Enhanced JSON error reporting in binder

Author: denyskoch

binder.go

@@ -36,7 +36,13 @@ func (b *binder) Bind(i interface{}, c Context) (err error) {
 	switch {
 	case strings.HasPrefix(ctype, MIMEApplicationJSON):
 		if err = json.NewDecoder(req.Body()).Decode(i); err != nil {
-			err = NewHTTPError(http.StatusBadRequest, err.Error())
+			if ute, ok := err.(*json.UnmarshalTypeError); ok {
+				err = NewHTTPError(http.StatusBadRequest, fmt.Sprintf("type mismatch: expected %v got %v at offset %v", ute.Type, ute.Value, ute.Offset))
+			} else if se, ok := err.(*json.SyntaxError); ok {
+				err = NewHTTPError(http.StatusBadRequest, fmt.Sprintf("syntax error at byte %v: %v", se.Offset, se.Error()))
+			} else {
+				err = NewHTTPError(http.StatusBadRequest, err.Error())
+			}
 		}
 	case strings.HasPrefix(ctype, MIMEApplicationXML):
 		if err = xml.NewDecoder(req.Body()).Decode(i); err != nil {