result.txt

main.Config{Foo:main.MaybeBool{b:false, isSet:true}, Bar:main.MaybeBool{b:false, isSet:false}}

yaml_bool.go

package main

import (
	"errors"
	"fmt"

	yaml "gopkg.in/yaml.v2"
)

type MaybeBool struct {
	b     bool
	isSet bool
}

func (b *MaybeBool) UnmarshalYAML(unmarshal func(interface{}) error) error {
	var aux interface{}
	if err := unmarshal(&aux); err != nil {
		return err
	}
	if x, ok := aux.(bool); ok {
		b.b = x
		b.isSet = true
	} else {
		return errors.New("not a bool value")
	}
	return nil
}

type Config struct {
	Foo MaybeBool `yaml:"foo"`
	Bar MaybeBool `yaml:"bar"`
}

func main() {
	var conf Config
	yaml.Unmarshal([]byte("foo: false\n"), &conf)
	fmt.Printf("%#v\n", conf)
}