Hey folks ;)

I think it would be nice if mapstructure would support an unmarshaling interface like for the json module, so custom marshalers can be defined on a per-type basis easily:

type MyType struct {
    number int
    numberTimesTwo int
}

// For example we call the interface function for mapstructure UnmarshalMap
func (t *MyType) UnmarshalMap(value interface{}) error {
    // Although not a real-world usecase, just multiply our read number by 2.
    intValue, ok := value.(int)

    if !ok {
        return errors.New("Can't convert value to int")
    }

    t.number = value
    t.numberTimesTwo = value * 2

    return nil
}

Specifically I tried to do custom marshaling inside viper, which internally uses mapstructure to unmarshal from its internal map-representation of config-key-values to my config-struct. But there are no possibilities there to "override" marshaling behaviour for my types and I have to write quite some boilerplate to circumvent that^^