Untagged

Implies a different encoding for unions where different alternatives are not tagged. This union type should be avoided whenever possible for performance reasons. However, some third party APIs use it so it is important to be able to represent it. The following union: structure One { a: Int } structure Two { b: String } union Test { one: One two: Two } would normally be encoded in JSON as { "one": { "a": 123 } } When it is annotated with ``@untagged, it is instead encoded as: { "a": 123 }. Therefore the parser will need to try each different alternative in the union before it can determine which one is appropriate.