Golang: time without date

timeonly.go

package main

import (
	"time"
	"log"
	"errors"
	"encoding/json"
)

var timeLayout = "15:04"
var TimeParseError = errors.New(`TimeParseError: should be a string formatted as "15:04:05"`)

type Time struct {
	time.Time
}

func (t Time) MarshalJSON() ([]byte, error) {
	return []byte(`"` + t.Format(timeLayout) + `"`), nil
}

func (t *Time) UnmarshalJSON(b []byte) error {
	s := string(b)
	// len(`"23:59"`) == 7
	if len(s) != 7 {
		return TimeParseError
	}
	ret, err := time.Parse(timeLayout, s[1:6])
	if err != nil {
		return err
	}
	t.Time = ret
	return nil
}

type Foo struct {
	Id   int  `json:"id"`
	Time Time `json:"time"`
}

type TimeRange [2]Time

type Bar struct {
	FooId  int `json:"foo_id"`
	Ranges []TimeRange `json:"ranges"`
}

func fatalExit(err error) {
	if err != nil {
		log.Fatal(err)
	}
}

func main() {
	o := &Foo{1, Time{time.Now()}}
	log.Println(o)
	b, err := json.Marshal(o)
	fatalExit(err)
	log.Println(string(b))
	blob := []byte(`{"id":1,"time":"01:40"}`)
	f := &Foo{}
	err = json.Unmarshal(blob, f)
	fatalExit(err)
	str := f.Time.Format(timeLayout)
	log.Printf("%+v, Time: %s\n", f, str)
	b = []byte(`
	{
		"foo_id": 1,
		"ranges": [
			["10:00", "12:00"],
			["13:00", "17:00"],
			["17:45", "20:45"],
			["22:00", "23:00"]
		]
	}`)
	bar := &Bar{}
	err = json.Unmarshal(b, bar)
	fatalExit(err)
	log.Println("%+v\n", bar)
	b, err = json.Marshal(bar)
	fatalExit(err)
	log.Println(string(b))
}