inferred type doesn't type-check (using type families)

[Daniel Fischer]

Am Dienstag 03 November 2009 19:28:55 schrieb Roland Zumkeller:
> Hi,
>
> Compiling
>
> > class WithT a where
> >   type T a
> >
> > f :: T a -> a -> T a
> > f = undefined
> >
> > g x = f x 42
>
> with -XTypeFamilies -fwarn-missing-signatures gives:
>
>              Inferred type: g :: forall a. (Num a) => T a -> T a
>
> Adding
>
> > g :: Num a => T a -> T a
>
> results in:
>
>     Couldn't match expected type `T a' against inferred type `T a1'
>     In the first argument of `f', namely `x'
>
> Is the inferred type not the right one? Is g typeable?

The type function T isn't injective (or, it isn't guaranteed to be), so there's no way to 
determine which type a to use for 42.

>
> Best,
>
> Roland