about predictable, legitimate shift/reduce
conflicts, use the %expect n declaration.
There will be no warning as long as the number of shift/reduce conflicts
is exactly n, and Bison will report an error if there is a
different number.
See Suppressing Conflict Warnings.
The definition of if_stmt above is solely to blame for the
conflict, but the conflict does not actually appear without additional
rules. Here is a complete Bison grammar file that actually manifests
the conflict:
%token IF THEN ELSE variable
%%
stmt: expr
| if_stmt
;
if_stmt:
IF expr THEN stmt
| IF expr THEN stmt ELSE stmt
;
expr: variable
;
Another situation where shift/reduce conflicts appear is in arithmetic expressions. Here shifting is not always the preferred resolution; the Bison declarations for operator precedence allow you to specify when to shift and when to reduce.
Consider the following ambiguous grammar fragment (ambiguous because the input ‘1 - 2 * 3’ can be parsed in two different ways):
expr: expr '-' expr
| expr '*' expr
| expr '<' expr
| '(' expr ')'
...
;
Suppose the parser has seen the tokens ‘1’, ‘-’ and ‘2’; should it reduce them via the rule for the subtraction operator? It depends on the next token. Of course, if the next token is ‘)’, we must reduce; shifting is invalid because no single rule can reduce the token sequence ‘- 2 )’ or anything starting with that. But if the next token is ‘*’ or ‘<’, we have a choice: either shifting or reduction would allow the parse to complete, but with different results.
To decide which one Bison should do, we must consider the results. If the next operator token op is shifted, then it must be reduced first in order to permit another opportunity to reduce the difference. The result is (in effect) ‘1 - (2 op 3)’. On the other hand, if the subtraction is reduced before shifting op, the result is ‘(1 - 2) op 3’. Clearly, then, the choice of shift or reduce should depend on the relative precedence of the operators ‘-’ and op: ‘*’ should be shifted first, but not ‘<’.
What about input such as ‘1 - 2 - 5’; should this be ‘(1 - 2) - 5’ or should it be ‘1 - (2 - 5)’? For most operators we prefer the former, which is called left association. The latter alternative, right association, is desirable for assignment operators. The choice of left or right association is a matter of whether the parser chooses to shift or reduce when the stack contains ‘1 - 2’ and the lookahead token is ‘-’: shifting makes right-associativity.
Bison allows you to specify these choices with the operator precedence
declarations %left and %right. Each such declaration
contains a list of tokens, which are operators whose precedence and
associativity is being declared. The %left declaration makes all
those operators left-associative and the %right declaration makes
them right-associative. A third alternative is %nonassoc, which
declares that it is a syntax error to find the same operator twice “in a
row”.
The relative precedence of different operators is controlled by the
order in which they are declared. The first %left or
%right declaration in the file declares the operators whose
precedence is lowest, the next such declaration declares the operators
whose precedence is a little higher, and so on.
In our example, we would want the following declarations:
%left '<'
%left '-'
%left '*'
In a more complete example, which supports other operators as well, we
would declare them in groups of equal precedence. For example, '+' is
declared with '-':
%left '<' '>' '=' NE LE GE
%left '+' '-'
%left '*' '/'
(Here NE and so on stand for the operators for “not equal”
and so on. We assume that these tokens are more than one character long
and therefore are represented by names, not character literals.)
The first effect of the precedence declarations is to assign precedence levels to the terminal symbols declared. The second effect is to assign precedence levels to certain rules: each rule gets its precedence from the last terminal symbol mentioned in the components. (You can also specify explicitly the precedence of a rule. See Context-Dependent Precedence.)
Finally, the resolution of conflicts works by comparing the precedence of the rule being considered with that of the lookahead token. If the token's precedence is higher, the choice is to shift. If the rule's precedence is higher, the choice is to reduce. If they have equal precedence, the choice is made based on the associativity of that precedence level. The verbose output file made by ‘-v’ (see Invoking Bison) says how each conflict was resolved.
Not all rules and not all tokens have precedence. If either the rule or the lookahead token has no precedence, then the default is to shift.
Often the precedence of an operator depends on the context. This sounds outlandish at first, but it is really very common. For example, a minus sign typically has a very high precedence as a unary operator, and a somewhat lower precedence (lower than multiplication) as a binary operator.
The Bison precedence declarations, %left, %right and
%nonassoc, can only be used once for a given token; so a token has
only one precedence declared in this way. For context-dependent
precedence, you need to use an additional mechanism: the %prec
modifier for rules.
The %prec modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that rule.
It's not necessary for that symbol to appear otherwise in the rule. The
modifier's syntax is:
%prec terminal-symbol
and it is written after the components of the rule. Its effect is to assign the rule the precedence of terminal-symbol, overriding the precedence that would be deduced for it in the ordinary way. The altered rule precedence then affects how conflicts involving that rule are resolved (see Operator Precedence).
Here is how %prec solves the problem of unary minus. First, declare
a precedence for a fictitious terminal symbol named UMINUS. There
are no tokens of this type, but the symbol serves to stand for its
precedence:
...
%left '+' '-'
%left '*'
%left UMINUS
Now the precedence of UMINUS can be used in specific rules:
exp: ...
| exp '-' exp
...
| '-' exp %prec UMINUS
The function yyparse is implemented using a finite-state machine.
The values pushed on the parser stack are not simply token type codes; they
represent the entire sequence of terminal and nonterminal symbols at or
near the top of the stack. The current state collects all the information
about previous input which is relevant to deciding what to do next.
Each time a lookahead token is read, the current parser state together with the type of lookahead token are looked up in a table. This table entry can say, “Shift the lookahead token.” In this case, it also specifies the new parser state, which is pushed onto the top of the parser stack. Or it can say, “Reduce using rule number n.” This means that a certain number of tokens or groupings are taken off the top of the stack, and replaced by one grouping. In other words, that number of states are popped from the stack, and one new state is pushed.
There is one other alternative: the table can say that the lookahead token is erroneous in the current state. This causes error processing to begin (see Error Recovery).
A reduce/reduce conflict occurs if there are two or more rules that apply to the same sequence of input. This usually indicates a serious error in the grammar.
For example, here is an erroneous attempt to define a sequence
of zero or more word groupings.
sequence: /* empty */
{ printf ("empty sequence\n"); }
| maybeword
| sequence word
{ printf ("added word %s\n", $2); }
;
maybeword: /* empty */
{ printf ("empty maybeword\n"); }
| word
{ printf ("single word %s\n", $1); }
;
The error is an ambiguity: there is more than one way to parse a single
word into a sequence. It could be reduced to a
maybeword and then into a sequence via the second rule.
Alternatively, nothing-at-all could be reduced into a sequence
via the first rule, and this could be combined with the word
using the third rule for sequence.
There is also more than one way to reduce nothing-at-all into a
sequence. This can be done directly via the first rule,
or indirectly via maybeword and then the second rule.
You might think that this is a distinction without a difference, because it does not change whether any particular input is valid or not. But it does affect which actions are run. One parsing order runs the second rule's action; the other runs the first rule's action and the third rule's action. In this example, the output of the program changes.
Bison resolves a reduce/reduce conflict by choosing to use the rule that
appears first in the grammar, but it is very risky to rely on this. Every
reduce/reduce conflict must be studied and usually eliminated. Here is the
proper way to define sequence:
sequence: /* empty */
{ printf ("empty sequence\n"); }
| sequence word
{ printf ("added word %s\n", $2); }
;
Here is another common error that yields a reduce/reduce conflict:
sequence: /* empty */
| sequence words
| sequence redirects
;
words: /* empty */
| words word
;
redirects:/* empty */
| redirects redirect
;
The intention here is to define a sequence which can contain either
word or redirect groupings. The individual definitions of
sequence, words and redirects are error-free, but the
three together make a subtle ambiguity: even an empty input can be parsed
in infinitely many ways!
Consider: nothing-at-all could be a words. Or it could be two
words in a row, or three, or any number. It could equally well be a
redirects, or two, or any number. Or it could be a words
followed by three redirects and another words. And so on.
Here are two ways to correct these rules. First, to make it a single level of sequence:
sequence: /* empty */
| sequence word
| sequence redirect
;
Second, to prevent either a words or a redirects
from being empty:
sequence: /* empty */
| sequence words
| sequence redirects
;
words: word
| words word
;
redirects:redirect
| redirects redirect
;
Sometimes reduce/reduce conflicts can occur that don't look warranted. Here is an example:
%token ID
%%
def: param_spec return_spec ','
;
param_spec:
type
| name_list ':' type
;
return_spec:
type
| name ':' type
;
type: ID
;
name: ID
;
name_list:
name
| name ',' name_list
;
It would seem that this grammar can be parsed with only a single token
of lookahead: when a param_spec is being read, an ID is
a name if a comma or colon follows, or a type if another
ID follows. In other words, this grammar is LR(1).
However, for historical reasons, Bison cannot by default handle all
LR(1) grammars.
In this grammar, two contexts, that after an ID at the beginning
of a param_spec and likewise at the beginning of a
return_spec, are similar enough that Bison assumes they are the
same.
They appear similar because the same set of rules would be
active—the rule for reducing to a name and that for reducing to
a type. Bison is unable to determine at that stage of processing
that the rules would require different lookahead tokens in the two
contexts, so it makes a single parser state for them both. Combining
the two contexts causes a conflict later. In parser terminology, this
occurrence means that the grammar is not LALR(1).
For many practical grammars (specifically those that fall into the non-LR(1) class), the limitations of LALR(1) result in difficulties beyond just mysterious reduce/reduce conflicts. The best way to fix all these problems is to select a different parser table construction algorithm. Either IELR(1) or canonical LR(1) would suffice, but the former is more efficient and easier to debug during development. See LR Table Construction, for details. (Bison's IELR(1) and canonical LR(1) implementations are experimental. More user feedback will help to stabilize them.)
If you instead wish to work around LALR(1)'s limitations, you
can often fix a mysterious conflict by identifying the two parser states
that are being confused, and adding something to make them look
distinct. In the above example, adding one rule to
return_spec as follows makes the problem go away:
%token BOGUS
...
%%
...
return_spec:
type
| name ':' type
/* This rule is never used. */
| ID BOGUS
;
This corrects the problem because it introduces the possibility of an
additional active rule in the context after the ID at the beginning of
return_spec. This rule is not active in the corresponding context
in a param_spec, so the two contexts receive distinct parser states.
As long as the token BOGUS is never generated by yylex,
the added rule cannot alter the way actual input is parsed.
In this particular example, there is another way to solve the problem:
rewrite the rule for return_spec to use ID directly
instead of via name. This also causes the two confusing
contexts to have different sets of active rules, because the one for
return_spec activates the altered rule for return_spec
rather than the one for name.
param_spec:
type
| name_list ':' type
;
return_spec:
type
| ID ':' type
;
For a more detailed exposition of LALR(1) parsers and parser generators, see DeRemer 1982.
The default behavior of Bison's LR-based parsers is chosen mostly for
historical reasons, but that behavior is often not robust. For example, in
the previous section, we discussed the mysterious conflicts that can be
produced by LALR(1), Bison's default parser table construction algorithm.
Another example is Bison's %error-verbose directive, which instructs
the generated parser to produce verbose syntax error messages, which can
sometimes contain incorrect information.
In this section, we explore several modern features of Bison that allow you to tune fundamental aspects of the generated LR-based parsers. Some of these features easily eliminate shortcomings like those mentioned above. Others can be helpful purely for understanding your parser.
Most of the features discussed in this section are still experimental. More user feedback will help to stabilize them.
For historical reasons, Bison constructs LALR(1) parser tables by default. However, LALR does not possess the full language-recognition power of LR. As a result, the behavior of parsers employing LALR parser tables is often mysterious. We presented a simple example of this effect in Mysterious Conflicts.
As we also demonstrated in that example, the traditional approach to eliminating such mysterious behavior is to restructure the grammar. Unfortunately, doing so correctly is often difficult. Moreover, merely discovering that LALR causes mysterious behavior in your parser can be difficult as well.
Fortunately, Bison provides an easy way to eliminate the possibility of such
mysterious behavior altogether. You simply need to activate a more powerful
parser table construction algorithm by using the %define lr.type
directive.
Specify the type of parser tables within the LR(1) family. The accepted values for TYPE are:
lalr(default)ielrcanonical-lr(This feature is experimental. More user feedback will help to stabilize it.)
For example, to activate IELR, you might add the following directive to you grammar file:
%define lr.type ielr
For the example in Mysterious Conflicts, the mysterious conflict is then eliminated, so there is no need to invest time in comprehending the conflict or restructuring the grammar to fix it. If, during future development, the grammar evolves such that all mysterious behavior would have disappeared using just LALR, you need not fear that continuing to use IELR will result in unnecessarily large parser tables. That is, IELR generates LALR tables when LALR (using a deterministic parsing algorithm) is sufficient to support the full language-recognition power of LR. Thus, by enabling IELR at the start of grammar development, you can safely and completely eliminate the need to consider LALR's shortcomings.
