外部ライブラリを利用せず、structをPretty Printしたい

外部ライブラリを利用せず、Go言語でstructをPretty Printする方法を2つ紹介します。

fmtパッケージを利用する

fmt.Printfを利用する場合、書式指定子に%+vを指定すればよいです。

package main

import (
	"fmt"
	"time"
)

type Employee struct {
	ID       int
	Name     string
	HireDate time.Time
}

type Department struct {
	ID        int
	Name      string
	Employees []Employee
}

func main() {
	department := Department{
		ID:   9876,
		Name: "営業部",
		Employees: []Employee{
			Employee{ID: 100, Name: "佐藤", HireDate: time.Date(2001, 1, 1, 1, 1, 1, 1, time.UTC)},
			Employee{ID: 200, Name: "田中", HireDate: time.Date(2002, 2, 2, 2, 2, 2, 2, time.UTC)},
			Employee{ID: 300, Name: "高橋", HireDate: time.Date(2003, 3, 3, 3, 3, 3, 3, time.UTC)},
		},
	}

	fmt.Printf("%+v\n", department)
}

実行結果は以下の通りです。

$ go run main.go
{ID:9876 Name:営業部 Employees:[{ID:100 Name:佐藤 HireDate:2001-01-01 01:01:01.000000001 +0000 UTC} {ID:200 Name:田中 HireDate:2002-02-02 02:02:02.000000002 +0000 UTC} {ID:300 Name:高橋 HireDate:2003-03-03 03:03:03.000000003 +0000 UTC}]}

jsonパッケージを利用する

structをjsonとして扱う方法もあります。

package main

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

type Employee struct {
	ID       int
	Name     string
	HireDate time.Time
}

type Department struct {
	ID        int
	Name      string
	Employees []Employee
}

func main() {
	department := Department{
		ID:   9876,
		Name: "営業部",
		Employees: []Employee{
			Employee{ID: 100, Name: "佐藤", HireDate: time.Date(2001, 1, 1, 1, 1, 1, 1, time.UTC)},
			Employee{ID: 200, Name: "田中", HireDate: time.Date(2002, 2, 2, 2, 2, 2, 2, time.UTC)},
			Employee{ID: 300, Name: "高橋", HireDate: time.Date(2003, 3, 3, 3, 3, 3, 3, time.UTC)},
		},
	}

	b, err := json.MarshalIndent(department, "", "  ")
	if err != nil {
		log.Fatalf("unexpected error: %w", err)
	}
	fmt.Println(string(b))
}

実行結果は以下の通りです。

$ go run main.go
{
  "ID": 9876,
  "Name": "営業部",
  "Employees": [
    {
      "ID": 100,
      "Name": "佐藤",
      "HireDate": "2001-01-01T01:01:01.000000001Z"
    },
    {
      "ID": 200,
      "Name": "田中",
      "HireDate": "2002-02-02T02:02:02.000000002Z"
    },
    {
      "ID": 300,
      "Name": "高橋",
      "HireDate": "2003-03-03T03:03:03.000000003Z"
    }
  ]
}

環境情報

$ go version
go version go1.22.1 linux/amd64