| // Copyright 2009 The Go Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style |
| // license that can be found in the LICENSE file. |
| |
| /* |
| |
| Yacc is a version of yacc for Go. |
| It is written in Go and generates parsers written in Go. |
| |
| Usage: |
| |
| go tool yacc args... |
| |
| It is largely transliterated from the Inferno version written in Limbo |
| which in turn was largely transliterated from the Plan 9 version |
| written in C and documented at |
| |
| https://9p.io/magic/man2html/1/yacc |
| |
| Adepts of the original yacc will have no trouble adapting to this |
| form of the tool. |
| |
| The directory $GOROOT/src/cmd/yacc/testdata/expr is a yacc program |
| for a very simple expression parser. See expr.y and main.go in that |
| directory for examples of how to write and build yacc programs. |
| |
| The generated parser is reentrant. The parsing function yyParse expects |
| to be given an argument that conforms to the following interface: |
| |
| type yyLexer interface { |
| Lex(lval *yySymType) int |
| Error(e string) |
| } |
| |
| Lex should return the token identifier, and place other token |
| information in lval (which replaces the usual yylval). |
| Error is equivalent to yyerror in the original yacc. |
| |
| Code inside the grammar actions may refer to the variable yylex, |
| which holds the yyLexer passed to yyParse. |
| |
| Clients that need to understand more about the parser state can |
| create the parser separately from invoking it. The function yyNewParser |
| returns a yyParser conforming to the following interface: |
| |
| type yyParser interface { |
| Parse(yyLex) int |
| Lookahead() int |
| } |
| |
| Parse runs the parser; the top-level call yyParse(yylex) is equivalent |
| to yyNewParser().Parse(yylex). |
| |
| Lookahead can be called during grammar actions to read (but not consume) |
| the value of the current lookahead token, as returned by yylex.Lex. |
| If there is no current lookahead token (because the parser has not called Lex |
| or has consumed the token returned by the most recent call to Lex), |
| Lookahead returns -1. Calling Lookahead is equivalent to reading |
| yychar from within in a grammar action. |
| |
| Multiple grammars compiled into a single program should be placed in |
| distinct packages. If that is impossible, the "-p prefix" flag to |
| yacc sets the prefix, by default yy, that begins the names of |
| symbols, including types, the parser, and the lexer, generated and |
| referenced by yacc's generated code. Setting it to distinct values |
| allows multiple grammars to be placed in a single package. |
| |
| */ |
| package main |