is a library for injecting values recursively into structs - a convenient 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 a JSON file - NewJSONFileProvider("./testdata/input.json")

• string, *string, []string, []*string
• bool, *bool, []bool, []*bool
• int, int8, int16, int32, int64 + slices of these types
• *int, *int8, *int16, *int32, *int64 + slices of these types
• uint, uint8, uint16, uint32, uint64 + slices of these types
• *uint, *uint8, *uint16, *uint32, *uint64 + slices of these types
• float32, float64 + slices of these types
• *float32, *float64 + slices of these types
• time.Duration from strings like 12ms, 2s etc.
• embedded structs and pointers to structs
• any custom type which satisfies FieldSetter interface

• your entire configuration can be defined in one model
• all metadata is in your model (defined with tags)
• easy to set/change a source of data for your configuration
• easy to set a priority of sources to fetch data from (e.g., 1.flags, 2.env, 3.default or another order)
• you can implement your custom provider
• no external dependencies
• complies with 12-factor app

Import path github.com/BoRuDar/configuration/v4

	// defining a struct
cfg := struct {
    Name     string `flag:"name"`
    LastName string `default:"defaultLastName"`
    Age      byte   `env:"AGE_ENV"    default:"-1"`
    BoolPtr  *bool  `default:"false"`
    
    ObjPtr *struct {
        F32       float32       `default:"32"`
        StrPtr    *string       `default:"str_ptr_test"`
        HundredMS time.Duration `default:"100ms"`
    }

    Obj struct {
        IntPtr     *int16   `default:"123"`
        Beta       int      `file_json:"inside.beta"   default:"24"`
        StrSlice   []string `default:"one;two"`
        IntSlice   []int64  `default:"3; 4"`
        unexported string   `xml:"ignored"`
    }
}{}

configurator := configuration.New(
    &cfg,
    // order of execution will be preserved: 
    configuration.NewFlagProvider(),             // 1st
    configuration.NewEnvProvider(),              // 2nd 
    configuration.NewJSONFileProvider(fileName), // 3rd 
    configuration.NewDefaultProvider(),          // 4th
)

if err := configurator.InitValues(); err != nil {
    log.Fatalf("unexpected error: ", err)
}

If you need only ENV variables and default values you can use a shorter form:

err := configuration.FromEnvAndDefault(&cfg)

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

[]Provider{
    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 - an application will be terminated. This behavior can be changed with configurator.OnFailFnOpt option:

err := configuration.New(
    &cfg,
    configuration.NewEnvProvider(),
    configuration.NewDefaultProvider()).
    SetOptions(
        configuration.OnFailFnOpt(func(err error) {
            log.Println(err)
        }),
    ).InitValues()

You can define a custom provider which should satisfy next interface:

type Provider interface {
    Name() string
    Init(ptr any) error
    Provide(field reflect.StructField, v reflect.Value) error
}

Looks for default tag and set value from it:

struct {
    // ...
    Name string `default:"defaultName"`
    // ...
}

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

struct {
    // ...
    Age      byte   `env:"AGE"`
    // ...
}

Name inside tag env:"<name>" must be unique for each field. Only UPPER register for ENV vars is accepted:

bad_env_var_name=bad
GOOD_ENV_VAR_NAME=good

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

struct {
    // ...
    Name     string `flag:"first_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:

Flags: 
	-first_name		"Description (default: default_value)"

And program execution will be terminated.

• WithFlagSet - sets a custom FlagSet

Requires file_json:"<path_to_json_field>" tag.

NewJSONFileProvider("./testdata/input.json")

For example, tag file_json:"cache.retention" will assume that you have this structure of your JSON file:

{
  "cache": {
    "retention": 1
  }
}

• YAML files

You can define how to set fields with any custom types:

type FieldSetter interface {
	SetField(field reflect.StructField, val reflect.Value, valStr string) error
}

Example:

type ipTest net.IP

func (it *ipTest) SetField(_ reflect.StructField, val reflect.Value, valStr string) error {
	i := ipTest(net.ParseIP(valStr))

	if val.Kind() == reflect.Pointer {
		val.Set(reflect.ValueOf(&i))
	} else {
		val.Set(reflect.ValueOf(i))
	}

	return nil
}

1. Open a feature request or a bug report in issues
2. Fork and create a PR into dev branch