This type declaration for 'm' probably doesn't mean what you think it does. I think what you want is "m takes an item of type 'a' and returns an item of a particular type in the Num class, but I'm not going to tell you which one", but what this declaration really means "m takes an item of type 'a' and will return an item of any type you wish, so long as it is in the Num class".
In this context 'a' and 'b' are very different kinds of type variables: 'a' is fixed, but 'b' is universally quantified.
Thank you Mr. Dockins for pointing that out, now I understand (I think) why my attempts failed.
[ ... ] {-# OPTIONS -fglasgow-exts #-} class Num b => Rect a b | a -> b where width :: a -> b height :: a -> b
instance Rect IRect Int where width ( IRect (x1, _ ) (x2, _ ) ) = abs(x2 - x1) height ( IRect ( _, y1) ( _, y2) ) = abs(y2 - y1)
This works perfectly! Anyone: However, I haven't been able to make PRect an instance of this class (with extensions). I might not have grasped this yet, but I came to think; if the old class declaration would say that "width" and "height" returns something with unfixed type in the "Num" class, then wouldn't it be possible to make PRect an instance of that class (since PRect has a type parameter)? Something like this: class Rect a where width :: (Num b) => a -> b height :: (Num b) => a -> b data Num a => PRect a = PRect (a, a) (a, a) deriving (Eq, Show) instance Rect PRect a where ... This (as well as my other attemps) fail with a "Kind error: `PRect' is not applied to enough type arguments" - error. Is there a way to do it, or am I lost here? regards /johan