fixup! refactor: define interfaces for JSON types

Author: tyru

dsl/types/types.go

@@ -11,6 +11,9 @@ type Type interface {
 
 // ===================== Void type ===================== //
 
+// VoidType is a void type
+var VoidType = &voidType{}
+
 type voidType struct{}
 
 func (*voidType) String() string {