Editing Yacc

{{Short description|Parser generator}}
{{Infobox software
| name = Yacc
| logo = 
| screenshot = 
| screenshot size = 
| caption = 
| other_names = Bell Labs Yacc; AT&T Yacc
| author = [[Stephen C. Johnson]]
| developer = 
| latest release version = 
| latest release date = 
| programming language = [[C (programming language)|C]]
| operating system = [[Unix]], [[Unix-like]], [[Plan 9 from Bell Labs|Plan 9]], [[Inferno (operating system)|Inferno]]
| platform = [[Cross-platform]]
| genre = [[Command (computing)|Command]]
| license = Plan 9: [[MIT License]]
| website = 
}}
'''Yacc''' ('''Yet Another Compiler-Compiler''') is a [[computer program]] for the [[Unix]] operating system developed by [[Stephen C. Johnson]].  It is a [[LALR parser generator|lookahead left-to-right rightmost derivation (LALR) parser generator]], generating a [[LALR parser]] (the part of a [[compiler]] that tries to make syntactic sense of the [[source code]]) based on a [[formal grammar]], written in a notation similar to [[Backus–Naur form]] (BNF).<ref>{{cite web |title=The A-Z of Programming Languages: YACC |publisher=Computerworld |url=http://www.techworld.com.au/article/252319/a-z_programming_languages_yacc/ |access-date=30 November 2012 |archive-date=31 January 2013 |archive-url=https://web.archive.org/web/20130131031011/http://www.techworld.com.au/article/252319/a-z_programming_languages_yacc/ |url-status=dead }}</ref> Yacc is supplied as a standard utility on BSD and AT&T Unix.<ref name="Levine 1992 p.xx">{{cite book | last=Levine | first=John | title=Lex & yacc | publisher=O'Reilly & Associates | publication-place=Sebastopol, CA | year=1992 | isbn=1-56592-000-7 | page=xx | url-access=registration | url=https://archive.org/details/lexyacc00levi }}</ref> [[GNU]]-based [[Linux]] distributions include [[GNU bison|Bison]], a [[forward-compatible]] Yacc replacement.<ref name="Levine 2009 p.xv">{{cite book | last=Levine | first=John | title=Flex & bison | publisher=O'Reilly Media | publication-place=Sebastopol, Calif | year=2009 | isbn=978-0-596-15597-1 | page=xv}}</ref>

== History ==
In the early 1970s, [[Stephen C. Johnson]], a computer scientist at [[Bell Labs]] / [[AT&T Corporation|AT&T]], developed Yacc because he wanted to insert an [[exclusive or]] operator into a [[B (programming language)|B language]] compiler<ref name="red">{{cite news|last1=Morris|first1=Richard|title=Stephen Curtis Johnson: Geek of the Week|url=https://www.red-gate.com/simple-talk/opinion/geek-of-the-week/stephen-curtis-johnson-geek-of-the-week/|access-date=19 January 2018|work=Red Gate Software|date=1 October 2009}}</ref> (developed using [[Douglas McIlroy|McIlroy]]'s [[TMG (language)|TMG]] compiler-compiler<ref name="Chistory"/>), but it turned out to be a hard task. As a result, he was directed by his colleague at Bell Labs [[Al Aho]] to [[Donald Knuth]]'s work on [[LR parser|LR parsing]], which served as the basis for Yacc.<ref name="red">{{cite news|last1=Morris|first1=Richard|title=Stephen Curtis Johnson: Geek of the Week|url=https://www.red-gate.com/simple-talk/opinion/geek-of-the-week/stephen-curtis-johnson-geek-of-the-week/|access-date=19 January 2018|work=Red Gate Software|date=1 October 2009}}</ref> Yacc was influenced by<ref name="Yacc: Yet Another Compiler-Compiler"/> and received its name in reference to TMG compiler-compiler.<ref>{{cite web|title=Early Translator Writing Systems |url=http://www.chilton-computing.org.uk/acl/applications/cc/p007.htm |publisher=Atlas Computer Laboratory}}</ref>

Yacc was originally written in the [[B (programming language)|B programming language]], but was soon rewritten in [[C (programming language)|C]] by Alan Snyder.<ref name="Chistory">{{cite conference
|last=Ritchie
|first=Dennis M.
|author-link=Dennis Ritchie
|date=April 1993
|title=The Development of the C Language
|publisher=Association for Computing Machinery, Inc.
|doi=10.1145/234286.1057834
|doi-access=free
|book-title=History of programming languages---II
|language=en
|isbn=0-201-89502-1
|publication-date=1996-01-01
|quote=After the TMG version of B was working, Thompson rewrote B in itself(a bootstrapping step).…When Johnson returned to Bell Labs in 1973, he was disconcerted to find that the language whose seeds he had brought to Canada had evolved back home; even his own yacc program had been rewritten in C, by Alan Snyder.
|quote-pages=675,684
}}
</ref> It appeared as part of [[Version 3 Unix]],<ref name="reader">{{cite tech report |first1=M. D. |last1=McIlroy |author-link1=Doug McIlroy |year=1987 |url=http://www.cs.dartmouth.edu/~doug/reader.pdf |title=A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 |series=CSTR |number=139 |institution=Bell Labs}}</ref> and a full description of Yacc was published in 1975.<ref name="Yacc: Yet Another Compiler-Compiler">{{cite tech report |last=Johnson|first=Stephen C.|author-link=Stephen C. Johnson|year=1975|title=Yacc: Yet Another Compiler-Compiler|number=32|publisher=AT&T Bell Laboratories|location=Murray Hill, New Jersey|url=http://dinosaur.compilertools.net/yacc/|access-date=31 January 2020}}</ref>

