yuanbohan / yacc-example

simple examples to illustrate how to use goyacc

goyacc examples

help you to understand how to use goyacc to write your own parser.

• if/then/else
• calculator
• json

Install

go install golang.org/x/tools/cmd/goyacc@latest

Usage

goyacc -h

Usage of goyacc:
  -l	disable line directives
  -o string
        parser output (default "y.go")
  -p string
        name prefix to use in generated code (default "yy")
  -v string
        create parsing tables (default "y.output")

Steps

• write your lexer

// The parser uses the type <prefix>Lex as a lexer. It must provide
// the methods Lex(*<prefix>SymType) int and Error(string).

type <prefix>Lex struct {
    line []byte
    peek rune
}

// The parser calls this method to get each new token. This
// implementation returns operators and NUM.
func (x *yyLex) Lex(yylval *yySymType) int {
...
}

// The parser calls this method on a parse error.
func (x *yyLex) Error(s string) {
    ....
}

• write your parser (*.y file)
• generate parser (*.go file) by goyacc

parser Grammer (*.y file)

use Backus Naur Form (BNF)

• %token

• %type

the name in %type MUST be defined in %union, and %type connects the type defined in %union with symbols

• %union

• func (x *yyLex) Lex(yylval *yySymType) int {}

  • return. the type of the token. integer.
  • real value. stored in yylval

• rule

$$: symbol on the left side of colon $1 $2 ...: symbol on the right side of colon { ... } : action invoked

...
expr:
    NUM
|   expr '+' NUM {
        $$ = $1 + $3
    }
...

$$ : expr (outer)
$1 : expr (inner)
$3 : NUM

• final call

func yyParse(yylex yyLexer) int {
    return yyNewParser().Parse(yylex)
}

yylex is your lex struct which implemented the yyLexer interface

parser Example

%union{
    num int                // union will be a yySymType struct, and all fields in union will be copied into struct
}

%type   <num> top expr     // indicate parser to use num field (which is (yylval *yySymType) in goyacc)
%token  '+' '-' '*' '/'    // will not use yylval. Only indicate parser to use the token as ASCII code
%token  <num> NUM          // indicate parser to use yylval.num

%%

top:
    expr {
        fmt.Println($$)
    }
expr:
    NUM
|   expr '+' NUM {
        $$ = $1 + $3
    }
|   expr '-' NUM {
        $$ = $1 - $3
    }
|   expr '*' NUM {
        $$ = $1 * $3
    }
|   expr '/' NUM {
        $$ = $1 / $3
    }

%%

Resources

• https://zh.wikipedia.org/wiki/LALR%E8%AF%AD%E6%B3%95%E5%88%86%E6%9E%90%E5%99%A8
• https://cloud.tencent.com/developer/article/1744609
• https://mp.weixin.qq.com/s/qSbftFcRfigqEl_2-bfw3A
• https://about.sourcegraph.com/blog/go/gophercon-2018-how-to-write-a-parser-in-go
• https://silcnitc.github.io/yacc.html
• https://github.com/sougou/parser_tutorial
• https://github.com/u2takey/learn-yacc
• https://github.com/golang/tools/tree/master/cmd/goyacc/testdata/expr