> Please, what is wrong here?
>
> -------------------------------------------
> class Project a b where prj :: b -> a -> b
>
> data Pol a = Pol [(a,Int)] Char
>
> instance Project (a,Int) (Pol a) where
> prj (Pol _ c) (a,p) = Pol [(a,p)] c
>
> f :: a -> Pol a -> Pol a
> f a g@(Pol ms _) = prj g (head ms)
>
> f' :: a -> Pol a -> Pol a
> f' a g = prj g (a,1)
> -------------------------------------------
The error message is
Foo.hs:14:
Could not deduce `Project (a, t) (Pol a)'
(arising from use of `prj' at Foo.hs:14)
from the context: ()
Probable cause: missing `Project (a, t) (Pol a)'
in type signature for `f''
or missing instance declaration for `Project (a, t) (Pol
a)'
When checking the type signature(s) for `f''
The reason is that GHC doesn't know what type you want '1' to
be. If you replaced '1' by '1::Int', it works fine.
You may say that, since there is only an instance for Project (a,Int)
if GHC wants to solve the constraint Project (a,t), it should bind t to Int.
That is possible (see Mark's thesis) but GHC doesn't do it. Solving
class constraints never binds type variables.
Simon
