[Haskell-cafe] Problems with type families

[Leza Morais Lutonda]

Hi All!
Consider the following definitions:

type family TFunIn  t e
type family TFunOut t e

data IdT
data ComposeT t1 t2
-- etc...

type instance TFunIn  (ComposeT t1 t2) e = TFunIn  t2 e
type instance TFunOut (ComposeT t1 t2) e = TFunOut t1 e
-- etc...

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

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

Due to the `TFunIn` and `TFunOut` type families instances for [(ComposeT 
t1 t2) e], the `transform` function for
[(ComposeT t1 t2) e] instance of SignalT should be inferred as:

transform :: t -> TFunIn (ComposeT t1 t2) e -> TFunOut (ComposeT t1 t2) e
    <==>   :: t -> TFunIn t2 e -> TFunOut t1 e

But I cannot figure out why it does not compile.

Any ideas?
Thanks.


-- 
Leza Morais Lutonda, Lemol-C
http://lemol.github.io



50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964  http://cujae.edu.cu