While IELR is almost always preferable, there are circumstances where LALR or the canonical LR parser tables described by Knuth (see Knuth 1965) can be useful. Here we summarize the relative advantages of each parser table construction algorithm within Bison:
There are at least two scenarios where LALR can be worthwhile:
When employing GLR parsers (see GLR Parsers), if you do not resolve any
conflicts statically (for example, with %left or %prec), then
the parser explores all potential parses of any given input. In this case,
the choice of parser table construction algorithm is guaranteed not to alter
the language accepted by the parser. LALR parser tables are the smallest
parser tables Bison can currently construct, so they may then be preferable.
Nevertheless, once you begin to resolve conflicts statically, GLR behaves
more like a deterministic parser in the syntactic contexts where those
conflicts appear, and so either IELR or canonical LR can then be helpful to
avoid LALR's mysterious behavior.
Occasionally during development, an especially malformed grammar with a major recurring flaw may severely impede the IELR or canonical LR parser table construction algorithm. LALR can be a quick way to construct parser tables in order to investigate such problems while ignoring the more subtle differences from IELR and canonical LR.
IELR (Inadequacy Elimination LR) is a minimal LR algorithm. That is, given any grammar (LR or non-LR), parsers using IELR or canonical LR parser tables always accept exactly the same set of sentences. However, like LALR, IELR merges parser states during parser table construction so that the number of parser states is often an order of magnitude less than for canonical LR. More importantly, because canonical LR's extra parser states may contain duplicate conflicts in the case of non-LR grammars, the number of conflicts for IELR is often an order of magnitude less as well. This effect can significantly reduce the complexity of developing a grammar.
While inefficient, canonical LR parser tables can be an interesting means to
explore a grammar because they possess a property that IELR and LALR tables
do not. That is, if %nonassoc is not used and default reductions are
left disabled (see Default Reductions), then, for every left context of
every canonical LR state, the set of tokens accepted by that state is
guaranteed to be the exact set of tokens that is syntactically acceptable in
that left context. It might then seem that an advantage of canonical LR
parsers in production is that, under the above constraints, they are
guaranteed to detect a syntax error as soon as possible without performing
any unnecessary reductions. However, IELR parsers that use LAC are also
able to achieve this behavior without sacrificing %nonassoc or
default reductions. For details and a few caveats of LAC, see LAC.
For a more detailed exposition of the mysterious behavior in LALR parsers and the benefits of IELR, see Denny 2008 March, and Denny 2010 November.
After parser table construction, Bison identifies the reduction with the largest lookahead set in each parser state. To reduce the size of the parser state, traditional Bison behavior is to remove that lookahead set and to assign that reduction to be the default parser action. Such a reduction is known as a default reduction.
Default reductions affect more than the size of the parser tables. They also affect the behavior of the parser:
yylex invocations.
A consistent state is a state that has only one possible parser
action. If that action is a reduction and is encoded as a default
reduction, then that consistent state is called a defaulted state.
Upon reaching a defaulted state, a Bison-generated parser does not bother to
invoke yylex to fetch the next token before performing the reduction.
In other words, whether default reductions are enabled in consistent states
determines how soon a Bison-generated parser invokes yylex for a
token: immediately when it reaches that token in the input or when it
eventually needs that token as a lookahead to determine the next
parser action. Traditionally, default reductions are enabled, and so the
parser exhibits the latter behavior.
The presence of defaulted states is an important consideration when
designing yylex and the grammar file. That is, if the behavior of
yylex can influence or be influenced by the semantic actions
associated with the reductions in defaulted states, then the delay of the
next yylex invocation until after those reductions is significant.
For example, the semantic actions might pop a scope stack that yylex
uses to determine what token to return. Thus, the delay might be necessary
to ensure that yylex does not look up the next token in a scope that
should already be considered closed.
When the parser fetches a new token by invoking yylex, it checks
whether there is an action for that token in the current parser state. The
parser detects a syntax error if and only if either (1) there is no action
for that token or (2) the action for that token is the error action (due to
the use of %nonassoc). However, if there is a default reduction in
that state (which might or might not be a defaulted state), then it is
impossible for condition 1 to exist. That is, all tokens have an action.
Thus, the parser sometimes fails to detect the syntax error until it reaches
a later state.
While default reductions never cause the parser to accept syntactically
incorrect sentences, the delay of syntax error detection can have unexpected
effects on the behavior of the parser. However, the delay can be caused
anyway by parser state merging and the use of %nonassoc, and it can
be fixed by another Bison feature, LAC. We discuss the effects of delayed
syntax error detection and LAC more in the next section (see LAC).
For canonical LR, the only default reduction that Bison enables by default
is the accept action, which appears only in the accepting state, which has
no other action and is thus a defaulted state. However, the default accept
action does not delay any yylex invocation or syntax error detection
because the accept action ends the parse.
For LALR and IELR, Bison enables default reductions in nearly all states by
default. There are only two exceptions. First, states that have a shift
action on the error token do not have default reductions because
delayed syntax error detection could then prevent the error token
from ever being shifted in that state. However, parser state merging can
cause the same effect anyway, and LAC fixes it in both cases, so future
versions of Bison might drop this exception when LAC is activated. Second,
GLR parsers do not record the default reduction as the action on a lookahead
token for which there is a conflict. The correct action in this case is to
split the parse instead.
To adjust which states have default reductions enabled, use the
%define lr.default-reductions directive.
Specify the kind of states that are permitted to contain default reductions. The accepted values of WHERE are:
most(default for LALR and IELR)consistentaccepting(default for canonical LR)(The ability to specify where default reductions are permitted is experimental. More user feedback will help to stabilize it.)
Canonical LR, IELR, and LALR can suffer from a couple of problems upon encountering a syntax error. First, the parser might perform additional parser stack reductions before discovering the syntax error. Such reductions can perform user semantic actions that are unexpected because they are based on an invalid token, and they cause error recovery to begin in a different syntactic context than the one in which the invalid token was encountered. Second, when verbose error messages are enabled (see Error Reporting), the expected token list in the syntax error message can both contain invalid tokens and omit valid tokens.
The culprits for the above problems are %nonassoc, default reductions
in inconsistent states (see Default Reductions), and parser state
merging. Because IELR and LALR merge parser states, they suffer the most.
Canonical LR can suffer only if %nonassoc is used or if default
reductions are enabled for inconsistent states.
LAC (Lookahead Correction) is a new mechanism within the parsing algorithm
that solves these problems for canonical LR, IELR, and LALR without
sacrificing %nonassoc, default reductions, or state merging. You can
enable LAC with the %define parse.lac directive.
Enable LAC to improve syntax error handling.
(This feature is experimental. More user feedback will help to stabilize it. Moreover, it is currently only available for deterministic parsers in C.)
none(default)full
Conceptually, the LAC mechanism is straight-forward. Whenever the parser fetches a new token from the scanner so that it can determine the next parser action, it immediately suspends normal parsing and performs an exploratory parse using a temporary copy of the normal parser state stack. During this exploratory parse, the parser does not perform user semantic actions. If the exploratory parse reaches a shift action, normal parsing then resumes on the normal parser stacks. If the exploratory parse reaches an error instead, the parser reports a syntax error. If verbose syntax error messages are enabled, the parser must then discover the list of expected tokens, so it performs a separate exploratory parse for each token in the grammar.
There is one subtlety about the use of LAC. That is, when in a consistent parser state with a default reduction, the parser will not attempt to fetch a token from the scanner because no lookahead is needed to determine the next parser action. Thus, whether default reductions are enabled in consistent states (see Default Reductions) affects how soon the parser detects a syntax error: immediately when it reaches an erroneous token or when it eventually needs that token as a lookahead to determine the next parser action. The latter behavior is probably more intuitive, so Bison currently provides no way to achieve the former behavior while default reductions are enabled in consistent states.
Thus, when LAC is in use, for some fixed decision of whether to enable default reductions in consistent states, canonical LR and IELR behave almost exactly the same for both syntactically acceptable and syntactically unacceptable input. While LALR still does not support the full language-recognition power of canonical LR and IELR, LAC at least enables LALR's syntax error handling to correctly reflect LALR's language-recognition power.
There are a few caveats to consider when using LAC:
IELR plus LAC does have one shortcoming relative to canonical LR. Some parsers generated by Bison can loop infinitely. LAC does not fix infinite parsing loops that occur between encountering a syntax error and detecting it, but enabling canonical LR or disabling default reductions sometimes does.
Because of internationalization considerations, Bison-generated parsers limit the size of the expected token list they are willing to report in a verbose syntax error message. If the number of expected tokens exceeds that limit, the list is simply dropped from the message. Enabling LAC can increase the size of the list and thus cause the parser to drop it. Of course, dropping the list is better than reporting an incorrect list.
Because LAC requires many parse actions to be performed twice, it can have a performance penalty. However, not all parse actions must be performed twice. Specifically, during a series of default reductions in consistent states and shift actions, the parser never has to initiate an exploratory parse. Moreover, the most time-consuming tasks in a parse are often the file I/O, the lexical analysis performed by the scanner, and the user's semantic actions, but none of these are performed during the exploratory parse. Finally, the base of the temporary stack used during an exploratory parse is a pointer into the normal parser state stack so that the stack is never physically copied. In our experience, the performance penalty of LAC has proven insignificant for practical grammars.
While the LAC algorithm shares techniques that have been recognized in the parser community for years, for the publication that introduces LAC, see Denny 2010 May.
If there exists no sequence of transitions from the parser's start state to some state s, then Bison considers s to be an unreachable state. A state can become unreachable during conflict resolution if Bison disables a shift action leading to it from a predecessor state.
By default, Bison removes unreachable states from the parser after conflict resolution because they are useless in the generated parser. However, keeping unreachable states is sometimes useful when trying to understand the relationship between the parser and the grammar.
Request that Bison allow unreachable states to remain in the parser tables. VALUE must be a Boolean. The default is
false.
There are a few caveats to consider:
Unreachable states may contain conflicts and may use rules not used in any other state. Thus, keeping unreachable states may induce warnings that are irrelevant to your parser's behavior, and it may eliminate warnings that are relevant. Of course, the change in warnings may actually be relevant to a parser table analysis that wants to keep unreachable states, so this behavior will likely remain in future Bison releases.
While Bison is able to remove unreachable states, it is not guaranteed to remove other kinds of useless states. Specifically, when Bison disables reduce actions during conflict resolution, some goto actions may become useless, and thus some additional states may become useless. If Bison were to compute which goto actions were useless and then disable those actions, it could identify such states as unreachable and then remove those states. However, Bison does not compute which goto actions are useless.
Bison produces deterministic parsers that choose uniquely when to reduce and which reduction to apply based on a summary of the preceding input and on one extra token of lookahead. As a result, normal Bison handles a proper subset of the family of context-free languages. Ambiguous grammars, since they have strings with more than one possible sequence of reductions cannot have deterministic parsers in this sense. The same is true of languages that require more than one symbol of lookahead, since the parser lacks the information necessary to make a decision at the point it must be made in a shift-reduce parser. Finally, as previously mentioned (see Mysterious Conflicts), there are languages where Bison's default choice of how to summarize the input seen so far loses necessary information.
When you use the ‘%glr-parser’ declaration in your grammar file, Bison generates a parser that uses a different algorithm, called Generalized LR (or GLR). A Bison GLR parser uses the same basic algorithm for parsing as an ordinary Bison parser, but behaves differently in cases where there is a shift-reduce conflict that has not been resolved by precedence rules (see Precedence) or a reduce-reduce conflict. When a GLR parser encounters such a situation, it effectively splits into a several parsers, one for each possible shift or reduction. These parsers then proceed as usual, consuming tokens in lock-step. Some of the stacks may encounter other conflicts and split further, with the result that instead of a sequence of states, a Bison GLR parsing stack is what is in effect a tree of states.
In effect, each stack represents a guess as to what the proper parse is. Additional input may indicate that a guess was wrong, in which case the appropriate stack silently disappears. Otherwise, the semantics actions generated in each stack are saved, rather than being executed immediately. When a stack disappears, its saved semantic actions never get executed. When a reduction causes two stacks to become equivalent, their sets of semantic actions are both saved with the state that results from the reduction. We say that two stacks are equivalent when they both represent the same sequence of states, and each pair of corresponding states represents a grammar symbol that produces the same segment of the input token stream.
Whenever the parser makes a transition from having multiple states to having one, it reverts to the normal deterministic parsing algorithm, after resolving and executing the saved-up actions. At this transition, some of the states on the stack will have semantic values that are sets (actually multisets) of possible actions. The parser tries to pick one of the actions by first finding one whose rule has the highest dynamic precedence, as set by the ‘%dprec’ declaration. Otherwise, if the alternative actions are not ordered by precedence, but there the same merging function is declared for both rules by the ‘%merge’ declaration, Bison resolves and evaluates both and then calls the merge function on the result. Otherwise, it reports an ambiguity.
