[Haskell-cafe] Type families versus functional dependencies question

Alexey Rodriguez mrchebas at gmail.com
Thu Jul 3 09:20:07 EDT 2008

Hi guys,

We are having trouble with the following program that uses type families:

> class Blah f a where
>   blah :: a -> T f f a

> class A f where
>   type T f :: (* -> *) -> * -> *

the following function does not type:

> wrapper :: forall a f . Blah f a => a -> T f f a
> wrapper x = blah x

GHC gives the error:

    Couldn't match expected type `T f1 f1 a'
           against inferred type `T f f a'
    In the expression: blah x
    In the definition of `wrapper': wrapper x = blah x

Maybe it is a problem with ambiguous types, namely "f" appears only in
applications of "T". But that is not the case, there is a "naked" f
appearing as the argument of "T f". But perhaps the type checker does not
want to unify those two f's precisely because they are the arguments of "T

I have tried to encode the above program using FunDeps, because this
procedure helps me understand type errors. But in this case I get no type

> class A' (f :: * -> *) (g :: (* -> *) -> * -> *) | f -> g where

> class Blah' f a where
>   blah' :: A' f g => a -> g f a

> wrapper' :: forall a f g . (Blah' f a,A' f g) => a -> g f a
> wrapper' x = blah' x

So my question is whether my encoding is correct. And if it is, why isn't
the type families version working?


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