Johnson used Yacc to create the [[Portable C Compiler]].<ref name="reader"/> [[Bjarne Stroustrup]] also attempted to use Yacc to create a formal specification of [[C++]], but "was defeated by C's syntax".<ref name="hopl2">{{cite web|last=Stroustrup|first=Bjarne |author-link=Bjarne Stroustrup |title=A History of C++: 1979−1991|url=http://www.stroustrup.com/hopl2.pdf}}</ref> While finding it unsuitable for a formal specification of the language, Stroustrup did proceed to use Yacc to implement [[Cfront]], the first implementation of C++.<ref name="oldcfront">{{cite web|last=Stroustrup|first=Bjarne |author-link=Bjarne Stroustrup |title=Cfront source code|url=http://www.softwarepreservation.org/projects/c_plus_plus/index.html#cfront}}</ref>

In a 2008 interview, Johnson reflected that "the contribution Yacc made to the spread of [[Unix]] and [[C (programming language)|C]] is what I'm proudest of".<ref name="cw">{{
cite news
|url=https://www.computerworld.com/article/2534771/yacc--unix--and-advice-from--bell-labs-alumni-stephen-johnson.html
|title=Yacc, Unix, and advice from Bell Labs alumni Stephen Johnson
|last1=Hamilton
|first1=Naomi
|date=2008-07-09
|website=www.computerworld.com
|access-date=2020-11-10
|url-status=live
|archive-url=https://web.archive.org/web/20200822224200/https://www.computerworld.com/article/2534771/yacc--unix--and-advice-from--bell-labs-alumni-stephen-johnson.html
|archive-date=2020-08-22
}}</ref>

== Description ==
The input to Yacc is a grammar with snippets of [[C (programming language)|C]] code (called "actions") attached to its rules. Its output is a [[shift-reduce parser]] in C that executes the C snippets associated with each rule as soon as the rule is recognized. Typical actions involve the construction of [[parse tree]]s. Using an example from Johnson, if the call {{mono|node(label, left, right)}} constructs a binary parse tree node with the specified {{mono|label}} and children, then the rule
<syntaxhighlight lang="C">
expr : expr '+' expr  { $$ = node('+', $1, $3); }
</syntaxhighlight>
recognizes summation expressions and constructs nodes for them. The special identifiers {{mono|$$}}, {{mono|$1}} and {{mono|$3}} refer to items on the parser's [[Push-down automaton|stack]].{{r|Yacc: Yet Another Compiler-Compiler}}

Yacc produces only a parser (phrase analyzer) which can be used alone in the case of [[scannerless parsing]] however, full syntactic analysis typically requires an external [[lexical analyzer]] to perform a tokenization stage first (word analysis), which is then followed by the parsing stage proper.<ref name="Yacc: Yet Another Compiler-Compiler"/> Lexical analyzer generators, such as [[Lex (software)|Lex]] or [[Flex (lexical analyser generator)|Flex]], are widely available for this purpose. The [[IEEE]] [[POSIX]] P1003.2 standard defines the functionality and requirements for both Lex and Yacc.<ref>{{man|cu|lex}}, {{man|cu|yacc}}.</ref>

Some versions of AT&T Yacc have become [[open-source software|open source]]. For example, [[source code]] is available with the standard distributions of [[Plan 9 from Bell Labs|Plan 9]].<ref>{{cite web|title=plan9: UC Berkeley release of Plan 9 under the GPLv2|website=[[GitHub]] |url=https://github.com/brho/plan9/blob/89d43d2262ad43eb4b26c2a8d6a27cfeddb33828/sys/src/cmd/yacc.c|access-date=2 January 2018|date=26 December 2017}}</ref>

== Impact ==
Yacc and similar programs (largely reimplementations) have been very popular. Yacc itself used to be available as the default parser generator on most Unix systems, though it has since been supplanted by more recent, largely compatible, programs such as [[Berkeley Yacc]], [[GNU bison|GNU Bison]], [[MKS Inc.|MKS]] Yacc, and Abraxas PCYACC. An updated version of the original AT&T Yacc is included as part of [[Sun Microsystems|Sun's]] [[OpenSolaris]] project. Each offers slight improvements and additional features over the original Yacc, but the concept and basic syntax have remained the same.<ref>[https://www.gnu.org/software/bison/manual/html_node/History.html#History Bison Manual: History]</ref>

