The pigeon command generates parsers based on a parsing expression grammar (PEG). Its grammar and syntax is inspired by the PEG.js project, while the implementation is loosely based on the parsing expression grammar for C# 3.0 article. It parses Unicode text encoded in UTF-8.
See the godoc page for detailed usage.
Provided you have Go correctly installed with the $GOPATH and $GOBIN environment variables set, run:
$ go get -u github.com/PuerkitoBio/pigeon
This will install or update the package, and the pigeon command will be installed in your $GOBIN directory. Neither this package nor the parsers generated by this command require any third-party dependency, unless such a dependency is used in the code blocks of the grammar.
$ pigeon [options] [PEG_GRAMMAR_FILE]
By default, the input grammar is read from stdin and the generated code is printed to stdout. You may save it in a file using the -o flag, but pigeon makes no attempt to format the generated code, nor does it try to generate the required imports, because such a tool already exists. The recommended way to generate a properly formatted and working parser is to pipe the output of pigeon through the goimports tool:
$ pigeon my_revolutionary_programming_language.peg | goimports > main.go
This way, the generated code has all the necessary imports and is properly formatted. You can install goimports using:
$ go get golang.org/x/tools/cmd/goimports
See the godoc page for detailed usage.
Given the following grammar:
{
// part of the initializer code block omitted for brevity
var ops = map[string]func(int, int) int {
"+": func(l, r int) int {
return l + r
},
"-": func(l, r int) int {
return l - r
},
"*": func(l, r int) int {
return l * r
},
"/": func(l, r int) int {
return l / r
},
}
func toIfaceSlice(v interface{}) []interface{} {
if v == nil {
return nil
}
return v.([]interface{})
}
func eval(first, rest interface{}) int {
l := first.(int)
restSl := toIfaceSlice(rest)
for _, v := range restSl {
restExpr := toIfaceSlice(v)
r := restExpr[3].(int)
op := restExpr[1].(string)
l = ops[op](l, r)
}
return l
}
}
Input <- expr:Expr EOF {
return expr, nil
}
Expr <- _ first:Term rest:( _ AddOp _ Term )* _ {
return eval(first, rest), nil
}
Term <- first:Factor rest:( _ MulOp _ Factor )* {
return eval(first, rest), nil
}
Factor <- '(' expr:Expr ')' {
return expr, nil
} / integer:Integer {
return integer, nil
}
AddOp <- ( '+' / '-' ) {
return string(c.text), nil
}
MulOp <- ( '*' / '/' ) {
return string(c.text), nil
}
Integer <- '-'? [0-9]+ {
return strconv.Atoi(string(c.text))
}
_ "whitespace" <- [ \n\t\r]*
EOF <- !.
The generated parser can parse simple arithmetic operations, e.g.:
18 + 3 - 27 * (-18 / -3)
=> -141
More examples can be found in the examples/ subdirectory.
See the godoc page for detailed usage.
See the CONTRIBUTING.md file.
The BSD 3-Clause license. See the LICENSE file.