Go - 状態と振る舞い - インターフェイス - interface型、要件、fmtパッケージ、Stringer interface、Stringメソッド、encodingインポートパス、jsonパッケージ、Marshaler interface、MarshalJSONメソッド

入門Goプログラミング
原著: Get Programming with Go
楽天ブックス Yahoo!

開発環境

• macOS Catalina - Apple (OS)
• Emacs (Text Editor)
• Windows 10 Pro (OS)
• Visual Studio Code (Text Editor)
• Go (プログラミング言語)

入門Goプログラミング (Nathan Youngman(著)、Roger Peppé(著)、吉川 邦夫(監修, 翻訳)、翔泳社)のUNIT 5(状態と振る舞い)、LESSON 24(インターフェイス)の練習問題の解答を求めてみる。

コード

package main

import (
 "encoding/json"
 "fmt"
 "os"
 "strings"
)

type coordinate struct {
 d, m, s float64
 h       rune
}

func (c coordinate) decimal() float64 {
 sign := 1.0
 switch c.h {
 case 'S', 'W', 's', 'w':
  sign = -1
 }
 return sign * (c.d + c.m/60 + c.s/3600)
}
func (c coordinate) String() string {
 return fmt.Sprintf("%v°%v'%.1f\" %v",
  c.d, c.m, c.s, strings.ToUpper(string(c.h)))
}
func (c coordinate) MarshalJSON() ([]byte, error) {
 s := fmt.Sprintf(
  `{"decimal":%v,"dms":"%v°%v'%.1f\" %v","degrees":%[2]v,"minutes":%[3]v,"seconds":%[4]v,"hemisphere":"%[5]v"}`,
  c.decimal(), c.h, c.m, c.s, strings.ToUpper(string(c.h)),
 )
 return []byte(s), nil
}

type location struct {
 name      string
 lat, long coordinate
}

func newLocation(name string, lat, long coordinate) location {
 return location{name: name, lat: lat, long: long}
}
func (l location) String() string {
 return fmt.Sprintf("%v is at %v, %v", l.name, l.lat, l.long)
}
func exitOnError(err error) {
 if err != nil {
  fmt.Fprintln(os.Stderr, err)
  os.Exit(1)
 }
}
func main() {
 elsyiumPlanitiaLat := coordinate{4, 30, 0, 'n'}
 elysiumPlanitiaLong := coordinate{135, 54, 0, 'e'}
 elsyiumPlanitia := newLocation(
  "Elysium Planitia",
  elsyiumPlanitiaLat,
  elysiumPlanitiaLong,
 )
 fmt.Println(elsyiumPlanitia)
 bytes, err := json.Marshal(elysiumPlanitiaLong)
 exitOnError(err)
 fmt.Println(string(bytes))
 bytes, err = json.MarshalIndent(elysiumPlanitiaLong, "", "  ")
 exitOnError(err)
 fmt.Println(string(bytes))
 bytes, err = json.MarshalIndent(elysiumPlanitiaLong, ">", " ")
 exitOnError(err)
 fmt.Println(string(bytes))
}

入出力結果(Zsh、PowerShell、Terminal)

% go run marshal.go
Elysium Planitia is at 4°30'0.0" N, 135°54'0.0" E
{"decimal":135.9,"dms":"101°54'0.0\" E","degrees":101,"minutes":54,"seconds":0,"hemisphere":"E"}
{
  "decimal": 135.9,
  "dms": "101°54'0.0\" E",
  "degrees": 101,
  "minutes": 54,
  "seconds": 0,
  "hemisphere": "E"
}
{
> "decimal": 135.9,
> "dms": "101°54'0.0\" E",
> "degrees": 101,
> "minutes": 54,
> "seconds": 0,
> "hemisphere": "E"
>}
%