It is possible to use a data structure for the GLR parsing tree that permits the processing of any LR(1) grammar in linear time (in the size of the input), any unambiguous (not necessarily LR(1)) grammar in quadratic worst-case time, and any general (possibly ambiguous) context-free grammar in cubic worst-case time. However, Bison currently uses a simpler data structure that requires time proportional to the length of the input times the maximum number of stacks required for any prefix of the input. Thus, really ambiguous or nondeterministic grammars can require exponential time and space to process. Such badly behaving examples, however, are not generally of practical interest. Usually, nondeterminism in a grammar is local—the parser is “in doubt” only for a few tokens at a time. Therefore, the current data structure should generally be adequate. On LR(1) portions of a grammar, in particular, it is only slightly slower than with the deterministic LR(1) Bison parser.
For a more detailed exposition of GLR parsers, see Scott 2000.
The Bison parser stack can run out of memory if too many tokens are shifted and
not reduced. When this happens, the parser function yyparse
calls yyerror and then returns 2.
Because Bison parsers have growing stacks, hitting the upper limit usually results from using a right recursion instead of a left recursion, See Recursive Rules.
By defining the macro YYMAXDEPTH, you can control how deep the
parser stack can become before memory is exhausted. Define the
macro with a value that is an integer. This value is the maximum number
of tokens that can be shifted (and not reduced) before overflow.
The stack space allowed is not necessarily allocated. If you specify a
large value for YYMAXDEPTH, the parser normally allocates a small
stack at first, and then makes it bigger by stages as needed. This
increasing allocation happens automatically and silently. Therefore,
you do not need to make YYMAXDEPTH painfully small merely to save
space for ordinary inputs that do not need much stack.
However, do not allow YYMAXDEPTH to be a value so large that
arithmetic overflow could occur when calculating the size of the stack
space. Also, do not allow YYMAXDEPTH to be less than
YYINITDEPTH.
The default value of YYMAXDEPTH, if you do not define it, is
10000.
You can control how much stack is allocated initially by defining the
macro YYINITDEPTH to a positive integer. For the deterministic
parser in C, this value must be a compile-time constant
unless you are assuming C99 or some other target language or compiler
that allows variable-length arrays. The default is 200.
Do not allow YYINITDEPTH to be greater than YYMAXDEPTH.
Because of semantic differences between C and C++, the deterministic
parsers in C produced by Bison cannot grow when compiled
by C++ compilers. In this precise case (compiling a C parser as C++) you are
suggested to grow YYINITDEPTH. The Bison maintainers hope to fix
this deficiency in a future release.
It is not usually acceptable to have a program terminate on a syntax error. For example, a compiler should recover sufficiently to parse the rest of the input file and check it for errors; a calculator should accept another expression.
In a simple interactive command parser where each input is one line, it may
be sufficient to allow yyparse to return 1 on error and have the
caller ignore the rest of the input line when that happens (and then call
yyparse again). But this is inadequate for a compiler, because it
forgets all the syntactic context leading up to the error. A syntax error
deep within a function in the compiler input should not cause the compiler
to treat the following line like the beginning of a source file.
You can define how to recover from a syntax error by writing rules to
recognize the special token error. This is a terminal symbol that
is always defined (you need not declare it) and reserved for error
handling. The Bison parser generates an error token whenever a
syntax error happens; if you have provided a rule to recognize this token
in the current context, the parse can continue.
For example:
stmnts: /* empty string */
| stmnts '\n'
| stmnts exp '\n'
| stmnts error '\n'
The fourth rule in this example says that an error followed by a newline
makes a valid addition to any stmnts.
What happens if a syntax error occurs in the middle of an exp? The
error recovery rule, interpreted strictly, applies to the precise sequence
of a stmnts, an error and a newline. If an error occurs in
the middle of an exp, there will probably be some additional tokens
and subexpressions on the stack after the last stmnts, and there
will be tokens to read before the next newline. So the rule is not
applicable in the ordinary way.
But Bison can force the situation to fit the rule, by discarding part of
the semantic context and part of the input. First it discards states
and objects from the stack until it gets back to a state in which the
error token is acceptable. (This means that the subexpressions
already parsed are discarded, back to the last complete stmnts.)
At this point the error token can be shifted. Then, if the old
lookahead token is not acceptable to be shifted next, the parser reads
tokens and discards them until it finds a token which is acceptable. In
this example, Bison reads and discards input until the next newline so
that the fourth rule can apply. Note that discarded symbols are
possible sources of memory leaks, see Freeing Discarded Symbols, for a means to reclaim this memory.
The choice of error rules in the grammar is a choice of strategies for error recovery. A simple and useful strategy is simply to skip the rest of the current input line or current statement if an error is detected:
stmnt: error ';' /* On error, skip until ';' is read. */
It is also useful to recover to the matching close-delimiter of an opening-delimiter that has already been parsed. Otherwise the close-delimiter will probably appear to be unmatched, and generate another, spurious error message:
primary: '(' expr ')'
| '(' error ')'
...
;
Error recovery strategies are necessarily guesses. When they guess wrong,
one syntax error often leads to another. In the above example, the error
recovery rule guesses that an error is due to bad input within one
stmnt. Suppose that instead a spurious semicolon is inserted in the
middle of a valid stmnt. After the error recovery rule recovers
from the first error, another syntax error will be found straightaway,
since the text following the spurious semicolon is also an invalid
stmnt.
To prevent an outpouring of error messages, the parser will output no error message for another syntax error that happens shortly after the first; only after three consecutive input tokens have been successfully shifted will error messages resume.
Note that rules which accept the error token may have actions, just
as any other rules can.
You can make error messages resume immediately by using the macro
yyerrok in an action. If you do this in the error rule's action, no
error messages will be suppressed. This macro requires no arguments;
‘yyerrok;’ is a valid C statement.
The previous lookahead token is reanalyzed immediately after an error. If
this is unacceptable, then the macro yyclearin may be used to clear
this token. Write the statement ‘yyclearin;’ in the error rule's
action.
See Special Features for Use in Actions.
For example, suppose that on a syntax error, an error handling routine is called that advances the input stream to some point where parsing should once again commence. The next symbol returned by the lexical scanner is probably correct. The previous lookahead token ought to be discarded with ‘yyclearin;’.
The expression YYRECOVERING () yields 1 when the parser
is recovering from a syntax error, and 0 otherwise.
Syntax error diagnostics are suppressed while recovering from a syntax
error.
The Bison paradigm is to parse tokens first, then group them into larger syntactic units. In many languages, the meaning of a token is affected by its context. Although this violates the Bison paradigm, certain techniques (known as kludges) may enable you to write Bison parsers for such languages.
(Actually, “kludge” means any technique that gets its job done but is neither clean nor robust.)
The C language has a context dependency: the way an identifier is used depends on what its current meaning is. For example, consider this:
foo (x);
This looks like a function call statement, but if foo is a typedef
name, then this is actually a declaration of x. How can a Bison
parser for C decide how to parse this input?
The method used in GNU C is to have two different token types,
IDENTIFIER and TYPENAME. When yylex finds an
identifier, it looks up the current declaration of the identifier in order
to decide which token type to return: TYPENAME if the identifier is
declared as a typedef, IDENTIFIER otherwise.
The grammar rules can then express the context dependency by the choice of
token type to recognize. IDENTIFIER is accepted as an expression,
but TYPENAME is not. TYPENAME can start a declaration, but
IDENTIFIER cannot. In contexts where the meaning of the identifier
is not significant, such as in declarations that can shadow a
typedef name, either TYPENAME or IDENTIFIER is
accepted—there is one rule for each of the two token types.
This technique is simple to use if the decision of which kinds of identifiers to allow is made at a place close to where the identifier is parsed. But in C this is not always so: C allows a declaration to redeclare a typedef name provided an explicit type has been specified earlier:
typedef int foo, bar;
int baz (void)
{
static bar (bar); /* redeclare bar as static variable */
extern foo foo (foo); /* redeclare foo as function */
return foo (bar);
}
Unfortunately, the name being declared is separated from the declaration construct itself by a complicated syntactic structure—the “declarator”.
As a result, part of the Bison parser for C needs to be duplicated, with all the nonterminal names changed: once for parsing a declaration in which a typedef name can be redefined, and once for parsing a declaration in which that can't be done. Here is a part of the duplication, with actions omitted for brevity:
initdcl:
declarator maybeasm '='
init
| declarator maybeasm
;
notype_initdcl:
notype_declarator maybeasm '='
init
| notype_declarator maybeasm
;
Here initdcl can redeclare a typedef name, but notype_initdcl
cannot. The distinction between declarator and
notype_declarator is the same sort of thing.
There is some similarity between this technique and a lexical tie-in (described next), in that information which alters the lexical analysis is changed during parsing by other parts of the program. The difference is here the information is global, and is used for other purposes in the program. A true lexical tie-in has a special-purpose flag controlled by the syntactic context.
One way to handle context-dependency is the lexical tie-in: a flag which is set by Bison actions, whose purpose is to alter the way tokens are parsed.
For example, suppose we have a language vaguely like C, but with a special
construct ‘hex (hex-expr)’. After the keyword hex comes
an expression in parentheses in which all integers are hexadecimal. In
particular, the token ‘a1b’ must be treated as an integer rather than
as an identifier if it appears in that context. Here is how you can do it:
%{
int hexflag;
int yylex (void);
void yyerror (char const *);
%}
%%
...
expr: IDENTIFIER
| constant
| HEX '('
{ hexflag = 1; }
expr ')'
{ hexflag = 0;
$$ = $4; }
| expr '+' expr
{ $$ = make_sum ($1, $3); }
...
;
constant:
INTEGER
| STRING
;
Here we assume that yylex looks at the value of hexflag; when
it is nonzero, all integers are parsed in hexadecimal, and tokens starting
with letters are parsed as integers if possible.
The declaration of hexflag shown in the prologue of the grammar
file is needed to make it accessible to the actions (see The Prologue). You must also write the code in yylex to obey
the flag.
Lexical tie-ins make strict demands on any error recovery rules you have. See Error Recovery.
The reason for this is that the purpose of an error recovery rule is to abort the parsing of one construct and resume in some larger construct. For example, in C-like languages, a typical error recovery rule is to skip tokens until the next semicolon, and then start a new statement, like this:
stmt: expr ';'
| IF '(' expr ')' stmt { ... }
...
error ';'
{ hexflag = 0; }
;
If there is a syntax error in the middle of a ‘hex (expr)’
construct, this error rule will apply, and then the action for the
completed ‘hex (expr)’ will never run. So hexflag would
remain set for the entire rest of the input, or until the next hex
keyword, causing identifiers to be misinterpreted as integers.
To avoid this problem the error recovery rule itself clears hexflag.
There may also be an error recovery rule that works within expressions. For example, there could be a rule which applies within parentheses and skips to the close-parenthesis:
expr: ...
| '(' expr ')'
{ $$ = $2; }
| '(' error ')'
...
If this rule acts within the hex construct, it is not going to abort
that construct (since it applies to an inner level of parentheses within
the construct). Therefore, it should not clear the flag: the rest of
the hex construct should be parsed with the flag still in effect.
What if there is an error recovery rule which might abort out of the
hex construct or might not, depending on circumstances? There is no
way you can write the action to determine whether a hex construct is
being aborted or not. So if you are using a lexical tie-in, you had better
make sure your error recovery rules are not of this kind. Each rule must
be such that you can be sure that it always will, or always won't, have to
clear the flag.
Developing a parser can be a challenge, especially if you don't understand the algorithm (see The Bison Parser Algorithm). Even so, sometimes a detailed description of the automaton can help (see Understanding Your Parser), or tracing the execution of the parser can give some insight on why it behaves improperly (see Tracing Your Parser).
As documented elsewhere (see The Bison Parser Algorithm) Bison parsers are shift/reduce automata. In some cases (much more frequent than one would hope), looking at this automaton is required to tune or simply fix a parser. Bison provides two different representation of it, either textually or graphically (as a DOT file).
The textual file is generated when the options --report or --verbose are specified, see See Invoking Bison. Its name is made by removing ‘.tab.c’ or ‘.c’ from the parser implementation file name, and adding ‘.output’ instead. Therefore, if the grammar file is foo.y, then the parser implementation file is called foo.tab.c by default. As a consequence, the verbose output file is called foo.output.
The following grammar file, calc.y, will be used in the sequel:
%token NUM STR
%left '+' '-'
%left '*'
%%
exp: exp '+' exp
| exp '-' exp
| exp '*' exp
| exp '/' exp
| NUM
;
useless: STR;
%%
bison reports:
calc.y: warning: 1 nonterminal useless in grammar
calc.y: warning: 1 rule useless in grammar
calc.y:11.1-7: warning: nonterminal useless in grammar: useless
calc.y:11.10-12: warning: rule useless in grammar: useless: STR
calc.y: conflicts: 7 shift/reduce
When given --report=state, in addition to calc.tab.c, it creates a file calc.output with contents detailed below. The order of the output and the exact presentation might vary, but the interpretation is the same.
The first section includes details on conflicts that were solved thanks to precedence and/or associativity:
Conflict in state 8 between rule 2 and token '+' resolved as reduce.
Conflict in state 8 between rule 2 and token '-' resolved as reduce.
Conflict in state 8 between rule 2 and token '*' resolved as shift.
...
The next section lists states that still have conflicts.