Yacc was also one of several UNIX tools available for Charles River Data Systems' [[UNOS (operating system)|UNOS]] operating system under [[Bell Laboratories]] license.<ref>{{Cite book|year=1983|title=The Insider's Guide To The Universe|publisher=Charles River Data Systems, Inc.|url=https://www.1000bit.it/ad/bro/charles/CharlesRiverSystem-Universe.pdf|page=13}}</ref>

Among the languages that were first implemented with Yacc are [[AWK]], [[C++]],<ref name="oldcfront">{{cite web|last=Stroustrup|first=Bjarne |author-link=Bjarne Stroustrup |title=Cfront source code|url=http://www.softwarepreservation.org/projects/c_plus_plus/index.html#cfront}}</ref> [[eqn (software)|eqn]] and [[Pic language|Pic]].<ref>{{cite web|url=https://www.youtube.com/watch?v=vT_J6xc-Az0  |archive-url=https://ghostarchive.org/varchive/youtube/20211211/vT_J6xc-Az0| archive-date=2021-12-11 |url-status=live|title=UNIX Special: Profs Kernighan & Brailsford |work=Computerphile |date=September 30, 2015 }}{{cbignore}}</ref><ref name="cw" /> Yacc was also used on Unix to implement the [[Portable C Compiler]], as well as parsers for such programming languages as [[FORTRAN 77]], [[Ratfor]], [[APL (programming language)|APL]], [[bc (programming language)|bc]], [[m4 (computer language)|m4]], etc.<ref name="reader"/><ref>{{cite book|last1=Kernighan|first1=Brian W.|last2=Pike|first2=Rob|title=[[The Unix Programming Environment]]|year=1984|publisher=Prentice Hall|isbn=0-13-937681-X}}</ref>

Yacc has also been rewritten for other languages, including [[OCaml]],<ref>{{cite web |title=OCaml User's Manual: Chapter 12 Lexer and parser generators (ocamllex, ocamlyacc) |url=http://caml.inria.fr/pub/docs/manual-ocaml-4.00/manual026.html |access-date=25 Nov 2013}}</ref> [[Ratfor]], [[ML programming language|ML]], [[Ada (programming language)|Ada]], [[Pascal (programming language)|Pascal]], [[Java (programming language)|Java]], [[PHP]],  [[Python (programming language)|Python]], [[Ruby (programming language)|Ruby]], [[Go (programming language)|Go]],<ref>{{cite web |title=Yacc.go: A version of Yacc for the Go Programming Language |url=https://godoc.org/golang.org/x/tools/cmd/goyacc |access-date=15 July 2017}}</ref> [[Common Lisp]]<ref>{{cite web|title=CL-Yacc: A Common Lisp version of Yacc |url=http://www.pps.univ-paris-diderot.fr/~jch/software/cl-yacc/}}</ref> and [[Erlang (programming language)|Erlang]].<ref>{{cite web |title=yecc: An Erlang implementation of Yacc |url=http://erlang.org/doc/man/yecc.html}}</ref>

* [[Berkeley Yacc]]: The Berkeley implementation of Yacc quickly became more popular than AT&T Yacc itself because of its performance and lack of reuse restrictions.<ref>{{citation |title=flex & bison |author=John Levine |publisher=O'Reilly Media |date=August 2009 }}</ref>
* [[LALR parser]]: The underlying parsing algorithm in Yacc-generated parsers.
* [[GNU bison|Bison]]: The GNU version of Yacc.
* [[Lex programming tool|Lex]] (and [[Flex lexical analyser]]), a token parser commonly used in conjunction with Yacc (and Bison).
* [[Backus-Naur form|BNF]] is a [[metasyntax]] used to express [[context-free grammar]]s: that is a formal way to describe context-free languages.
* [[PLY (Python Lex-Yacc)]] is an alternative implementation of Lex and Yacc in Python.

== See also ==
* [[Compiler-compiler]]
* [[hoc (programming language)]]

== References ==
{{Reflist}}

==External links==
* [https://pacc.interitty.org Playground] environment for learning and testing syntax

{{Wikibooks|Guide to Unix|Commands}}
* {{man|cu|yacc|SUS}}
* {{man|1|yacc|Plan 9}}
* {{man|1|yacc|Inferno}}
* {{man|1|yacc|ManKier}}

{{Unix commands}}
{{Plan 9 commands}}

{{Authority control}}

[[Category:Compiling tools]]
[[Category:Parser generators]]
[[Category:Unix programming tools]]
[[Category:Unix SUS2008 utilities]]
[[Category:Plan 9 commands]]
[[Category:Inferno (operating system) commands]]