Quoting "Matthew Steele" <mdste...@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
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