Hello Daniel, On Thu, Apr 30, 2009 at 05:17:42PM +0200, Daniel Fischer wrote:
In
nonsense t = case nonsense t of Nothing -> Just empty
, which type has the Nothing? It can have the type Maybe s1 for all s1 belonging to SUBST, that is the ambiguous type variable.
thanks for the explanation. Maybe I'm starting to understand what is going on. Now I understand it thus the call of `nonsense` inside the case construct can potentially result in a different `SUBST`-type `s1` than the top-level `nonsense`. That is why it has to be explicitly typed. But I am still not following why the type inference works fine without the signature. Isn't it still ambiguous?
{-# LANGUAGE ScopedTypeVariables #-}
nonsense :: forall s. SUBST s => t -> Maybe s nonsense t = case nonsense t :: Maybe s of Nothing -> Just empty
Great, ScopedTypeVariables is exactly what I was looking for. It solves all my problems. Thank you, Jan. -- Heriot-Watt University is a Scottish charity registered under charity number SC000278.