Hello list, I've got a collection of types which come from generated code (it's protocol buffer stuff). I'd like to write a polymorphic function which maps the protocol buffer generated data types to native data types. I've tried something like this: class FromProto a where type InputDataType fromProto :: InputDataType -> a instance ToProto SomeNativeType where type OutputDataType = SomeGeneratedType toProto = {- etc -} instance FromProto SomeOtherNativeType where type InputDataType = SomeOtherGeneratedType fromProto = {- etc -} Which works fine for mapping one native data type to *one* generated data type, but breaks when I want to define different *InputDataType*s for different instances of *FromProto*. I feel like I'm making this more complicated than I need to. Is there an easy way to get the data type to data type mapping that I'm looking for? Cheers, Alex