You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: env.go
    path of file: ./repos/go-gin-api/pkg/env
    
    the code of the file until where you have to start completion: package env

import (
	"flag"
	"fmt"
	"strings"
)

var (
	active Environment
	dev    Environment = &environment{value: "dev"}
	fat    Environment = &environment{value: "fat"}
	uat    Environment = &environment{value: "uat"}
	pro    Environment = &environment{value: "
    
func init() {
	env := flag.String("env", "", "请输入运行环境:\n dev:开发环境\n fat:测试环境\n uat:预上线环境\n pro:正式环境\n")
	flag.Parse()

	switch strings.ToLower(strings.TrimSpace(*env)) {
	case "dev":
		active = dev
	case "fat":
		active = fat
	case "uat":
		active = uat
	case "pro":
		active = pro
	default:
		active = fat
		fmt.Println("Warning: '-env' cannot be found, or it is illegal. The default 'fat' will be used.")
	}
}