mapstructure

README

mapstructure

mapstructure is a Go library for decoding generic map values to structures and vice versa, while providing helpful error handling.

This library is most useful when decoding values from some data stream (JSON, Gob, etc.) where you don't quite know the structure of the underlying data until you read a part of it. You can therefore read a map[string]interface{} and use this library to decode it into the proper underlying native Go structure.

Installation

Standard go get:

$ go get github.com/mitchellh/mapstructure

Usage & Example

For usage and examples see the Godoc.

The Decode function has examples associated with it there.

But Why?!

Go offers fantastic standard libraries for decoding formats such as JSON. The standard method is to have a struct pre-created, and populate that struct from the bytes of the encoded format. This is great, but the problem is if you have configuration or an encoding that changes slightly depending on specific fields. For example, consider this JSON:

{
  "type": "person",
  "name": "Mitchell"
}

Perhaps we can't populate a specific structure without first reading the "type" field from the JSON. We could always do two passes over the decoding of the JSON (reading the "type" first, and the rest later). However, it is much simpler to just decode this into a map[string]interface{} structure, read the "type" key, then use something like this library to decode it into the proper structure.

Documentation

Overview

The mapstructure package exposes functionality to convert an abitrary map[string]interface{} into a native Go structure.

The Go structure can be arbitrarily complex, containing slices, other structs, etc. and the decoder will properly decode nested maps and so on into the proper structures in the native Go struct. See the examples to see what the decoder is capable of.

Index

• func Decode(m interface{}, rawVal interface{}) error
• func WeakDecode(input, output interface{}) error
• func WeaklyTypedHook(f reflect.Kind, t reflect.Kind, data interface{}) (interface{}, error)
• type DecodeHookFunc
  • func ComposeDecodeHookFunc(fs ...DecodeHookFunc) DecodeHookFunc
  • func StringToSliceHookFunc(sep string) DecodeHookFunc
• type Decoder
  • func NewDecoder(config *DecoderConfig) (*Decoder, error)
  • func (d *Decoder) Decode(raw interface{}) error
• type DecoderConfig
• type Error
  • func (e *Error) Error() string
• type Metadata

Examples

• Decode
• Decode (Errors)
• Decode (Metadata)
• Decode (Tags)
• Decode (WeaklyTypedInput)

Functions

func Decode

func Decode(m interface{}, rawVal interface{}) error

Decode takes a map and uses reflection to convert it into the given Go native structure. val must be a pointer to a struct.

Example

type Person struct {
	Name   string
	Age    int
	Emails []string
	Extra  map[string]string
}

// This input can come from anywhere, but typically comes from
// something like decoding JSON where we're not quite sure of the
// struct initially.
input := map[string]interface{}{
	"name":   "Mitchell",
	"age":    91,
	"emails": []string{"one", "two", "three"},
	"extra": map[string]string{
		"twitter": "mitchellh",
	},
}

var result Person
err := Decode(input, &result)
if err != nil {
	panic(err)
}

fmt.Printf("%#v", result)

Output:

mapstructure.Person{Name:"Mitchell", Age:91, Emails:[]string{"one", "two", "three"}, Extra:map[string]string{"twitter":"mitchellh"}}

Example (Errors)

type Person struct {
	Name   string
	Age    int
	Emails []string
	Extra  map[string]string
}

// This input can come from anywhere, but typically comes from
// something like decoding JSON where we're not quite sure of the
// struct initially.
input := map[string]interface{}{
	"name":   123,
	"age":    "bad value",
	"emails": []int{1, 2, 3},
}

var result Person
err := Decode(input, &result)
if err == nil {
	panic("should have an error")
}

fmt.Println(err.Error())

Output:

5 error(s) decoding:

* 'Name' expected type 'string', got unconvertible type 'int'
* 'Age' expected type 'int', got unconvertible type 'string'
* 'Emails[0]' expected type 'string', got unconvertible type 'int'
* 'Emails[1]' expected type 'string', got unconvertible type 'int'
* 'Emails[2]' expected type 'string', got unconvertible type 'int'

Example (Metadata)

type Person struct {
	Name string
	Age  int
}

// This input can come from anywhere, but typically comes from
// something like decoding JSON where we're not quite sure of the
// struct initially.
input := map[string]interface{}{
	"name":  "Mitchell",
	"age":   91,
	"email": "foo@bar.com",
}

// For metadata, we make a more advanced DecoderConfig so we can
// more finely configure the decoder that is used. In this case, we
// just tell the decoder we want to track metadata.
var md Metadata
var result Person
config := &DecoderConfig{
	Metadata: &md,
	Result:   &result,
}

decoder, err := NewDecoder(config)
if err != nil {
	panic(err)
}

if err := decoder.Decode(input); err != nil {
	panic(err)
}

fmt.Printf("Unused keys: %#v", md.Unused)

Output:

Unused keys: []string{"email"}

Example (Tags)

