2009/11/3 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.

I think (untested) that in this particular case you can get around the problem using scoped type variables:

g :: forall a. Num a => T a -> T a g x = f x (42 :: a)

In fact, this seems to be the general pattern for fixing problems like this with type families: add extra "witness" arguments which GHC can use to unify type variables that are "hidden" inside type family applications. Cheers, Max

| I think (untested) that in this particular case you can get around the | problem using scoped type variables: | | > g :: forall a. Num a => T a -> T a | > g x = f x (42 :: a) | | In fact, this seems to be the general pattern for fixing problems like | this with type families: add extra "witness" arguments which GHC can | use to unify type variables that are "hidden" inside type family | applications. But in this case all you've done is to make g have an ambiguous type. I've drafted a FAQ about this at http://haskell.org/haskellwiki/GHC/Type_families#Frequently_asked_questions Please, everyone, edit and extend. Simon