When generating swagger with Query Parameters, Neil realised that the TypeDecoder could not decode some special types like GreaterThan. This is due to the fact that these types contain a Operator Enum field.

When decoding, the decoder has to return a value in a specific type. In this example, the value returned is a String:

value = container.decode(Sting.self, key: .key)

In the case of the TypeDecoder, it returns dummy values to keep the compiler happy. When decoding an Enum is returns a dummy value. This causes an error because that dummy value is not of the same type as that Enum. Furthermore, we could not construct an enum from rawValue because none of the cases match the empty string.

To solve this issue, we implement a custom decoder for these special types where we decide to not decode the Operator Enum field since it already has a default value. We will only decode the value field associated with those types.