| 1 | // Copyright 2009 The Go Authors. All rights reserved. |
|---|---|
| 2 | // Use of this source code is governed by a BSD-style |
| 3 | // license that can be found in the LICENSE file. |
| 4 | |
| 5 | /* |
| 6 | Goyacc is a version of yacc for Go. |
| 7 | It is written in Go and generates parsers written in Go. |
| 8 | |
| 9 | Usage: |
| 10 | |
| 11 | goyacc args... |
| 12 | |
| 13 | It is largely transliterated from the Inferno version written in Limbo |
| 14 | which in turn was largely transliterated from the Plan 9 version |
| 15 | written in C and documented at |
| 16 | |
| 17 | https://9p.io/magic/man2html/1/yacc |
| 18 | |
| 19 | Adepts of the original yacc will have no trouble adapting to this |
| 20 | form of the tool. |
| 21 | |
| 22 | The directory $GOPATH/src/golang.org/x/tools/cmd/goyacc/testdata/expr |
| 23 | is a yacc program for a very simple expression parser. See expr.y and |
| 24 | main.go in that directory for examples of how to write and build |
| 25 | goyacc programs. |
| 26 | |
| 27 | The generated parser is reentrant. The parsing function yyParse expects |
| 28 | to be given an argument that conforms to the following interface: |
| 29 | |
| 30 | type yyLexer interface { |
| 31 | Lex(lval *yySymType) int |
| 32 | Error(e string) |
| 33 | } |
| 34 | |
| 35 | Lex should return the token identifier, and place other token |
| 36 | information in lval (which replaces the usual yylval). |
| 37 | Error is equivalent to yyerror in the original yacc. |
| 38 | |
| 39 | Code inside the grammar actions may refer to the variable yylex, |
| 40 | which holds the yyLexer passed to yyParse. |
| 41 | |
| 42 | Clients that need to understand more about the parser state can |
| 43 | create the parser separately from invoking it. The function yyNewParser |
| 44 | returns a yyParser conforming to the following interface: |
| 45 | |
| 46 | type yyParser interface { |
| 47 | Parse(yyLex) int |
| 48 | Lookahead() int |
| 49 | } |
| 50 | |
| 51 | Parse runs the parser; the top-level call yyParse(yylex) is equivalent |
| 52 | to yyNewParser().Parse(yylex). |
| 53 | |
| 54 | Lookahead can be called during grammar actions to read (but not consume) |
| 55 | the value of the current lookahead token, as returned by yylex.Lex. |
| 56 | If there is no current lookahead token (because the parser has not called Lex |
| 57 | or has consumed the token returned by the most recent call to Lex), |
| 58 | Lookahead returns -1. Calling Lookahead is equivalent to reading |
| 59 | yychar from within in a grammar action. |
| 60 | |
| 61 | Multiple grammars compiled into a single program should be placed in |
| 62 | distinct packages. If that is impossible, the "-p prefix" flag to |
| 63 | goyacc sets the prefix, by default yy, that begins the names of |
| 64 | symbols, including types, the parser, and the lexer, generated and |
| 65 | referenced by yacc's generated code. Setting it to distinct values |
| 66 | allows multiple grammars to be placed in a single package. |
| 67 | */ |
| 68 | package main |
| 69 |
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