State 8 conflicts: 1 shift/reduce
State 9 conflicts: 1 shift/reduce
State 10 conflicts: 1 shift/reduce
State 11 conflicts: 4 shift/reduce
The next section reports useless tokens, nonterminal and rules. Useless nonterminals and rules are removed in order to produce a smaller parser, but useless tokens are preserved, since they might be used by the scanner (note the difference between “useless” and “unused” below):
Nonterminals useless in grammar:
useless
Terminals unused in grammar:
STR
Rules useless in grammar:
#6 useless: STR;
The next section reproduces the exact grammar that Bison used:
Grammar
Number, Line, Rule
0 5 $accept -> exp $end
1 5 exp -> exp '+' exp
2 6 exp -> exp '-' exp
3 7 exp -> exp '*' exp
4 8 exp -> exp '/' exp
5 9 exp -> NUM
and reports the uses of the symbols:
Terminals, with rules where they appear
$end (0) 0
'*' (42) 3
'+' (43) 1
'-' (45) 2
'/' (47) 4
error (256)
NUM (258) 5
Nonterminals, with rules where they appear
$accept (8)
on left: 0
exp (9)
on left: 1 2 3 4 5, on right: 0 1 2 3 4
Bison then proceeds onto the automaton itself, describing each state with it set of items, also known as pointed rules. Each item is a production rule together with a point (marked by ‘.’) that the input cursor.
state 0
$accept -> . exp $ (rule 0)
NUM shift, and go to state 1
exp go to state 2
This reads as follows: “state 0 corresponds to being at the very
beginning of the parsing, in the initial rule, right before the start
symbol (here, exp). When the parser returns to this state right
after having reduced a rule that produced an exp, the control
flow jumps to state 2. If there is no such transition on a nonterminal
symbol, and the lookahead is a NUM, then this token is shifted on
the parse stack, and the control flow jumps to state 1. Any other
lookahead triggers a syntax error.”
Even though the only active rule in state 0 seems to be rule 0, the
report lists NUM as a lookahead token because NUM can be
at the beginning of any rule deriving an exp. By default Bison
reports the so-called core or kernel of the item set, but if
you want to see more detail you can invoke bison with
--report=itemset to list all the items, include those that can
be derived:
state 0
$accept -> . exp $ (rule 0)
exp -> . exp '+' exp (rule 1)
exp -> . exp '-' exp (rule 2)
exp -> . exp '*' exp (rule 3)
exp -> . exp '/' exp (rule 4)
exp -> . NUM (rule 5)
NUM shift, and go to state 1
exp go to state 2
In the state 1...
state 1
exp -> NUM . (rule 5)
$default reduce using rule 5 (exp)
the rule 5, ‘exp: NUM;’, is completed. Whatever the lookahead token (‘$default’), the parser will reduce it. If it was coming from state 0, then, after this reduction it will return to state 0, and will jump to state 2 (‘exp: go to state 2’).
state 2
$accept -> exp . $ (rule 0)
exp -> exp . '+' exp (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
$ shift, and go to state 3
'+' shift, and go to state 4
'-' shift, and go to state 5
'*' shift, and go to state 6
'/' shift, and go to state 7
In state 2, the automaton can only shift a symbol. For instance, because of the item ‘exp -> exp . '+' exp’, if the lookahead if ‘+’, it will be shifted on the parse stack, and the automaton control will jump to state 4, corresponding to the item ‘exp -> exp '+' . exp’. Since there is no default action, any other token than those listed above will trigger a syntax error.
The state 3 is named the final state, or the accepting state:
state 3
$accept -> exp $ . (rule 0)
$default accept
the initial rule is completed (the start symbol and the end of input were read), the parsing exits successfully.
The interpretation of states 4 to 7 is straightforward, and is left to the reader.
state 4
exp -> exp '+' . exp (rule 1)
NUM shift, and go to state 1
exp go to state 8
state 5
exp -> exp '-' . exp (rule 2)
NUM shift, and go to state 1
exp go to state 9
state 6
exp -> exp '*' . exp (rule 3)
NUM shift, and go to state 1
exp go to state 10
state 7
exp -> exp '/' . exp (rule 4)
NUM shift, and go to state 1
exp go to state 11
As was announced in beginning of the report, ‘State 8 conflicts: 1 shift/reduce’:
state 8
exp -> exp . '+' exp (rule 1)
exp -> exp '+' exp . (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
Indeed, there are two actions associated to the lookahead ‘/’: either shifting (and going to state 7), or reducing rule 1. The conflict means that either the grammar is ambiguous, or the parser lacks information to make the right decision. Indeed the grammar is ambiguous, as, since we did not specify the precedence of ‘/’, the sentence ‘NUM + NUM / NUM’ can be parsed as ‘NUM + (NUM / NUM)’, which corresponds to shifting ‘/’, or as ‘(NUM + NUM) / NUM’, which corresponds to reducing rule 1.
Because in deterministic parsing a single decision can be made, Bison arbitrarily chose to disable the reduction, see Shift/Reduce Conflicts. Discarded actions are reported in between square brackets.
Note that all the previous states had a single possible action: either shifting the next token and going to the corresponding state, or reducing a single rule. In the other cases, i.e., when shifting and reducing is possible or when several reductions are possible, the lookahead is required to select the action. State 8 is one such state: if the lookahead is ‘*’ or ‘/’ then the action is shifting, otherwise the action is reducing rule 1. In other words, the first two items, corresponding to rule 1, are not eligible when the lookahead token is ‘*’, since we specified that ‘*’ has higher precedence than ‘+’. More generally, some items are eligible only with some set of possible lookahead tokens. When run with --report=lookahead, Bison specifies these lookahead tokens:
state 8
exp -> exp . '+' exp (rule 1)
exp -> exp '+' exp . [$, '+', '-', '/'] (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
The remaining states are similar:
state 9
exp -> exp . '+' exp (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp '-' exp . (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 2 (exp)]
$default reduce using rule 2 (exp)
state 10
exp -> exp . '+' exp (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp '*' exp . (rule 3)
exp -> exp . '/' exp (rule 4)
'/' shift, and go to state 7
'/' [reduce using rule 3 (exp)]
$default reduce using rule 3 (exp)
state 11
exp -> exp . '+' exp (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
exp -> exp '/' exp . (rule 4)
'+' shift, and go to state 4
'-' shift, and go to state 5
'*' shift, and go to state 6
'/' shift, and go to state 7
'+' [reduce using rule 4 (exp)]
'-' [reduce using rule 4 (exp)]
'*' [reduce using rule 4 (exp)]
'/' [reduce using rule 4 (exp)]
$default reduce using rule 4 (exp)
Observe that state 11 contains conflicts not only due to the lack of precedence of ‘/’ with respect to ‘+’, ‘-’, and ‘*’, but also because the associativity of ‘/’ is not specified.
If a Bison grammar compiles properly but doesn't do what you want when it
runs, the yydebug parser-trace feature can help you figure out why.
There are several means to enable compilation of trace facilities:
YYDEBUGYYDEBUG to a nonzero value when you compile the
parser. This is compliant with POSIX Yacc. You could use
‘-DYYDEBUG=1’ as a compiler option or you could put ‘#define
YYDEBUG 1’ in the prologue of the grammar file (see The Prologue).
%debug directive (see Bison Declaration Summary). This is a Bison extension, which will prove
useful when Bison will output parsers for languages that don't use a
preprocessor. Unless POSIX and Yacc portability matter to
you, this is
the preferred solution.
We suggest that you always enable the debug option so that debugging is always possible.
The trace facility outputs messages with macro calls of the form
YYFPRINTF (stderr, format, args) where
format and args are the usual printf format and variadic
arguments. If you define YYDEBUG to a nonzero value but do not
define YYFPRINTF, <stdio.h> is automatically included
and YYFPRINTF is defined to fprintf.
Once you have compiled the program with trace facilities, the way to
request a trace is to store a nonzero value in the variable yydebug.
You can do this by making the C code do it (in main, perhaps), or
you can alter the value with a C debugger.
Each step taken by the parser when yydebug is nonzero produces a
line or two of trace information, written on stderr. The trace
messages tell you these things:
yylex, what kind of token was read.
To make sense of this information, it helps to refer to the listing file produced by the Bison ‘-v’ option (see Invoking Bison). This file shows the meaning of each state in terms of positions in various rules, and also what each state will do with each possible input token. As you read the successive trace messages, you can see that the parser is functioning according to its specification in the listing file. Eventually you will arrive at the place where something undesirable happens, and you will see which parts of the grammar are to blame.
The parser implementation file is a C program and you can use C debuggers on it, but it's not easy to interpret what it is doing. The parser function is a finite-state machine interpreter, and aside from the actions it executes the same code over and over. Only the values of variables show where in the grammar it is working.
The debugging information normally gives the token type of each token
read, but not its semantic value. You can optionally define a macro
named YYPRINT to provide a way to print the value. If you define
YYPRINT, it should take three arguments. The parser will pass a
standard I/O stream, the numeric code for the token type, and the token
value (from yylval).
Here is an example of YYPRINT suitable for the multi-function
calculator (see Declarations for mfcalc):
%{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(file, type, value) print_token_value (file, type, value)
%}
... %% ... %% ...
static void
print_token_value (FILE *file, int type, YYSTYPE value)
{
if (type == VAR)
fprintf (file, "%s", value.tptr->name);
else if (type == NUM)
fprintf (file, "%d", value.val);
}
The usual way to invoke Bison is as follows:
bison infile
Here infile is the grammar file name, which usually ends in ‘.y’. The parser implementation file's name is made by replacing the ‘.y’ with ‘.tab.c’ and removing any leading directory. Thus, the ‘bison foo.y’ file name yields foo.tab.c, and the ‘bison hack/foo.y’ file name yields foo.tab.c. It's also possible, in case you are writing C++ code instead of C in your grammar file, to name it foo.ypp or foo.y++. Then, the output files will take an extension like the given one as input (respectively foo.tab.cpp and foo.tab.c++). This feature takes effect with all options that manipulate file names like ‘-o’ or ‘-d’.
For example :
bison -d infile.yxx
will produce infile.tab.cxx and infile.tab.hxx, and
bison -d -o output.c++ infile.y
will produce output.c++ and outfile.h++.
For compatibility with POSIX, the standard Bison distribution also contains a shell script called yacc that invokes Bison with the -y option.
Bison supports both traditional single-letter options and mnemonic long option names. Long option names are indicated with ‘--’ instead of ‘-’. Abbreviations for option names are allowed as long as they are unique. When a long option takes an argument, like ‘--file-prefix’, connect the option name and the argument with ‘=’.
Here is a list of options that can be used with Bison, alphabetized by short option. It is followed by a cross key alphabetized by long option.
Operations modes:
#define statements in addition to an enum to associate
token numbers with token names. Thus, the following shell script can
substitute for Yacc, and the Bison distribution contains such a script
for compatibility with POSIX:
#! /bin/sh
bison -y "$@"
The -y/--yacc option is intended for use with
traditional Yacc grammars. If your grammar uses a Bison extension
like ‘%glr-parser’, Bison might not be Yacc-compatible even if
this option is specified.
midrule-values$2 in:
exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
Also warn about mid-rule values that are used but not set.
For example, warn about unset $$ in the mid-rule action in:
exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
These warnings are not enabled by default since they sometimes prove to
be false alarms in existing grammars employing the Yacc constructs
$0 or $-n (where n is some positive integer).
yaccconflicts-srconflicts-rr%expect or %expect-rr directive is specified, an
unexpected number of conflicts is an error, and an expected number of
conflicts is not reported, so -W and --warning then have
no effect on the conflict report.
otherThis category is provided merely for the sake of completeness. Future
releases of Bison may move warnings from this category to new, more specific
categories.
allnoneerrorA category can be turned off by prefixing its name with ‘no-’. For instance, -Wno-yacc will hide the warnings about POSIX Yacc incompatibilities.
Tuning the parser:
YYDEBUG to
1 if it is not already defined, so that the debugging facilities are
compiled. See Tracing Your Parser.
-D or
--define, Bison reports an error for any %define
definition for name.
-F or
--force-define instead, Bison quietly ignores all %define
definitions for name.
%define
definitions for name.
You should avoid using -F and --force-define in your
make files unless you are confident that it is safe to quietly ignore
any conflicting %define that may be added to the grammar file.
%language was specified (see Bison Declaration Summary). Currently supported languages include C, C++, and Java.
language is case-insensitive.
This option is experimental and its effect may be modified in future
releases.
%locations was specified. See Decl Summary.
%name-prefix "prefix" was specified.
See Decl Summary.
#line preprocessor commands in the parser
implementation file. Ordinarily Bison puts them in the parser
implementation file so that the C compiler and debuggers will
associate errors with your source file, the grammar file. This option
causes them to associate errors with the parser implementation file,
treating it as an independent source file in its own right.
%skeleton
(see Bison Declaration Summary).
If file does not contain a /, file is the name of a skeleton
file in the Bison installation directory.
If it does, file is an absolute file name or a file name relative to the
current working directory.
This is similar to how most shells resolve commands.
%token-table was specified. See Decl Summary.
Adjust the output:
%defines was specified, i.e., write an extra output
file containing macro definitions for the token type names defined in
the grammar, as well as a few other declarations. See Decl Summary.
