Allow override decode semantics through decode middleware

[smyrman]

As shown by #191 and #115, it is not possible through the current hook system to specify override semantics because hooks only does pre-processing.

To my experience, middleware is always more powerful than hooks, because you can control the flow a lot better.

I think it would make sense to introduce a middleware for:

func (d *Decoder) decode(name string, input interface{}, outVal reflect.Value) error

Example implementation stub:

type DecodeFunc func(name string, input interface{}, outVal reflect.Value) error

type DecoderConfig struct{
   ...
   // if defined, pass in Decoder.decode to get a new DecodeFunc.
   // Fallback to call Decoder.decode directly.
   DecodeMiddleware func(DecodeFunc) DecodeFunc
}

With an example implementation for preserving time.Time as described in #191 (although this should probably be handled by default as well):

func PreseveTimeMiddlware(next DecodeFunc) DecodeFunc {
    return func(name string, input interface{}, outVal reflect.Value) error {
        inVal := reflect.ValueOf(input)
        if _, ok := input.(time.Time); ok && inVal.Type(input).AssignableTo(outVal.Type()) {
            outVal.Set(inVal)
            return nil
        }
        return next(name, input, outVal)
    }
}

[smyrman]

PS! As for what to do with the existing hook types, and functions, they would still work, but it would make sense (to me) to document them as deprecated; they really just do half the job, and they add complexity...

[smyrman]

Implementation wise it's probably not exactly as straight-farward as to wrap the entire Decoder.decode, but rather, a sub-set of it. The middleware must of-course run recrusivly from the inner funciton, so the bit to replace, is probably the code here:

mapstructure/mapstructure.go

Line 423 in 14428cd

switch outputKind {

Put the switch-case inside a new function, and pass it to the middlware func before calling.
The lower bit with the meta-data update must still happen after. Figure out how to handle the addMetaKey Boolean.

[mitchellh]

Thanks for the suggestion. I believe everything you want to do would be possible by composing decode hooks using the new #183 PR (I saw you responded there as well). In general, I'm a fan of middleware, but the aforementioned PR fits the existing "shape" of this project so I wanted to give that a shot. Let me know if that doesn't solve things for you!