Haskell: Use typeclasses for types that can be encoded and decoded by typedefs.

[andrevidela]

Right now, the Haskell backend relies on explicitly calling serialisation function for types that depend on other types. For example:

(name List (mu (Nil 1) (Cons (* (var 1) (var 0)))))

generates

encodeList :: Serialiser x0 -> Serialiser (List x0)
decodeList :: Deserialiser x0 -> Deserialiser (List x0)

which makes it very difficult to use from the haskell side since every call has to pass the x0's serialiser explicitly.

Using a typeclass we could generate the following code instead.

class TypedefCodec t where
    encode :: Serialiser t
    decode :: Deserialiser t

instance (TypedefCodec x) => TypdefCodec (List x) where
    encode = …
    decode = …

which allows to use encode and decode on List a without having to pass the serialiser for a explicitly as long as there is an instance for a.