--defines except -d does not accept a
file argument since POSIX Yacc requires that -d can be bundled
with other short options.
%file-prefix was specified, i.e., specify prefix to use
for all Bison output file names. See Decl Summary.
statelookaheadstate and augments the description of the automaton with
each rule's lookahead set.
itemsetstate and augments the description of the automaton with
the full set of items for each state, instead of its core only.
%verbose was specified, i.e., write an extra output
file containing verbose descriptions of the grammar and
parser. See Decl Summary.
The other output files' names are constructed from file as
described under the ‘-v’ and ‘-d’ options.
Here is a list of options, alphabetized by long option, to help you find the corresponding short option and directive.
| Long Option | Short Option | Bison Directive
|
|---|---|---|
| --debug | -t | %debug
|
| --define=name[=value] | -D name[=value] | %define name ["value"]
|
| --defines[=file] | -d | %defines ["file"]
|
| --file-prefix=prefix | -b prefix | %file-prefix "prefix"
|
| --force-define=name[=value] | -F name[=value] | %define name ["value"]
|
| --graph[=file] | -g [file] |
|
| --help | -h |
|
| --language=language | -L language | %language "language"
|
| --locations | %locations
| |
| --name-prefix=prefix | -p prefix | %name-prefix "prefix"
|
| --no-lines | -l | %no-lines
|
| --output=file | -o file | %output "file"
|
| --print-datadir |
| |
| --print-localedir |
| |
| --report-file=file |
| |
| --report=things | -r things |
|
| --skeleton=file | -S file | %skeleton "file"
|
| --token-table | -k | %token-table
|
| --verbose | -v | %verbose
|
| --version | -V |
|
| --warnings[=category] | -W [category] |
|
| --xml[=file] | -x [file] |
|
| --yacc | -y | %yacc
|
The Yacc library contains default implementations of the
yyerror and main functions. These default
implementations are normally not useful, but POSIX requires
them. To use the Yacc library, link your program with the
-ly option. Note that Bison's implementation of the Yacc
library is distributed under the terms of the GNU General
Public License (see Copying).
If you use the Yacc library's yyerror function, you should
declare yyerror as follows:
int yyerror (char const *);
Bison ignores the int value returned by this yyerror.
If you use the Yacc library's main function, your
yyparse function should have the following type signature:
int yyparse (void);
The C++ deterministic parser is selected using the skeleton directive, ‘%skeleton "lalr1.cc"’, or the synonymous command-line option --skeleton=lalr1.cc. See Decl Summary.
When run, bison will create several entities in the ‘yy’ namespace. Use the ‘%define namespace’ directive to change the namespace name, see namespace. The various classes are generated in the following files:
position and location,
used for location tracking. See C++ Location Values.
stack used by the parser.
The header is mandatory; you must either pass -d/--defines to bison, or use the ‘%defines’ directive.
All these files are documented using Doxygen; run doxygen for a complete and accurate documentation.
The %union directive works as for C, see The Collection of Value Types. In particular it produces a genuine
union1, which have a few specific features in C++.
YYSTYPE is defined but its use is discouraged: rather
you should refer to the parser's encapsulated type
yy::parser::semantic_type.
Because objects have to be stored via pointers, memory is not
reclaimed automatically: using the %destructor directive is the
only means to avoid leaks. See Freeing Discarded Symbols.
When the directive %locations is used, the C++ parser supports
location tracking, see Locations Overview. Two
auxiliary classes define a position, a single point in a file,
and a location, a range composed of a pair of
positions (possibly spanning several files).
The name of the file. It will always be handled as a pointer, the parser will never duplicate nor deallocate it. As an experimental feature you may change it to ‘type*’ using ‘%define filename_type "type"’.
Advance by height lines, resetting the column number.
Advance by width columns, without changing the line number.
Various forms of syntactic sugar for
columns.
Report p on o like this: ‘file:line.column’, or ‘line.column’ if file is null.
The first, inclusive, position of the range, and the first beyond.
Advance the
endposition.
Various forms of syntactic sugar.
The output files output.hh and output.cc
declare and define the parser class in the namespace yy. The
class name defaults to parser, but may be changed using
‘%define parser_class_name "name"’. The interface of
this class is detailed below. It can be extended using the
%parse-param feature: its semantics is slightly changed since
it describes an additional member of the parser class, and an
additional argument for its constructor.
The types for semantics value and locations.
A structure that contains (only) the definition of the tokens as the
yytokentypeenumeration. To refer to the tokenFOO, the scanner should useyy::parser::token::FOO. The scanner can use ‘typedef yy::parser::token token;’ to “import” the token enumeration (see Calc++ Scanner).
Build a new parser object. There are no arguments by default, unless ‘%parse-param {type1 arg1}’ was used.
Get or set the stream used for tracing the parsing. It defaults to
std::cerr.
Get or set the tracing level. Currently its value is either 0, no trace, or nonzero, full tracing.
The definition for this member function must be supplied by the user: the parser uses it to report a parser error occurring at l, described by m.
The parser invokes the scanner by calling yylex. Contrary to C
parsers, C++ parsers are always pure: there is no point in using the
%define api.pure directive. Therefore the interface is as follows.
Return the next token. Its type is the return value, its semantic value and location being yylval and yylloc. Invocations of ‘%lex-param {type1 arg1}’ yield additional arguments.
This section demonstrates the use of a C++ parser with a simple but complete example. This example should be available on your system, ready to compile, in the directory ../bison/examples/calc++. It focuses on the use of Bison, therefore the design of the various C++ classes is very naive: no accessors, no encapsulation of members etc. We will use a Lex scanner, and more precisely, a Flex scanner, to demonstrate the various interaction. A hand written scanner is actually easier to interface with.
Of course the grammar is dedicated to arithmetics, a single
expression, possibly preceded by variable assignments. An
environment containing possibly predefined variables such as
one and two, is exchanged with the parser. An example
of valid input follows.
three := 3
seven := one + two * three
seven * seven
To support a pure interface with the parser (and the scanner) the technique of the “parsing context” is convenient: a structure containing all the data to exchange. Since, in addition to simply launch the parsing, there are several auxiliary tasks to execute (open the file for parsing, instantiate the parser etc.), we recommend transforming the simple parsing context structure into a fully blown parsing driver class.
The declaration of this driver class, calc++-driver.hh, is as follows. The first part includes the CPP guard and imports the required standard library components, and the declaration of the parser class.
#ifndef CALCXX_DRIVER_HH
# define CALCXX_DRIVER_HH
# include <string>
# include <map>
# include "calc++-parser.hh"
Then comes the declaration of the scanning function. Flex expects
the signature of yylex to be defined in the macro
YY_DECL, and the C++ parser expects it to be declared. We can
factor both as follows.
// Tell Flex the lexer's prototype ...
# define YY_DECL \
yy::calcxx_parser::token_type \
yylex (yy::calcxx_parser::semantic_type* yylval, \
yy::calcxx_parser::location_type* yylloc, \
calcxx_driver& driver)
// ... and declare it for the parser's sake.
YY_DECL;
The calcxx_driver class is then declared with its most obvious
members.
// Conducting the whole scanning and parsing of Calc++.
class calcxx_driver
{
public:
calcxx_driver ();
virtual ~calcxx_driver ();
std::map<std::string, int> variables;
int result;
To encapsulate the coordination with the Flex scanner, it is useful to have two members function to open and close the scanning phase.
// Handling the scanner.
void scan_begin ();
void scan_end ();
bool trace_scanning;
Similarly for the parser itself.
// Run the parser. Return 0 on success.
int parse (const std::string& f);
std::string file;
bool trace_parsing;
To demonstrate pure handling of parse errors, instead of simply dumping them on the standard error output, we will pass them to the compiler driver using the following two member functions. Finally, we close the class declaration and CPP guard.
// Error handling.
void error (const yy::location& l, const std::string& m);
void error (const std::string& m);
};
#endif // ! CALCXX_DRIVER_HH
The implementation of the driver is straightforward. The parse
member function deserves some attention. The error functions
are simple stubs, they should actually register the located error
messages and set error state.
#include "calc++-driver.hh"
#include "calc++-parser.hh"
calcxx_driver::calcxx_driver ()
: trace_scanning (false), trace_parsing (false)
{
variables["one"] = 1;
variables["two"] = 2;
}
calcxx_driver::~calcxx_driver ()
{
}
int
calcxx_driver::parse (const std::string &f)
{
file = f;
scan_begin ();
yy::calcxx_parser parser (*this);
parser.set_debug_level (trace_parsing);
int res = parser.parse ();
scan_end ();
return res;
}
void
calcxx_driver::error (const yy::location& l, const std::string& m)
{
std::cerr << l << ": " << m << std::endl;
}
void
calcxx_driver::error (const std::string& m)
{
std::cerr << m << std::endl;
}
The grammar file calc++-parser.yy starts by asking for the C++ deterministic parser skeleton, the creation of the parser header file, and specifies the name of the parser class. Because the C++ skeleton changed several times, it is safer to require the version you designed the grammar for.
%skeleton "lalr1.cc" /* -*- C++ -*- */
%require "2.5"
%defines
%define parser_class_name "calcxx_parser"
Then come the declarations/inclusions needed to define the
%union. Because the parser uses the parsing driver and
reciprocally, both cannot include the header of the other. Because the
driver's header needs detailed knowledge about the parser class (in
particular its inner types), it is the parser's header which will simply
use a forward declaration of the driver.
See %code Summary.
%code requires {
# include <string>
class calcxx_driver;
}
The driver is passed by reference to the parser and to the scanner. This provides a simple but effective pure interface, not relying on global variables.
// The parsing context.
%parse-param { calcxx_driver& driver }
%lex-param { calcxx_driver& driver }
Then we request the location tracking feature, and initialize the first location's file name. Afterward new locations are computed relatively to the previous locations: the file name will be automatically propagated.
%locations
%initial-action
{
// Initialize the initial location.
@$.begin.filename = @$.end.filename = &driver.file;
};
Use the two following directives to enable parser tracing and verbose error messages. However, verbose error messages can contain incorrect information (see LAC).
%debug
%error-verbose
Semantic values cannot use “real” objects, but only pointers to them.
// Symbols.
%union
{
int ival;
std::string *sval;
};
The code between ‘%code {’ and ‘}’ is output in the *.cc file; it needs detailed knowledge about the driver.
%code {
# include "calc++-driver.hh"
}
The token numbered as 0 corresponds to end of file; the following line
allows for nicer error messages referring to “end of file” instead
of “$end”. Similarly user friendly named are provided for each
symbol. Note that the tokens names are prefixed by TOKEN_ to
avoid name clashes.
%token END 0 "end of file"
%token ASSIGN ":="
%token <sval> IDENTIFIER "identifier"
%token <ival> NUMBER "number"
%type <ival> exp
To enable memory deallocation during error recovery, use
%destructor.
%printer { debug_stream () << *$$; } "identifier"
%destructor { delete $$; } "identifier"
%printer { debug_stream () << $$; } <ival>
The grammar itself is straightforward.
%%
%start unit;
unit: assignments exp { driver.result = $2; };
assignments: assignments assignment {}
| /* Nothing. */ {};
assignment:
"identifier" ":=" exp
{ driver.variables[*$1] = $3; delete $1; };
%left '+' '-';
%left '*' '/';
exp: exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp { $$ = $1 / $3; }
| "identifier" { $$ = driver.variables[*$1]; delete $1; }
| "number" { $$ = $1; };
%%
Finally the error member function registers the errors to the
driver.
void
yy::calcxx_parser::error (const yy::calcxx_parser::location_type& l,
const std::string& m)
{
driver.error (l, m);
}
The Flex scanner first includes the driver declaration, then the parser's to get the set of defined tokens.
%{ /* -*- C++ -*- */
# include <cstdlib>
# include <cerrno>
# include <climits>
# include <string>
# include "calc++-driver.hh"
# include "calc++-parser.hh"
/* Work around an incompatibility in flex (at least versions
2.5.31 through 2.5.33): it generates code that does
not conform to C89. See Debian bug 333231
<http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>. */
# undef yywrap
# define yywrap() 1
/* By default yylex returns int, we use token_type.
Unfortunately yyterminate by default returns 0, which is
not of token_type. */
#define yyterminate() return token::END
%}
Because there is no #include-like feature we don't need
yywrap, we don't need unput either, and we parse an
actual file, this is not an interactive session with the user.
Finally we enable the scanner tracing features.
%option noyywrap nounput batch debug
Abbreviations allow for more readable rules.
id [a-zA-Z][a-zA-Z_0-9]*
int [0-9]+
blank [ \t]
The following paragraph suffices to track locations accurately. Each
time yylex is invoked, the begin position is moved onto the end
position. Then when a pattern is matched, the end position is
advanced of its width. In case it matched ends of lines, the end
cursor is adjusted, and each time blanks are matched, the begin cursor
is moved onto the end cursor to effectively ignore the blanks
preceding tokens. Comments would be treated equally.
