Add a value hook func

This commit adds a third hook func type which has access to the
reflect.Value representations of both the `from` and the `to`
objects. This gives all the same power of the existing hook functions
(types, kinds, and data can all be retrieved from values), but
with the additional benefit of being able to modify the `to` object.

This change is largely backwards compatible. The only change of the
public API (other than adding a new hook type) is a change in the
signature of `DecodeHookExec`.

decode_hooks.go

@@ -16,10 +16,11 @@ func typedDecodeHook(h DecodeHookFunc) DecodeHookFunc {
 	// Create variables here so we can reference them with the reflect pkg
 	var f1 DecodeHookFuncType
 	var f2 DecodeHookFuncKind
+	var f3 DecodeHookFuncValue
 
 	// Fill in the variables into this interface and the rest is done
 	// automatically using the reflect package.
-	potential := []interface{}{f1, f2}
+	potential := []interface{}{f1, f2, f3}
 
 	v := reflect.ValueOf(h)
 	vt := v.Type()
@@ -38,13 +39,15 @@ func typedDecodeHook(h DecodeHookFunc) DecodeHookFunc {
 // that took reflect.Kind instead of reflect.Type.
 func DecodeHookExec(
 	raw DecodeHookFunc,
-	from reflect.Type, to reflect.Type,
-	data interface{}) (interface{}, error) {
+	from reflect.Value, to reflect.Value) (interface{}, error) {
+
 	switch f := typedDecodeHook(raw).(type) {
 	case DecodeHookFuncType:
-		return f(from, to, data)
+		return f(from.Type(), to.Type(), from.Interface())
 	case DecodeHookFuncKind:
-		return f(from.Kind(), to.Kind(), data)
+		return f(from.Kind(), to.Kind(), from.Interface())
+	case DecodeHookFuncValue:
+		return f(from, to)
 	default:
 		return nil, errors.New("invalid decode hook signature")
 	}
@@ -56,22 +59,16 @@ func DecodeHookExec(
 // The composed funcs are called in order, with the result of the
 // previous transformation.
 func ComposeDecodeHookFunc(fs ...DecodeHookFunc) DecodeHookFunc {
-	return func(
-		f reflect.Type,
-		t reflect.Type,
-		data interface{}) (interface{}, error) {
+	return func(f reflect.Value, t reflect.Value) (interface{}, error) {
 		var err error
+		var data interface{}
+		newFrom := f
 		for _, f1 := range fs {
-			data, err = DecodeHookExec(f1, f, t, data)
+			data, err = DecodeHookExec(f1, newFrom, t)
 			if err != nil {
 				return nil, err
 			}
-
-			// Modify the from kind to be correct with the new data
-			f = nil
-			if val := reflect.ValueOf(data); val.IsValid() {
-				f = val.Type()
-			}
+			newFrom = reflect.ValueOf(data)
 		}
 
 		return data, nil
@@ -215,3 +212,21 @@ func WeaklyTypedHook(
 
 	return data, nil
 }
+
+func RecursiveStructToMapHookFunc() DecodeHookFunc {
+	return func(f reflect.Value, t reflect.Value) (interface{}, error) {
+		if f.Kind() != reflect.Struct {
+			return f.Interface(), nil
+		}
+
+		var i interface{} = struct{}{}
+		if t.Type() != reflect.TypeOf(&i).Elem() {
+			return f.Interface(), nil
+		}
+
+		m := make(map[string]interface{})
+		t.Set(reflect.ValueOf(m))
+
+		return f.Interface(), nil
+	}
+}