Here is the list of features you may be interested in:

• ✔️ Simple
• ✔️ Supports JSON (Using encoding/json)
• ✔️ Supports YAML (Using https://gopkg.in/yaml.v2)
• ✔️ Supports TOML (Using https://github.com/BurntSushi/toml)
• ✔️ Supports INI (Using https://gopkg.in/ini.v1)
• ✔️ Aliases, subfolders and on-demand lookup in home folder (github.com/mitchellh/go-homedir) and /etc/
• ✔️ Data validation
• ❌ Placeholders
• ❌ Configuration file chaining
• ❌ Autoreload

First of all, you need some structure, that will hold configuration for your application. Like this:

type Config struct {
    Token string `json:"token" yaml:"token" toml:"token"`
}

Just import and call config.ReadDefault.

package main

import "github.com/mono83/config"

func main() {
  var c Config
  if err := config.ReadDefault(&c); err != nil {
    panic(err)
  }

  fmt.Println(c)
}

Module will make attempt to find one of these files: config.json, config.yaml, config.toml in current folder and unmarshal them using corresponding unmarshaller.

You can specify filename and other options using config.Source structure (event config.ReadDefault uses it intenally):

if err := (config.Source{FileName: "app.json"}).Read(&c); err != nil {
  panic(err)
}

There can be multiple aliases for configuration files (module reads only one - that was specified/found first, no joining applied):

if err := (config.Source{
    FileNames: []string{"app.json", "config.json"},
  }).Read(&c); err != nil {
  panic(err)
}

You can specify subfolder for configuration files:

if err := (config.Source{
    Subfolder: "configuration",
    FileNames: []string{"app.json", "config.json"},
  }).Read(&c); err != nil {
  panic(err)
}

Or even allow lookup in homedir and /etc/

if err := (config.Source{
    Subfolder: "configuration",
    FileNames: []string{"app.json", "config.json"},
    LookupHome: true,
    LookupEtc: true,
  }).Read(&c); err != nil {
  panic(err)
}

If structure, that is used in configuration has method Validate() error, configuration reader will automatically invoke it:

type Config struct {
    Token string `json:"token" yaml:"token" toml:"token"`
}

// Validate will be automatically invoked by mono83/config
func (c Config) Validate() error {
  if len(c.Token) == 0 {
    return errors.New("empty access token")
  }

  return nil
}

This behaviour can be disabled during config.Source initialization:

(config.Source{SkipValidation: true, ...}).Read(...)