G'day all. On Wed, Oct 16, 2002 at 07:54:17AM -0700, Hal Daume III wrote:

I was reading the HFL libs, namely Control.Monad.Logic, and there's a definition in there:

newtype Logic a = Logic { mkLogic :: (forall b. (a -> b -> b) -> b -> b) }

I'm curious why this is legal, but

newtype Logic2 a = forall b . Logic2 ((a -> b -> b) -> b -> b)

is not...

Firstly, it's not the same type, but you probably worked that much out by yourself. You don't want an existentially quantified constructor at all, what you want is a constructor with a single argument which has an existentially quantified function type. Secondly, I've changed the definition of Logic since you looked at it, so this is no longer valid. We'll look at a simpler example anyway, using data instead of newtype: data CPS a = forall b. CPS ((a -> b) -> b) What should the type of this function be? mkCPS (CPS x) = x Cheers, Andrew Bromage