%{
# define YY_USER_ACTION yylloc->columns (yyleng);
%}
%%
%{
yylloc->step ();
%}
{blank}+ yylloc->step ();
[\n]+ yylloc->lines (yyleng); yylloc->step ();
The rules are simple, just note the use of the driver to report errors.
It is convenient to use a typedef to shorten
yy::calcxx_parser::token::identifier into
token::identifier for instance.
%{
typedef yy::calcxx_parser::token token;
%}
/* Convert ints to the actual type of tokens. */
[-+*/] return yy::calcxx_parser::token_type (yytext[0]);
":=" return token::ASSIGN;
{int} {
errno = 0;
long n = strtol (yytext, NULL, 10);
if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
driver.error (*yylloc, "integer is out of range");
yylval->ival = n;
return token::NUMBER;
}
{id} yylval->sval = new std::string (yytext); return token::IDENTIFIER;
. driver.error (*yylloc, "invalid character");
%%
Finally, because the scanner related driver's member function depend on the scanner's data, it is simpler to implement them in this file.
void
calcxx_driver::scan_begin ()
{
yy_flex_debug = trace_scanning;
if (file == "-")
yyin = stdin;
else if (!(yyin = fopen (file.c_str (), "r")))
{
error (std::string ("cannot open ") + file);
exit (1);
}
}
void
calcxx_driver::scan_end ()
{
fclose (yyin);
}
The top level file, calc++.cc, poses no problem.
#include <iostream>
#include "calc++-driver.hh"
int
main (int argc, char *argv[])
{
calcxx_driver driver;
for (++argv; argv[0]; ++argv)
if (*argv == std::string ("-p"))
driver.trace_parsing = true;
else if (*argv == std::string ("-s"))
driver.trace_scanning = true;
else if (!driver.parse (*argv))
std::cout << driver.result << std::endl;
}
(The current Java interface is experimental and may evolve. More user feedback will help to stabilize it.)
The Java parser skeletons are selected using the %language "Java"
directive or the -L java/--language=java option.
When generating a Java parser, bison basename.y will
create a single Java source file named basename.java
containing the parser implementation. Using a grammar file without a
.y suffix is currently broken. The basename of the parser
implementation file can be changed by the %file-prefix
directive or the -p/--name-prefix option. The
entire parser implementation file name can be changed by the
%output directive or the -o/--output option.
The parser implementation file contains a single class for the parser.
You can create documentation for generated parsers using Javadoc.
Contrary to C parsers, Java parsers do not use global variables; the
state of the parser is always local to an instance of the parser class.
Therefore, all Java parsers are “pure”, and the %pure-parser
and %define api.pure directives does not do anything when used in
Java.
Push parsers are currently unsupported in Java and %define
api.push-pull have no effect.
GLR parsers are currently unsupported in Java. Do not use the
glr-parser directive.
No header file can be generated for Java parsers. Do not use the
%defines directive or the -d/--defines options.
Currently, support for debugging and verbose errors are always compiled
in. Thus the %debug and %token-table directives and the
-t/--debug and -k/--token-table
options have no effect. This may change in the future to eliminate
unused code in the generated parser, so use %debug and
%verbose-error explicitly if needed. Also, in the future the
%token-table directive might enable a public interface to
access the token names and codes.
There is no %union directive in Java parsers. Instead, the
semantic values' types (class names) should be specified in the
%type or %token directive:
%type <Expression> expr assignment_expr term factor
%type <Integer> number
By default, the semantic stack is declared to have Object members,
which means that the class types you specify can be of any class.
To improve the type safety of the parser, you can declare the common
superclass of all the semantic values using the %define stype
directive. For example, after the following declaration:
%define stype "ASTNode"
any %type or %token specifying a semantic type which
is not a subclass of ASTNode, will cause a compile-time error.
Types used in the directives may be qualified with a package name. Primitive data types are accepted for Java version 1.5 or later. Note that in this case the autoboxing feature of Java 1.5 will be used. Generic types may not be used; this is due to a limitation in the implementation of Bison, and may change in future releases.
Java parsers do not support %destructor, since the language
adopts garbage collection. The parser will try to hold references
to semantic values for as little time as needed.
Java parsers do not support %printer, as toString()
can be used to print the semantic values. This however may change
(in a backwards-compatible way) in future versions of Bison.
When the directive %locations is used, the Java parser
supports location tracking, see Locations Overview.
An auxiliary user-defined class defines a position, a single point
in a file; Bison itself defines a class representing a location,
a range composed of a pair of positions (possibly spanning several
files). The location class is an inner class of the parser; the name
is Location by default, and may also be renamed using
%define location_type "class-name".
The location class treats the position as a completely opaque value.
By default, the class name is Position, but this can be changed
with %define position_type "class-name". This class must
be supplied by the user.
The first, inclusive, position of the range, and the first beyond.
Create a
Locationdenoting an empty range located at a given point.
Create a
Locationfrom the endpoints of the range.
Prints the range represented by the location. For this to work properly, the position class should override the
equalsandtoStringmethods appropriately.
The name of the generated parser class defaults to YYParser. The
YY prefix may be changed using the %name-prefix directive
or the -p/--name-prefix option. Alternatively, use
%define parser_class_name "name" to give a custom name to
the class. The interface of this class is detailed below.
By default, the parser class has package visibility. A declaration
%define public will change to public visibility. Remember that,
according to the Java language specification, the name of the .java
file should match the name of the class in this case. Similarly, you can
use abstract, final and strictfp with the
%define declaration to add other modifiers to the parser class.
The Java package name of the parser class can be specified using the
%define package directive. The superclass and the implemented
interfaces of the parser class can be specified with the %define
extends and %define implements directives.
The parser class defines an inner class, Location, that is used
for location tracking (see Java Location Values), and a inner
interface, Lexer (see Java Scanner Interface). Other than
these inner class/interface, and the members described in the interface
below, all the other members and fields are preceded with a yy or
YY prefix to avoid clashes with user code.
The parser class can be extended using the %parse-param
directive. Each occurrence of the directive will add a protected
final field to the parser class, and an argument to its constructor,
which initialize them automatically.
Token names defined by %token and the predefined EOF token
name are added as constant fields to the parser class.
Build a new parser object with embedded
%code lexer. There are no parameters, unless%parse-params and/or%lex-params are used.
Build a new parser object using the specified scanner. There are no additional parameters unless
%parse-params are used.If the scanner is defined by
%code lexer, this constructor is declaredprotectedand is called automatically with a scanner created with the correct%lex-params.
Run the syntactic analysis, and return
trueon success,falseotherwise.
During the syntactic analysis, return
trueif recovering from a syntax error. See Error Recovery.
Get or set the stream used for tracing the parsing. It defaults to
System.err.
Get or set the tracing level. Currently its value is either 0, no trace, or nonzero, full tracing.
There are two possible ways to interface a Bison-generated Java parser
with a scanner: the scanner may be defined by %code lexer, or
defined elsewhere. In either case, the scanner has to implement the
Lexer inner interface of the parser class.
In the first case, the body of the scanner class is placed in
%code lexer blocks. If you want to pass parameters from the
parser constructor to the scanner constructor, specify them with
%lex-param; they are passed before %parse-params to the
constructor.
In the second case, the scanner has to implement the Lexer interface,
which is defined within the parser class (e.g., YYParser.Lexer).
The constructor of the parser object will then accept an object
implementing the interface; %lex-param is not used in this
case.
In both cases, the scanner has to implement the following methods.
This method is defined by the user to emit an error message. The first parameter is omitted if location tracking is not active. Its type can be changed using
%define location_type "class-name".
Return the next token. Its type is the return value, its semantic value and location are saved and returned by the their methods in the interface.
Use
%define lex_throwsto specify any uncaught exceptions. Default isjava.io.IOException.
Return respectively the first position of the last token that
yylexreturned, and the first position beyond it. These methods are not needed unless location tracking is active.The return type can be changed using
%define position_type "class-name".
Return the semantic value of the last token that yylex returned.
The return type can be changed using
%define stype "class-name".
The following special constructs can be uses in Java actions. Other analogous C action features are currently unavailable for Java.
Use %define throws to specify any uncaught exceptions from parser
actions, and initial actions specified by %initial-action.
The semantic value for the nth component of the current rule. This may not be assigned to. See Java Semantic Values.
The semantic value for the grouping made by the current rule. As a value, this is in the base type (
Objector as specified by%define stype) as in not cast to the declared subtype because casts are not allowed on the left-hand side of Java assignments. Use an explicit Java cast if the correct subtype is needed. See Java Semantic Values.
Same as
$$since Java always allow assigning to the base type. Perhaps we should use this and$<>$for the value and$$for setting the value but there is currently no easy way to distinguish these constructs. See Java Semantic Values.
The location information of the nth component of the current rule. This may not be assigned to. See Java Location Values.
The location information of the grouping made by the current rule. See Java Location Values.
Return immediately from the parser, indicating failure. See Java Parser Interface.
Return immediately from the parser, indicating success. See Java Parser Interface.
Start error recovery without printing an error message. See Error Recovery.
Return whether error recovery is being done. In this state, the parser reads token until it reaches a known state, and then restarts normal operation. See Error Recovery.
Print an error message using the
yyerrormethod of the scanner instance in use.
The different structure of the Java language forces several differences between C/C++ grammars, and grammars designed for Java parsers. This section summarizes these differences.
YYERROR, YYACCEPT,
YYABORT symbols (see Table of Symbols) cannot obviously be
macros. Instead, they should be preceded by return when they
appear in an action. The actual definition of these symbols is
opaque to the Bison grammar, and it might change in the future. The
only meaningful operation that you can do, is to return them.
See see Java Action Features.
Note that of these three symbols, only YYACCEPT and
YYABORT will cause a return from the yyparse
method2.
%union has no effect. Instead, semantic
values have a common base type: Object or as specified by
‘%define stype’. Angle brackets on %token, type,
$n and $$ specify subtypes rather than fields of
an union. The type of $$, even with angle brackets, is the base
type since Java casts are not allow on the left-hand side of assignments.
Also, $n and @n are not allowed on the
left-hand side of assignments. See see Java Semantic Values and
see Java Action Features.
%code importspackage declarations, it is
suggested to use %define package instead.
%code%code lexerOther %code blocks are not supported in Java parsers.
In particular, %{ ... %} blocks should not be used
and may give an error in future versions of Bison.
The epilogue has the same meaning as in C/C++ code and it can be used to define other classes used by the parser outside the parser class.
This summary only include declarations specific to Java or have special meaning when used in a Java parser.
A parameter for the lexer class defined by
%code lexeronly, added as parameters to the lexer constructor and the parser constructor that creates a lexer. Default is none. See Java Scanner Interface.
The prefix of the parser class name prefix
Parserif%define parser_class_nameis not used. Default isYY. See Java Bison Interface.
A parameter for the parser class added as parameters to constructor(s) and as fields initialized by the constructor(s). Default is none. See Java Parser Interface.
Declare tokens. Note that the angle brackets enclose a Java type. See Java Semantic Values.
Declare the type of nonterminals. Note that the angle brackets enclose a Java type. See Java Semantic Values.
Code appended to the inside of the parser class. See Java Differences.
Code inserted just after the
packagedeclaration. See Java Differences.
Code added to the body of a inner lexer class within the parser class. See Java Scanner Interface.
Code (after the second
%%) appended to the end of the file, outside the parser class. See Java Differences.
Whether the parser class is declared
abstract. Default is false. See Java Bison Interface.
The superclass of the parser class. Default is none. See Java Bison Interface.
Whether the parser class is declared
final. Default is false. See Java Bison Interface.
The implemented interfaces of the parser class, a comma-separated list. Default is none. See Java Bison Interface.
The exceptions thrown by the
yylexmethod of the lexer, a comma-separated list. Default isjava.io.IOException. See Java Scanner Interface.
The name of the class used for locations (a range between two positions). This class is generated as an inner class of the parser class by bison. Default is
Location. See Java Location Values.
The package to put the parser class in. Default is none. See Java Bison Interface.
The name of the parser class. Default is
YYParseror name-prefixParser. See Java Bison Interface.
The name of the class used for positions. This class must be supplied by the user. Default is
Position. See Java Location Values.
Whether the parser class is declared
public. Default is false. See Java Bison Interface.
The base type of semantic values. Default is
Object. See Java Semantic Values.
Whether the parser class is declared
strictfp. Default is false. See Java Bison Interface.
The exceptions thrown by user-supplied parser actions and
%initial-action, a comma-separated list. Default is none. See Java Parser Interface.
Several questions about Bison come up occasionally. Here some of them are addressed.
My parser returns with error with a ‘memory exhausted’
message. What can I do?
This question is already addressed elsewhere, See Recursive Rules.
The following phenomenon has several symptoms, resulting in the following typical questions:
I invokeyyparseseveral times, and on correct input it works properly; but when a parse error is found, all the other calls fail too. How can I reset the error flag ofyyparse?
or
My parser includes support for an ‘#include’-like feature, in which case I runyyparsefromyyparse. This fails although I did specify%define api.pure.