// Note that the mapstructure tags defined in the struct type
// can indicate which fields the values are mapped to.
type Person struct {
	Name string `mapstructure:"person_name"`
	Age  int    `mapstructure:"person_age"`
}

input := map[string]interface{}{
	"person_name": "Mitchell",
	"person_age":  91,
}

var result Person
err := Decode(input, &result)
if err != nil {
	panic(err)
}

fmt.Printf("%#v", result)

Output:

mapstructure.Person{Name:"Mitchell", Age:91}

Example (WeaklyTypedInput)

type Person struct {
	Name   string
	Age    int
	Emails []string
}

// This input can come from anywhere, but typically comes from
// something like decoding JSON, generated by a weakly typed language
// such as PHP.
input := map[string]interface{}{
	"name":   123,                      // number => string
	"age":    "42",                     // string => number
	"emails": map[string]interface{}{}, // empty map => empty array
}

var result Person
config := &DecoderConfig{
	WeaklyTypedInput: true,
	Result:           &result,
}

decoder, err := NewDecoder(config)
if err != nil {
	panic(err)
}

err = decoder.Decode(input)
if err != nil {
	panic(err)
}

fmt.Printf("%#v", result)

Output:

mapstructure.Person{Name:"123", Age:42, Emails:[]string{}}

func WeakDecode

func WeakDecode(input, output interface{}) error

WeakDecode is the same as Decode but is shorthand to enable WeaklyTypedInput. See DecoderConfig for more info.

func WeaklyTypedHook

func WeaklyTypedHook(
	f reflect.Kind,
	t reflect.Kind,
	data interface{}) (interface{}, error)

Types

type DecodeHookFunc

type DecodeHookFunc func(
	from reflect.Kind,
	to reflect.Kind,
	data interface{}) (interface{}, error)

DecodeHookFunc is the callback function that can be used for data transformations. See "DecodeHook" in the DecoderConfig struct.

func ComposeDecodeHookFunc

func ComposeDecodeHookFunc(fs ...DecodeHookFunc) DecodeHookFunc

ComposeDecodeHookFunc creates a single DecodeHookFunc that automatically composes multiple DecodeHookFuncs.

The composed funcs are called in order, with the result of the previous transformation.

func StringToSliceHookFunc

func StringToSliceHookFunc(sep string) DecodeHookFunc

StringToSliceHookFunc returns a DecodeHookFunc that converts string to []string by splitting on the given sep.

type Decoder

type Decoder struct {
	// contains filtered or unexported fields
}

A Decoder takes a raw interface value and turns it into structured data, keeping track of rich error information along the way in case anything goes wrong. Unlike the basic top-level Decode method, you can more finely control how the Decoder behaves using the DecoderConfig structure. The top-level Decode method is just a convenience that sets up the most basic Decoder.

func NewDecoder

func NewDecoder(config *DecoderConfig) (*Decoder, error)

NewDecoder returns a new decoder for the given configuration. Once a decoder has been returned, the same configuration must not be used again.

func (*Decoder) Decode

func (d *Decoder) Decode(raw interface{}) error

Decode decodes the given raw interface to the target pointer specified by the configuration.

type DecoderConfig

type DecoderConfig struct {
	// DecodeHook, if set, will be called before any decoding and any
	// type conversion (if WeaklyTypedInput is on). This lets you modify
	// the values before they're set down onto the resulting struct.
	//
	// If an error is returned, the entire decode will fail with that
	// error.
	DecodeHook DecodeHookFunc

	// If ErrorUnused is true, then it is an error for there to exist
	// keys in the original map that were unused in the decoding process
	// (extra keys).
	ErrorUnused bool

	// If WeaklyTypedInput is true, the decoder will make the following
	// "weak" conversions:
	//
	//   - bools to string (true = "1", false = "0")
	//   - numbers to string (base 10)
	//   - bools to int/uint (true = 1, false = 0)
	//   - strings to int/uint (base implied by prefix)
	//   - int to bool (true if value != 0)
	//   - string to bool (accepts: 1, t, T, TRUE, true, True, 0, f, F,
	//     FALSE, false, False. Anything else is an error)
	//   - empty array = empty map and vice versa
	//
	WeaklyTypedInput bool

	// Metadata is the struct that will contain extra metadata about
	// the decoding. If this is nil, then no metadata will be tracked.
	Metadata *Metadata

	// Result is a pointer to the struct that will contain the decoded
	// value.
	Result interface{}

	// The tag name that mapstructure reads for field names. This
	// defaults to "mapstructure"
	TagName string
}

DecoderConfig is the configuration that is used to create a new decoder and allows customization of various aspects of decoding.

type Error

type Error struct {
	Errors []string
}

Error implements the error interface and can represents multiple errors that occur in the course of a single decode.

func (*Error) Error

func (e *Error) Error() string

type Metadata

type Metadata struct {
	// Keys are the keys of the structure which were successfully decoded
	Keys []string

	// Unused is a slice of keys that were found in the raw value but
	// weren't decoded since there was no matching field in the result interface
	Unused []string
}

Metadata contains information about decoding a structure that is tedious or difficult to get otherwise.