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.
I forked this library from the upstream version out of frustration that useful PRs were being submitted by users and never merged by the maintainer. This fork includes the following two PRs:
Specifically, you can specify ErrorUnset: true in your DecoderConfig to tell
the library that having fields defined in your destination struct that aren't
in the source data is not allowed.
Separately, you can also specify required in your field tags to indicate that
a specific field is required:
type Foo struct {
ImportantField string `mapstructure:"important_field,required"`
OptionalStuff interface{} `mapstructure:"optional"`
}When decoding into the struct defined in the above example, if important_field
is missing in the input, an error will be returned. If optional is missing,
no error will be returned.
Note that using the required tag in conjunction with ErrorUnset doesn't make
a whole lot of sense, and is untested/not supported.
Aside from the two PRs linked above, no other changes have been made to the source.
Standard go get:
$ go get github.com/cmatthias/mapstructure
For usage and examples see the Godoc.
The Decode function has examples associated with it there.
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.