These problems typically come not from Bison itself, but from Lex-generated scanners. Because these scanners use large buffers for speed, they might not notice a change of input file. As a demonstration, consider the following source file, first-line.l:
%{
#include <stdio.h>
#include <stdlib.h>
%}
%%
.*\n ECHO; return 1;
%%
int
yyparse (char const *file)
{
yyin = fopen (file, "r");
if (!yyin)
exit (2);
/* One token only. */
yylex ();
if (fclose (yyin) != 0)
exit (3);
return 0;
}
int
main (void)
{
yyparse ("input");
yyparse ("input");
return 0;
}
If the file input contains
input:1: Hello, input:2: World!
then instead of getting the first line twice, you get:
$ flex -ofirst-line.c first-line.l
$ gcc -ofirst-line first-line.c -ll
$ ./first-line
input:1: Hello,
input:2: World!
Therefore, whenever you change yyin, you must tell the
Lex-generated scanner to discard its current buffer and switch to the
new one. This depends upon your implementation of Lex; see its
documentation for more. For Flex, it suffices to call
‘YY_FLUSH_BUFFER’ after each change to yyin. If your
Flex-generated scanner needs to read from several input streams to
handle features like include files, you might consider using Flex
functions like ‘yy_switch_to_buffer’ that manipulate multiple
input buffers.
If your Flex-generated scanner uses start conditions (see Start conditions), you might also want to reset the scanner's state, i.e., go back to the initial start condition, through a call to ‘BEGIN (0)’.
My parser seems to destroy old strings, or maybe it loses track of
them. Instead of reporting ‘"foo", "bar"’, it reports
‘"bar", "bar"’, or even ‘"foo\nbar", "bar"’.
This error is probably the single most frequent “bug report” sent to Bison lists, but is only concerned with a misunderstanding of the role of the scanner. Consider the following Lex code:
%{
#include <stdio.h>
char *yylval = NULL;
%}
%%
.* yylval = yytext; return 1;
\n /* IGNORE */
%%
int
main ()
{
/* Similar to using $1, $2 in a Bison action. */
char *fst = (yylex (), yylval);
char *snd = (yylex (), yylval);
printf ("\"%s\", \"%s\"\n", fst, snd);
return 0;
}
If you compile and run this code, you get:
$ flex -osplit-lines.c split-lines.l
$ gcc -osplit-lines split-lines.c -ll
$ printf 'one\ntwo\n' | ./split-lines
"one
two", "two"
this is because yytext is a buffer provided for reading
in the action, but if you want to keep it, you have to duplicate it
(e.g., using strdup). Note that the output may depend on how
your implementation of Lex handles yytext. For instance, when
given the Lex compatibility option -l (which triggers the
option ‘%array’) Flex generates a different behavior:
$ flex -l -osplit-lines.c split-lines.l
$ gcc -osplit-lines split-lines.c -ll
$ printf 'one\ntwo\n' | ./split-lines
"two", "two"
My simple calculator supports variables, assignments, and functions,
but how can I implement gotos, or loops?
Although very pedagogical, the examples included in the document blur the distinction to make between the parser—whose job is to recover the structure of a text and to transmit it to subsequent modules of the program—and the processing (such as the execution) of this structure. This works well with so called straight line programs, i.e., precisely those that have a straightforward execution model: execute simple instructions one after the others.
If you want a richer model, you will probably need to use the parser to construct a tree that does represent the structure it has recovered; this tree is usually called the abstract syntax tree, or AST for short. Then, walking through this tree, traversing it in various ways, will enable treatments such as its execution or its translation, which will result in an interpreter or a compiler.
This topic is way beyond the scope of this manual, and the reader is invited to consult the dedicated literature.
I have several closely related grammars, and I would like to share their
implementations. In fact, I could use a single grammar but with
multiple entry points.
Bison does not support multiple start-symbols, but there is a very
simple means to simulate them. If foo and bar are the two
pseudo start-symbols, then introduce two new tokens, say
START_FOO and START_BAR, and use them as switches from the
real start-symbol:
%token START_FOO START_BAR;
%start start;
start: START_FOO foo
| START_BAR bar;
These tokens prevents the introduction of new conflicts. As far as the parser goes, that is all that is needed.
Now the difficult part is ensuring that the scanner will send these
tokens first. If your scanner is hand-written, that should be
straightforward. If your scanner is generated by Lex, them there is
simple means to do it: recall that anything between ‘%{ ... %}’
after the first %% is copied verbatim in the top of the generated
yylex function. Make sure a variable start_token is
available in the scanner (e.g., a global variable or using
%lex-param etc.), and use the following:
/* Prologue. */ %% %{ if (start_token) { int t = start_token; start_token = 0; return t; } %} /* The rules. */
Is Bison secure? Does it conform to POSIX?
If you're looking for a guarantee or certification, we don't provide it. However, Bison is intended to be a reliable program that conforms to the POSIX specification for Yacc. If you run into problems, please send us a bug report.
I can't build Bison because make complains that
msgfmt is not found.
What should I do?
Like most GNU packages with internationalization support, that feature is turned on by default. If you have problems building in the po subdirectory, it indicates that your system's internationalization support is lacking. You can re-configure Bison with --disable-nls to turn off this support, or you can install GNU gettext from ftp://ftp.gnu.org/gnu/gettext/ and re-configure Bison. See the file ABOUT-NLS for more information.
I'm having trouble using Bison. Where can I find help?
First, read this fine manual. Beyond that, you can send mail to help-bison@gnu.org. This mailing list is intended to be populated with people who are willing to answer questions about using and installing Bison. Please keep in mind that (most of) the people on the list have aspects of their lives which are not related to Bison (!), so you may not receive an answer to your question right away. This can be frustrating, but please try not to honk them off; remember that any help they provide is purely voluntary and out of the kindness of their hearts.
I found a bug. What should I include in the bug report?
Before you send a bug report, make sure you are using the latest version. Check ftp://ftp.gnu.org/pub/gnu/bison/ or one of its mirrors. Be sure to include the version number in your bug report. If the bug is present in the latest version but not in a previous version, try to determine the most recent version which did not contain the bug.
If the bug is parser-related, you should include the smallest grammar you can which demonstrates the bug. The grammar file should also be complete (i.e., I should be able to run it through Bison without having to edit or add anything). The smaller and simpler the grammar, the easier it will be to fix the bug.
Include information about your compilation environment, including your operating system's name and version and your compiler's name and version. If you have trouble compiling, you should also include a transcript of the build session, starting with the invocation of `configure'. Depending on the nature of the bug, you may be asked to send additional files as well (such as `config.h' or `config.cache').
Patches are most welcome, but not required. That is, do not hesitate to send a bug report just because you can not provide a fix.
Send bug reports to bug-bison@gnu.org.
Will Bison ever have C++ and Java support? How about insert your
favorite language here?
C++ and Java support is there now, and is documented. We'd love to add other languages; contributions are welcome.
What is involved in being a beta tester?
It's not terribly involved. Basically, you would download a test release, compile it, and use it to build and run a parser or two. After that, you would submit either a bug report or a message saying that everything is okay. It is important to report successes as well as failures because test releases eventually become mainstream releases, but only if they are adequately tested. If no one tests, development is essentially halted.
Beta testers are particularly needed for operating systems to which the developers do not have easy access. They currently have easy access to recent GNU/Linux and Solaris versions. Reports about other operating systems are especially welcome.
How do I join the help-bison and bug-bison mailing lists?
In an action, the location of the left-hand side of the rule. See Locations Overview.
In an action, the location of the n-th symbol of the right-hand side of the rule. See Locations Overview.
In an action, the location of a symbol addressed by name. See Locations Overview.
In an action, the semantic value of the n-th symbol of the right-hand side of the rule. See Actions.
Delimiter used to separate the grammar rule section from the Bison declarations section or the epilogue. See The Overall Layout of a Bison Grammar.
All code listed between ‘%{’ and ‘%}’ is copied verbatim to the parser implementation file. Such code forms the prologue of the grammar file. See Outline of a Bison Grammar.
Separates alternate rules for the same result nonterminal. See Syntax of Grammar Rules.
Used to define a default tagged
%destructoror default tagged%printer.This feature is experimental. More user feedback will help to determine whether it should become a permanent feature.
Used to define a default tagless
%destructoror default tagless%printer.This feature is experimental. More user feedback will help to determine whether it should become a permanent feature.
The predefined nonterminal whose only rule is ‘$accept: start $end’, where start is the start symbol. See The Start-Symbol. It cannot be used in the grammar.
Insert code verbatim into the output parser source at the default location or at the location specified by qualifier. See %code Summary.
Define a variable to adjust Bison's behavior. See %define Summary.
Bison declaration to create a parser header file, which is usually meant for the scanner. See Decl Summary.
Same as above, but save in the file defines-file. See Decl Summary.
Specify how the parser should reclaim the memory associated to discarded symbols. See Freeing Discarded Symbols.
Bison declaration to assign a precedence to a rule that is used at parse time to resolve reduce/reduce conflicts. See Writing GLR Parsers.
The predefined token marking the end of the token stream. It cannot be used in the grammar.
A token name reserved for error recovery. This token may be used in grammar rules so as to allow the Bison parser to recognize an error in the grammar without halting the process. In effect, a sentence containing an error may be recognized as valid. On a syntax error, the token
errorbecomes the current lookahead token. Actions corresponding toerrorare then executed, and the lookahead token is reset to the token that originally caused the violation. See Error Recovery.
Bison declaration to request verbose, specific error message strings when
yyerroris called. See Error Reporting.
Bison declaration to set the prefix of the output files. See Decl Summary.
Bison declaration to assign left associativity to token(s). See Operator Precedence.
Bison declaration to specifying an additional parameter that
yylexshould accept. See Calling Conventions for Pure Parsers.
Bison declaration to assign a merging function to a rule. If there is a reduce/reduce conflict with a rule having the same merging function, the function is applied to the two semantic values to get a single result. See Writing GLR Parsers.
Bison declaration to rename the external symbols. See Decl Summary.
Bison declaration to avoid generating
#linedirectives in the parser implementation file. See Decl Summary.
Bison declaration to assign nonassociativity to token(s). See Operator Precedence.
Bison declaration to set the name of the parser implementation file. See Decl Summary.
Bison declaration to specifying an additional parameter that
yyparseshould accept. See The Parser Functionyyparse.
Bison declaration to assign a precedence to a specific rule. See Context-Dependent Precedence.
Deprecated version of
%define api.pure(see api.pure), for which Bison is more careful to warn about unreasonable usage.
Require version version or higher of Bison. See Require a Version of Bison.
Bison declaration to assign right associativity to token(s). See Operator Precedence.
Bison declaration to declare token(s) without specifying precedence. See Token Type Names.
Bison declaration to include a token name table in the parser implementation file. See Decl Summary.
The predefined token onto which all undefined values returned by
yylexare mapped. It cannot be used in the grammar, rather, useerror.
Bison declaration to specify several possible data types for semantic values. See The Collection of Value Types.
Macro to pretend that an unrecoverable syntax error has occurred, by making
yyparsereturn 1 immediately. The error reporting functionyyerroris not called. See The Parser Functionyyparse.For Java parsers, this functionality is invoked using
return YYABORT;instead.
Macro to pretend that a complete utterance of the language has been read, by making
yyparsereturn 0 immediately. See The Parser Functionyyparse.For Java parsers, this functionality is invoked using
return YYACCEPT;instead.
Macro to discard a value from the parser stack and fake a lookahead token. See Special Features for Use in Actions.
External integer variable that contains the integer value of the lookahead token. (In a pure parser, it is a local variable within
yyparse.) Error-recovery rule actions may examine this variable. See Special Features for Use in Actions.
Macro used in error-recovery rule actions. It clears the previous lookahead token. See Error Recovery.
External integer variable set to zero by default. If
yydebugis given a nonzero value, the parser will output information on input symbols and parser action. See Tracing Your Parser.
Macro to cause parser to recover immediately to its normal mode after a syntax error. See Error Recovery.
Macro to pretend that a syntax error has just been detected: call
yyerrorand then perform normal error recovery if possible (see Error Recovery), or (if recovery is impossible) makeyyparsereturn 1. See Error Recovery.For Java parsers, this functionality is invoked using
return YYERROR;instead.
User-supplied function to be called by
yyparseon error. See The Error Reporting Functionyyerror.
An obsolete macro that you define with
#definein the prologue to request verbose, specific error message strings whenyyerroris called. It doesn't matter what definition you use forYYERROR_VERBOSE, just whether you define it. Using%error-verboseis preferred. See Error Reporting.
Macro for specifying the initial size of the parser stack. See Memory Management.
User-supplied lexical analyzer function, called with no arguments to get the next token. See The Lexical Analyzer Function
yylex.
An obsolete macro for specifying an extra argument (or list of extra arguments) for
yyparseto pass toyylex. The use of this macro is deprecated, and is supported only for Yacc like parsers. See Calling Conventions for Pure Parsers.
External variable in which
yylexshould place the line and column numbers associated with a token. (In a pure parser, it is a local variable withinyyparse, and its address is passed toyylex.) You can ignore this variable if you don't use the ‘@’ feature in the grammar actions. See Textual Locations of Tokens. In semantic actions, it stores the location of the lookahead token. See Actions and Locations.
Data type of
yylloc; by default, a structure with four members. See Data Types of Locations.
