Re: [Haskell-cafe] Rigid skolem type variable escaping scope

On Aug 22, 2012, at 3:02 PM, Lauri Alanko wrote:

Quoting "Matthew Steele" :

...

{-# 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.

Ah ha, thank you! Yes, this solves my problem. However, I confess that I am still struggling to understand why unpacking earlier, as I originally tried, is invalid here. The two implementations are: 1) bar ifn = case ifn of IntFn fn -> foo fn 2) bar ifn = foo (case ifn of IntFn fn -> fn) Why is (1) invalid while (2) is valid? Is is possible to make (1) valid by e.g. adding a type signature somewhere, or is there something fundamentally wrong with it? (I tried a few things that I thought might work, but had no luck.) I can't help feeling like maybe I am missing some small but important piece from my mental model of how rank-2 types work. (-: Maybe there's some paper somewhere I need to read? Cheers, -Matt