[Haskell-cafe] Rigid skolem type variable escaping scope

[Lauri Alanko]

Quoting "Matthew Steele" <mdsteele at alum.mit.edu>:

>     {-# LANGUAGE Rank2Types #-}
>
>     class FooClass a where ...
>
>     foo :: (forall a. (FooClass a) => a -> Int) -> Bool
>     foo fn = ...

>     newtype IntFn a = IntFn (a -> Int)
>
>     bar :: (forall a. (FooClass a) => IntFn a) -> Bool
>     bar (IntFn fn) = foo fn

In case you hadn't yet discovered it, the solution here is to unpack  
the IntFn a bit later in a context where the required type argument is  
known:

bar ifn = foo (case ifn of IntFn fn -> fn)

Hope this helps.


Lauri