Load TOML config and pick environment from a environmental variable in Go

config.toml

[dev]
    url = "http://dev.example.com"
    username = "dev.account"
    password = "devdev"

[test]
    url = "http://test.example.com"
    username = "test.account"
    password = "testtest"

[prod]
    url = "http://prod.example.com"
    username = "prod.account"
    password = "prodprod"

main.go

package main

import (
	"fmt"
	"github.com/naoina/toml"
	"io/ioutil"
	"os"
	"reflect"
)

type AppConfig struct {
	Url      string
	Username string
	Password string
}

type Config struct {
	Dev  AppConfig
	Test AppConfig
	Prod AppConfig
}

const CONFIG = "config.toml"

func loadAppConfig(config Config, env string) AppConfig {
	r := reflect.ValueOf(config)
	return reflect.Indirect(r).FieldByName(env).Interface().(AppConfig)
}

func main() {
	fmt.Printf("Loading config file: %s\n", CONFIG)
	configData, err := ioutil.ReadFile(CONFIG)
	if err != nil {
		panic(err)
	}

	var config Config
	err = toml.Unmarshal(configData, &config)
	if err != nil {
		panic(err)
	}

	env := os.Getenv("APP_ENV")
	if env == "" {
		env = "Dev"
	}
	appConfig := loadAppConfig(config, env)
	fmt.Printf("%#v\n", appConfig)
	fmt.Printf("Environment set to %s\n", env)
}

// go build -o app
// ./app
// APP_ENV=Test ./app
// APP_ENV=Prod ./app