main.go

package main

import (
	"os"

	"github.com/alecthomas/repr"

	"github.com/alecthomas/participle/v2"
	"github.com/alecthomas/participle/v2/lexer"
)

// A custom lexer for INI files. This illustrates a relatively complex Regexp lexer, as well
// as use of the Unquote filter, which unquotes string tokens.
var (
	iniLexer = lexer.MustSimple([]lexer.SimpleRule{
		{`Ident`, `[a-zA-Z][a-zA-Z_\d]*`},
		{`String`, `"(?:\\.|[^"])*"`},
		{`Float`, `\d+(?:\.\d+)?`},
		{`Punct`, `[][=]`},
		{"comment", `[#;][^\n]*`},
		{"whitespace", `\s+`},
	})
	parser = participle.MustBuild[INI](
		participle.Lexer(iniLexer),
		participle.Unquote("String"),
		participle.Union[Value](String{}, Number{}),
	)
)

type INI struct {
	Properties []*Property `@@*`
	Sections   []*Section  `@@*`
}

type Section struct {
	Identifier string      `"[" @Ident "]"`
	Properties []*Property `@@*`
}

type Property struct {
	Key   string `@Ident "="`
	Value Value  `@@`
}

type Value interface{ value() }

type String struct {
	String string `@String`
}

func (String) value() {}

type Number struct {
	Number float64 `@Float`
}

func (Number) value() {}

func main() {
	ini, err := parser.Parse("", os.Stdin)
	repr.Println(ini, repr.Indent("  "), repr.OmitEmpty(true))
	if err != nil {
		panic(err)
	}
}