Support decoding nested structs

mapstructure.go

diff --git a/mapstructure.go b/mapstructure.go
index f1a0c80..937c205 100644
--- a/mapstructure.go
+++ b/mapstructure.go
@@ -24,8 +24,79 @@
 		return errors.New("val must be an addressable struct")
 	}
 
-	valType := val.Type()
+	return decode("root", m, val)
+}
 
+// Decodes an unknown data type into a specific reflection value.
+func decode(name string, data interface{}, val reflect.Value) error {
+	k := val.Kind()
+
+	// Some shortcuts because we treat all ints and uints the same way
+	if k >= reflect.Int && k <= reflect.Int64 {
+		k = reflect.Int
+	} else if k >= reflect.Uint && k <= reflect.Uint64 {
+		k = reflect.Uint
+	}
+
+	switch k {
+	case reflect.Bool:
+		fallthrough
+	case reflect.Int:
+		fallthrough
+	case reflect.String:
+		fallthrough
+	case reflect.Uint:
+		return decodeBasic(name, data, val)
+	case reflect.Struct:
+		return decodeStruct(name, data, val)
+	}
+
+	// If we reached this point then we weren't able to decode it
+	return fmt.Errorf("unsupported type: %s", k)
+}
+
+// This decodes a basic type (bool, int, string, etc.) and sets the
+// value to "data" of that type.
+func decodeBasic(name string, data interface{}, val reflect.Value) error {
+	dataVal := reflect.ValueOf(data)
+	if !dataVal.IsValid() {
+		// This should never happen because upstream makes sure it is valid
+		panic("data is invalid")
+	}
+
+	dataValType := dataVal.Type()
+	if !dataValType.AssignableTo(val.Type()) {
+		return fmt.Errorf(
+			"'%s' expected type '%s', got '%s'",
+			name, val.Type(), dataValType)
+	}
+
+	val.Set(dataVal)
+	return nil
+}
+
+func decodeStruct(name string, data interface{}, val reflect.Value) error {
+	dataVal := reflect.ValueOf(data)
+	dataValKind := dataVal.Kind()
+	if dataValKind != reflect.Map {
+		return fmt.Errorf("'%s' expected a map, got '%s'", name, dataValKind)
+	}
+
+	dataValType := dataVal.Type()
+	if dataValType.Key().Kind() != reflect.String {
+		return fmt.Errorf(
+			"'%s' needs a map with string keys, has '%s' keys",
+			name, dataValType.Key().Kind())
+	}
+
+	// At this point we know that data is a map with string keys, so
+	// we can properly cast it here.
+	m, ok := data.(map[string]interface{})
+	if !ok {
+		panic("data could not be cast as map[string]interface{}")
+	}
+
+	valType := val.Type()
 	for i := 0; i < valType.NumField(); i++ {
 		fieldType := valType.Field(i)
 		fieldName := fieldType.Name
@@ -54,23 +125,10 @@
 			panic("field is not valid")
 		}
 
-		mapVal := reflect.ValueOf(rawMapVal)
-		if !mapVal.IsValid() {
-			// This should never happen because we got the value out
-			// of the map.
-			panic("map value is not valid")
+		fieldName = fmt.Sprintf("%s.%s", name, fieldName)
+		if err := decode(fieldName, rawMapVal, field); err != nil {
+			return err
 		}
-
-		mapValType := mapVal.Type()
-		if !mapValType.AssignableTo(field.Type()) {
-			// If the value in the map can't be assigned to the field
-			// in the struct, then this is a problem...
-			return fmt.Errorf(
-				"field '%s' expected type '%s', got '%s'",
-				fieldName, field.Type(), mapValType)
-		}
-
-		field.Set(mapVal)
 	}
 
 	return nil

mapstructure_test.go

diff --git a/mapstructure_test.go b/mapstructure_test.go
index 3ec2a3a..a55953b 100644
--- a/mapstructure_test.go
+++ b/mapstructure_test.go
@@ -9,6 +9,11 @@
 	Vextra  string
 }
 
+type Nested struct {
+	Vfoo string
+	Vbar Basic
+}
+
 func TestBasicTypes(t *testing.T) {
 	t.Parallel()
 
@@ -42,6 +47,46 @@
 	}
 }
 
+func TestNestedType(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"vfoo": "foo",
+		"vbar": map[string]interface{}{
+			"vstring": "foo",
+			"vint":    42,
+			"vbool":   true,
+		},
+	}
+
+	var result Nested
+	err := MapToStruct(input, &result)
+	if err != nil {
+		t.Errorf("got an err: %s", err.Error())
+		t.FailNow()
+	}
+
+	if result.Vfoo != "foo" {
+		t.Errorf("vfoo value should be 'foo': %#v", result.Vfoo)
+	}
+
+	if result.Vbar.Vstring != "foo" {
+		t.Errorf("vstring value should be 'foo': %#v", result.Vbar.Vstring)
+	}
+
+	if result.Vbar.Vint != 42 {
+		t.Errorf("vint value should be 42: %#v", result.Vbar.Vint)
+	}
+
+	if result.Vbar.Vbool != true {
+		t.Errorf("vbool value should be true: %#v", result.Vbar.Vbool)
+	}
+
+	if result.Vbar.Vextra != "" {
+		t.Errorf("vextra value should be empty: %#v", result.Vbar.Vextra)
+	}
+}
+
 func TestInvalidType(t *testing.T) {
 	t.Parallel()
 
@@ -56,7 +101,7 @@
 		t.FailNow()
 	}
 
-	if err.Error() != "field 'Vstring' expected type 'string', got 'int'" {
+	if err.Error() != "'root.Vstring' expected type 'string', got 'int'" {
 		t.Errorf("got unexpected error: %s", err)
 	}
 }