枚举类型的惯用写法（包括和字符串的相互转换）

enum.go

package main

import (
	"errors"
	"fmt"
	"strings"
)

var ErrInvalidFruit = errors.New("invalid fruit")

type Fruit int

const (
	InvalidFruit Fruit = iota - 1
	Apple
	Orange
	Banana
	Pear
	Grape
)

var fruitNames = [...]string{
	Apple:  "apple",
	Orange: "orange",
	Banana: "banana",
	Pear:   "pear",
	Grape:  "grape",
}

var fruitStrings = map[string]Fruit{
	"apple":  Apple,
	"orange": Orange,
	"banana": Banana,
	"pear":   Pear,
	"grape":  Grape,
}

func (f Fruit) String() string {
	return fruitNames[f]
}

func ParseFruit(s string) (Fruit, error) {
	f, ok := fruitStrings[strings.ToLower(s)]
	if !ok {
		return InvalidFruit, ErrInvalidFruit
	}
	return f, nil
}

func main() {
	f, _ := ParseFruit("Apple")
	fmt.Println(f)
	// output: apple
	fmt.Println(Orange)
	// output: orange
}