External variable in which
yylexshould place the semantic value associated with a token. (In a pure parser, it is a local variable withinyyparse, and its address is passed toyylex.) See Semantic Values of Tokens. In semantic actions, it stores the semantic value of the lookahead token. See Actions.
Macro for specifying the maximum size of the parser stack. See Memory Management.
Global variable which Bison increments each time it reports a syntax error. (In a pure parser, it is a local variable within
yyparse. In a pure push parser, it is a member of yypstate.) See The Error Reporting Functionyyerror.
The parser function produced by Bison; call this function to start parsing. See The Parser Function
yyparse.
The function to delete a parser instance, produced by Bison in push mode; call this function to delete the memory associated with a parser. See The Parser Delete Function
yypstate_delete. (The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
The function to create a parser instance, produced by Bison in push mode; call this function to create a new parser. See The Parser Create Function
yypstate_new. (The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
The parser function produced by Bison in push mode; call this function to parse the rest of the input stream. See The Pull Parser Function
yypull_parse. (The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
The parser function produced by Bison in push mode; call this function to parse a single token. See The Push Parser Function
yypush_parse. (The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
An obsolete macro for specifying the name of a parameter that
yyparseshould accept. The use of this macro is deprecated, and is supported only for Yacc like parsers. See Calling Conventions for Pure Parsers.
The expression
YYRECOVERING ()yields 1 when the parser is recovering from a syntax error, and 0 otherwise. See Special Features for Use in Actions.
Macro used to control the use of
allocawhen the deterministic parser in C needs to extend its stacks. If defined to 0, the parser will usemallocto extend its stacks. If defined to 1, the parser will usealloca. Values other than 0 and 1 are reserved for future Bison extensions. If not defined,YYSTACK_USE_ALLOCAdefaults to 0.In the all-too-common case where your code may run on a host with a limited stack and with unreliable stack-overflow checking, you should set
YYMAXDEPTHto a value that cannot possibly result in unchecked stack overflow on any of your target hosts whenallocais called. You can inspect the code that Bison generates in order to determine the proper numeric values. This will require some expertise in low-level implementation details.
%nonassoc. Delayed syntax error detection results in
unexpected semantic actions, initiation of error recovery in the wrong
syntactic context, and an incorrect list of expected tokens in a verbose
syntax error message. See LAC.
if statement.
See Languages and Context-Free Grammars.
yylex.
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
http://fsf.org/
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.
The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.
Examples of suitable formats for Transparent copies include plain ascii without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.
The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.
The “publisher” means any person or entity that distributes copies of the Document to the public.
A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display copies.
If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.
You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document.
“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
$: Table of Symbols$: Java Action Features$: Action Features$$: Table of Symbols$$: Java Action Features$$: Action Features$$: Actions$<: Java Action Features$<: Action Features$[name]: Actions$accept: Table of Symbols$end: Table of Symbols$n: Actions$name: Actions$undefined: Table of Symbols%: Table of Symbols%: Java Declarations Summary%%: Table of Symbols%%: Java Declarations Summary%code: Table of Symbols%code: Java Declarations Summary%code: Calc++ Parser%code: %code Summary%code: Decl Summary%code: Prologue Alternatives%code imports: Java Declarations Summary%code imports: %code Summary%code lexer: Java Declarations Summary%code provides: %code Summary%code provides: Decl Summary%code provides: Prologue Alternatives%code requires: Calc++ Parser%code requires: %code Summary%code requires: Decl Summary%code requires: Prologue Alternatives%code top: %code Summary%code top: Prologue Alternatives%debug: Table of Symbols%debug: Tracing%debug: Decl Summary%define: Table of Symbols%define: %define Summary%define: Decl Summary%define abstract: Java Declarations Summary%define api.pure: %define Summary%define api.pure: Pure Decl%define api.push-pull: %define Summary%define api.push-pull: Push Decl%define extends: Java Declarations Summary%define final: Java Declarations Summary%define implements: Java Declarations Summary%define lex_throws: Java Declarations Summary%define location_type: Java Declarations Summary%define lr.default-reductions: Default Reductions%define lr.default-reductions: %define Summary%define lr.default-reductions WHERE: Default Reductions%define lr.keep-unreachable-states: Unreachable States%define lr.keep-unreachable-states: %define Summary%define lr.keep-unreachable-states VALUE: Unreachable States%define lr.type: LR Table Construction%define lr.type: %define Summary%define lr.type TYPE: LR Table Construction%define namespace: C++ Bison Interface%define namespace: %define Summary%define package: Java Declarations Summary%define parse.lac: LAC%define parse.lac: %define Summary%define parse.lac VALUE: LAC%define parser_class_name: Java Declarations Summary%define position_type: Java Declarations Summary%define public: Java Declarations Summary%define strictfp: Java Declarations Summary%define stype: Java Declarations Summary%define throws: Java Declarations Summary%defines: Table of Symbols%defines: Decl Summary%destructor: Table of Symbols%destructor: Decl Summary%destructor: Destructor Decl%destructor: Mid-Rule Actions%dprec: Table of Symbols%dprec: Merging GLR Parses%error-verbose: Table of Symbols%error-verbose: Error Reporting%expect: Decl Summary%expect: Expect Decl%expect-rr: Expect Decl%expect-rr: Simple GLR Parsers%file-prefix: Table of Symbols%file-prefix: Decl Summary%glr-parser: Table of Symbols%glr-parser: Simple GLR Parsers%glr-parser: GLR Parsers%initial-action: Table of Symbols%initial-action: Initial Action Decl%language: Table of Symbols%language: Decl Summary%language "Java": Java Declarations Summary%left: Table of Symbols%left: Using Precedence%left: Decl Summary%lex-param: Table of Symbols%lex-param: Java Declarations Summary%lex-param: Pure Calling%locations: Decl Summary%merge: Table of Symbols%merge: Merging GLR Parses%name-prefix: Table of Symbols%name-prefix: Java Declarations Summary%name-prefix: Decl Summary%no-lines: Table of Symbols%no-lines: Decl Summary%nonassoc: Table of Symbols%nonassoc: Default Reductions%nonassoc: LR Table Construction%nonassoc: Using Precedence%nonassoc: Decl Summary%output: Table of Symbols%output: Decl Summary%parse-param: Table of Symbols%parse-param: Java Declarations Summary%parse-param: Parser Function%prec: Table of Symbols%prec: Contextual Precedence%pure-parser: Table of Symbols%pure-parser: Decl Summary%require: Table of Symbols%require: Decl Summary%require: Require Decl%right: Table of Symbols%right: Using Precedence%right: Decl Summary%skeleton: Table of Symbols%skeleton: Decl Summary%start: Table of Symbols%start: Decl Summary%start: Start Decl%token: Table of Symbols%token: Java Declarations Summary%token: Decl Summary%token: Token Decl%token-table: Table of Symbols%token-table: Decl Summary%type: Table of Symbols%type: Java Declarations Summary%type: Decl Summary%type: Type Decl%union: Table of Symbols%union: Decl Summary%union: Union Decl%verbose: Decl Summary%yacc: Decl Summary*yypstate_new: Parser Create Function/*: Table of Symbols:: Table of Symbols;: Table of Symbols<*>: Table of Symbols<*>: Destructor Decl<>: Table of Symbols<>: Destructor Decl@$: Table of Symbols@$: Java Action Features@$: Action Features@$: Actions and Locations@[: Table of Symbols@[name]: Actions and Locations@n: Table of Symbols@n: Java Action Features@n: Action Features@n: Actions and Locations@name: Table of Symbols@name: Actions and Locationsbegin of Location: Java Location Valuesbegin on location: C++ Location ValuesBISON_I18N: InternationalizationBISON_LOCALEDIR: Internationalizationcalc: Infix Calccolumn on position: C++ Location Valuescolumns on location: C++ Location Valuescolumns on position: C++ Location Valueselse: Shift/Reducedebug_level on parser: C++ Parser Interfacedebug_stream on parser: C++ Parser Interfaceelse, dangling: Shift/Reduceend of Location: Java Location Valuesend on location: C++ Location Valueserror: Table of Symbolserror: Error Recoveryerror on parser: C++ Parser Interfacefile on position: C++ Location ValuesgetDebugLevel on YYParser: Java Parser InterfacegetDebugStream on YYParser: Java Parser InterfacegetEndPos on Lexer: Java Scanner InterfacegetLVal on Lexer: Java Scanner InterfacegetStartPos on Lexer: Java Scanner Interfaceinline: Compiler Requirementsyychar: GLR Semantic Actionsyyclearin: GLR Semantic ActionsYYERROR: GLR Semantic Actionsyylloc: GLR Semantic ActionsYYLLOC_DEFAULT: Location Default Actionyylval: GLR Semantic Actionsinline: Compiler Requirementslex-param: Pure Callingline on position: C++ Location Valueslines on location: C++ Location Valueslines on position: C++ Location ValuesLocation on Location: Java Location Valueslocation_type: C++ Parser Interfaceltcalc: Location Tracking Calcmfcalc: Multi-function Calcoperator+ on location: C++ Location Valuesoperator+ on position: C++ Location Valuesoperator+= on location: C++ Location Valuesoperator+= on position: C++ Location Valuesoperator- on position: C++ Location Valuesoperator-= on position: C++ Location Valuesoperator<< on position: C++ Location Valuesparse on parser: C++ Parser Interfaceparse on YYParser: Java Parser Interfaceparser on parser: C++ Parser Interfacerecovering: Java Action Featuresrecovering on YYParser: Java Parser Interfacereturn YYABORT;: Java Action Featuresreturn YYACCEPT;: Java Action Featuresreturn YYERROR;: Java Action Featuresrpcalc: RPN Calcsemantic_type: C++ Parser Interfaceset_debug_level on parser: C++ Parser Interfaceset_debug_stream on parser: C++ Parser InterfacesetDebugLevel on YYParser: Java Parser InterfacesetDebugStream on YYParser: Java Parser Interfacestep on location: C++ Location Valuestoken: C++ Parser InterfacetoString on Location: Java Location ValuesYYABORT: Table of SymbolsYYABORT: Parser FunctionYYABORT;: Action FeaturesYYACCEPT: Table of SymbolsYYACCEPT: Parser FunctionYYACCEPT;: Action FeaturesYYBACKUP: Table of SymbolsYYBACKUP: Action Featuresyychar: Table of Symbolsyychar: Lookaheadyychar: Action Featuresyychar: GLR Semantic Actionsyyclearin: Table of Symbolsyyclearin: Error Recoveryyyclearin: GLR Semantic Actionsyyclearin;: Action Featuresyydebug: Table of SymbolsYYDEBUG: Table of SymbolsYYDEBUG: Tracingyydebug: TracingYYEMPTY: Action FeaturesYYENABLE_NLS: InternationalizationYYEOF: Action Featuresyyerrok: Table of Symbolsyyerrok: Error Recoveryyyerrok;: Action Featuresyyerror: Table of SymbolsYYERROR: Table of Symbolsyyerror: Java Action FeaturesYYERROR: Action Featuresyyerror: Error ReportingYYERROR: GLR Semantic Actionsyyerror on Lexer: Java Scanner InterfaceYYERROR;: Action FeaturesYYERROR_VERBOSE: Table of SymbolsYYINITDEPTH: Table of SymbolsYYINITDEPTH: Memory Managementyylex: Table of Symbolsyylex: Lexicalyylex on Lexer: Java Scanner Interfaceyylex on parser: C++ Scanner InterfaceYYLEX_PARAM: Table of Symbolsyylloc: Table of Symbolsyylloc: Lookaheadyylloc: Action Featuresyylloc: Token Locationsyylloc: Actions and Locationsyylloc: GLR Semantic ActionsYYLLOC_DEFAULT: Location Default ActionYYLTYPE: Table of SymbolsYYLTYPE: Token Locationsyylval: Table of Symbolsyylval: Lookaheadyylval: Action Featuresyylval: Token Valuesyylval: Actionsyylval: GLR Semantic ActionsYYMAXDEPTH: Table of SymbolsYYMAXDEPTH: Memory Managementyynerrs: Table of Symbolsyynerrs: Error Reportingyyparse: Table of Symbolsyyparse: Parser FunctionYYPARSE_PARAM: Table of SymbolsYYParser on YYParser: Java Parser InterfaceYYPRINT: Tracingyypstate_delete: Table of Symbolsyypstate_delete: Parser Delete Functionyypstate_new: Table of Symbolsyypstate_new: Parser Create Functionyypull_parse: Table of Symbolsyypull_parse: Pull Parser Functionyypush_parse: Table of Symbolsyypush_parse: Push Parser FunctionYYRECOVERING: Table of SymbolsYYRECOVERING: Error RecoveryYYRECOVERING: Action FeaturesYYSTACK_USE_ALLOCA: Table of SymbolsYYSTYPE: Table of Symbols|: Table of Symbols|: Rules[1] In the future techniques to allow complex types within pseudo-unions (similar to Boost variants) might be implemented to alleviate these issues.
[2] Java parsers include the actions in a separate
method than yyparse in order to have an intuitive syntax that
corresponds to these C macros.