all: refactor attributes

errors.go

@@ -15,6 +15,12 @@ type DecodeErr struct {
 	Message string
 }
 
+// IsInvalidCookie returns true if error means that magic cookie
+// value is invalid.
+func (e DecodeErr) IsInvalidCookie() bool {
+	return e.Place == DecodeErrPlace{"message", "cookie"}
+}
+
 // IsPlaceParent reports if error place parent is p.
 func (e DecodeErr) IsPlaceParent(p string) bool {
 	return e.Place.Parent == p
@@ -50,3 +56,8 @@ func newDecodeErr(parent, children, message string) *DecodeErr {
 		Message: message,
 	}
 }
+
+// TODO(ar): rewrite errors to be more precise.
+func newAttrDecodeErr(children, message string) *DecodeErr {
+	return newDecodeErr("attribute", children, message)
+}