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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 ¶
Examples ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
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"}
Types ¶
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.
type DecoderConfig ¶
type DecoderConfig struct {
// 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 convert values between
// the following types:
//
// - numbers and bools
// - strings and numbers
// - strings and bools
// - empty arrays/slices and empty maps
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.
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.
Source Files