Editing GNU Bison (section)

{{Short description|Yacc-compatible parser generator program}}
{{Infobox software
| name                   = GNU Bison
| logo                   = Heckert GNU white.svg
| logo size              = 100px
| author                 = Robert Corbett
| developer              = The [[GNU Project]]
| released               = {{Start date and age|1985|06}}<ref name="Corbett85"/>
| discontinued           = <!-- Set to yes if software is discontinued, otherwise omit. -->
| ver layout             = <!-- simple (default) or stacked -->
| latest release version = {{wikidata|property|preferred|references|edit|Q1071969|P348|P548=Q2804309}}
| latest release date    = {{wikidata|qualifier|preferred|single|Q1071969|P348|P548=Q2804309|P577}}
| latest preview version = {{wikidata|property|preferred|references|edit|Q1071969|P348|P548=Q51930650}}
| latest preview date    = {{wikidata|qualifier|preferred|single|Q1071969|P348|P548=Q51930650|P577}}
| programming language   = [[C (programming language)|C]] and [[m4 (computer language)|m4]]
| operating system       = [[Unix-like]]
| platform               = 
| genre                  = [[Parser generator]]
| license                = [[GNU General Public License|GPL]]
}}

'''GNU Bison''', commonly known as '''Bison''', is a [[parser generator]] that is part of the [[GNU Project]]. Bison reads a specification in Bison syntax (described as "machine-readable [[Backus–Naur form|BNF]]"<ref>{{Cite web|title=Language and Grammar (Bison 3.8.1)|url=https://www.gnu.org/software/bison/manual/html_node/Language-and-Grammar.html|access-date=2021-12-26|website=www.gnu.org}}</ref>), warns about any [[parsing]] ambiguities, and generates a parser that reads sequences of [[Lexical analysis#Token|token]]s and decides whether the sequence conforms to the syntax specified by the grammar.

The generated parsers are portable: they do not require any specific compilers.  Bison by default generates [[LALR parser|LALR(1) parsers]] but it can also generate [[canonical LR parser|canonical LR]], IELR(1) and [[GLR parser|GLR]] parsers.<ref>[https://www.gnu.org/software/bison/manual/html_node/Introduction.html Bison Manual: Introduction.]</ref>

In [[POSIX]] mode, Bison is compatible with [[Yacc]], but also has several extensions over this earlier program, including
* Generation of counterexamples for conflicts
* Location tracking (e.g., file, line, column)
* Rich and internationalizable syntax error messages in the generated parsers
* Customizable syntax error generation,
* Reentrant parsers
* Push parsers, with autocompletion
* Support for named references
* Several types of reports (graphical, XML) on the generated parser
* Support for several programming languages ([[C (programming language)|C]], [[C++]], [[D (programming language)|D]], or [[Java (programming language)|Java]])

[[Flex (lexical analyser generator)|Flex]], an automatic [[Lexical analysis|lexical analyser]], is often used with Bison, to tokenise input data and provide Bison with tokens.<ref>{{Cite book | last=Levine | first=John | author-link=John R. Levine | title=flex & bison | publisher = O'Reilly Media | date=August 2009 | url=http://oreilly.com/catalog/9780596155988 | isbn=978-0-596-15597-1}}</ref>

Bison was originally written by Robert Corbett in 1985.<ref name="Corbett85">{{cite thesis |last1=Corbett |first1=Robert Paul |date=June 1985 |title=Static Semantics and Compiler Error Recovery |type=Ph.D. |publisher=[[University of California, Berkeley]] |id=[[Defense Technical Information Center|DTIC]] [https://web.archive.org/web/20170608112235/http://www.dtic.mil/docs/citations/ADA611756 ADA611756]{{cbignore|bot=medic}}}}</ref> Later, in 1989, Robert Corbett released another parser generator named [[Berkeley Yacc]]. Bison was made Yacc-compatible by [[Richard Stallman]].<ref name="authors">{{cite web |title=AUTHORS |url=http://git.savannah.gnu.org/cgit/bison.git/tree/AUTHORS |department=bison.git |work=[[GNU Savannah]] |access-date=2017-08-26}}</ref>

Bison is [[free software]] and is available under the [[GNU General Public License]], with an exception (discussed below) allowing its generated code to be used without triggering the [[copyleft]] requirements of the licence.