[Haskell-cafe] Problems with type families

[Silvio Frischknecht]

> type instance TFunIn DoubleT e = (Signal e, Signal e)


I give up. I'm not sure it's possible to tell to the compiler that when
using transform function (the IdT version in the implementation of
ComposeT) that it should use the one with the same e. Not sure it's
possible. Anyway a workaround would be.

class SignalT t e
    where
        transform :: e -> t -> TFunIn t e -> TFunOut t e

instance (
    SignalT t1 e,
    SignalT t2 e,
    TFunOut t2 e ~ TFunIn t1 e
    ) => SignalT (ComposeT t1 t2) e
    where
        transform _ _ x =
            (transform (undefined :: e) (undefined :: t1))
            (transform (undefined :: e) (undefined :: t2))
            (x :: TFunIn (ComposeT t1 t2) e)


But you could save your self a lot of hurt by just using

newtype DoubleSignal a = DoubleSignal (Signal a, Signal a)

you could even remove the e from the SignalT class.


Silvio