Yacc is a computer program for the Unix operating system. It is a LALR parser
generator, generating a parser, the part of a compiler that tries to make
syntactic sense of the source code, specifically a LALR parser, based on an
analytic grammar written in a notation similar to BNF. Yacc itself used to be
available as the default parser generator on most Unix systems, though
it has since been supplanted as the default by more recent, largely
compatible, programs. Description
YACC is an acronym for "Yet Another Compiler Compiler". It is a LALR parser
generator, generating a parser, the part of a compiler that tries to make
syntactic sense of the source code, specifically a LALR parser, based on an
analytic grammar written in a notation similar to BNF. It was originally
developed in the early 1970s by Stephen C. Johnson at AT&T Corporation and
written in the B programming language, but soon rewritten in C. It appeared as
part of Version 3 Unix, and a full description of Yacc was published in
1975. The input to Yacc is a grammar with
snippets of C code 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 trees.
Using an example from Johnson, if the call node(label, left, right) constructs
a binary parse tree node with the specified label and children, then the
rule recognizes summation expressions and
constructs nodes for them. The special identifiers $$, $1 and $3 refer to items
on the parser's stack. Yacc and similar programs 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 as the
default by more recent, largely compatible, programs such as Berkeley
Yacc, GNU bison, MKS Yacc and Abraxas PCYACC. An updated version of the
original AT&T version is included as part of Sun's OpenSolaris project. Each
offers slight improvements and additional features over the original
Yacc, but the concept and syntax have remained the same. Yacc has also been
rewritten for other languages, including OCaml, Ratfor, ML, Ada, Pascal, Java,
Python, Ruby, Go and Common Lisp. Yacc produces only a parser; for full
syntactic analysis this requires an external lexical analyzer to perform the
first tokenization stage, which is then followed by the parsing stage proper.
Lexical analyzer generators, such as Lex or Flex are widely available. The IEEE
POSIX P1003.2 standard defines the functionality and requirements for both
Lex and Yacc. Some versions of AT&T Yacc have become
open source. For example, source code is available with the standard
distributions of Plan 9 and OpenSolaris. See also
Berkeley Yacc: The Berkeley implementation of Yacc quickly became
more popular than AT&T Yacc itself because of lack of reuse restrictions
and performance. LALR parser: The underlying parsing
algorithm in Yacc-generated parsers. Bison: The GNU version of Yacc.
Lex, the token parser commonly used in conjunction with Yacc.
BNF, is a metasyntax used to express context-free grammars: that is, a formal
way to describe context-free languages. References
External links Computerworld Interview with Stephen C.
Johnson on YACC ML-Yacc a Yacc version for the Standard
ML language. CL-Yacc, a LALR(1)parser generator for
Common Lisp. PLY a Yacc version for Python
Yacc theory ocamlyacc a Yacc version for OCaml.
Racc a Yacc version for Ruby. Paper "Parsing Non-LR(k) Grammars with
Yacc" by Gary H. Merrill ANSI C Yacc grammar