Programming language: Go
Tags: Configuration    
Latest version: v1.2.0

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is a library for injecting values recursively into structs - a convinient way of setting up a configuration object. Available features :

  • setting default values for struct fields - NewDefaultProvider()
  • setting values from environment variables - NewEnvProvider()
  • setting values from command line flags - NewFlagProvider(&cfg)
  • setting values from files (JSON or YAML) - NewFileProvider("./testdata/input.yml")

Supported types:

  • string, *string, []string
  • bool, *bool, []bool
  • int, int8, int16, int32, int64 + slices of these types
  • *int, *int8, *int16, *int32, *int64
  • uint, uint8, uint16, uint32, uint64 + slices of these types
  • *uint, *uint8, *uint16, *uint32, *uint64
  • float32, float64 + slices of these types
  • *float32, *float64
  • embedded structs and pointers to structs

Quick start

// define a configuration object
    cfg := struct {
        Name     string `json:"name"          default:"defaultName"         flag:"name"`
        LastName string `json:"last_name"     default:"defaultLastName"`
        Age      byte   `json:"age"           env:"AGE_ENV"`
        IsDebug  bool   `json:"is_debug"      default:"false"`
        Object   struct {
            One string  `json:"one"            default:"defaultValForOne"`
            Two float32 `json:"two"            default:"33"`
        StrPtr  *string `json:"str_ptr"         default:"str_ptr_test"`
        IntPtr  *int    `json:"int_ptr"         default:"123"`
        BoolPtr *bool   `json:"bool_ptr"        default:"true"`

    configurator, err := New(
        &cfg, // pointer to the object
        []Provider{ // list of providers
            NewFlagProvider(&cfg), // flag provider expects pointer to the object to initialize flags
        false, // logging if true
        false, // fail fast if cannot set any field in the given object
    if err != nil {


You can specify one or more providers. They will be executed in order of definition:

    NewFlagProvider(&cfg), // 1
    NewEnvProvider(), // 2
    NewDefaultProvider(), // 3

If provider set value successfully next ones will not be executed (if flag provider from the sample above found a value env and default providers are skipped). The value of first successfully executed provider will be set. If none of providers found value - "zero" value of a field remains. You can define a custom provider which should satisfy next interface:

type Provider interface {
    Provide(field reflect.StructField, v reflect.Value, pathToField ...string) bool

Default provider

Looks for default tag and set value from it:

    struct {
        // ...
        Name string `json:"name"          default:"defaultName"`
        // ...

Env provider

Looks for env tag and tries to find an ENV variable with the name from the tag (AGE_ENV in this example):

    struct {
        // ...
        Age      byte   `json:"age"           env:"AGE_ENV"`
        // ...

Name inside tag env:"<name>" must be unique for each field.

Flag provider

Looks for flag tag and tries to set value from the command line flag -name

    struct {
        // ...
        Name     string `json:"name"  flag:"name|default_value|Description"`
        // ...

Name inside tag flag:"<name>" must be unique for each field. default_value and description sections are optional and can be omitted. NewFlagProvider(&cfg) expects a pointer to the same object for initialization.

Note: if program is executed with -help or -h flag you will see all available flags with description:

    -flag_name      "Description (default: default_value)"

And program execution will be terminated.

File provider

Doesn't require any specific tags. JSON and YAML formats of files are supported.