Felipe Lessa wrote:
class Shape a where
....whatever....
class (Shape a, Shape b) => Intersectable a b where
intersect :: a -> b -> Bool
This looks nice at first sight, but is it usefull in practice? I can
somehow express the type "any shape wich is intersectable with a given
other shape", but it is a pain:
data Intersectable a = forall b . Intersectable a b => Intersectable b
instance Intersectable a (Intersectable b) where
intersect a (Intersectable b) = intersect a b
Now consider I write another binary function as a type class:
class (Shape a, Shape b) => Containment a b where
contains :: a -> b -> Bool
data Containment a = forall b . Containment a b => Containment b
instance Containment a (Containment b) where
contains a (Containment b) = contains a b
I cannot combine these types to express "any shape wich is intersectable
and can be united with a given other shape" without writing another
existiential wrapper.
I cannot express "a list of pairwise intersectable shapes" either.
Things get even worse if we consider a different definition of intersect:
class (Shape a, Shape b, Shape c) => Intersect a b c | a b -> c where
intersect :: a -> b -> c
My conclusion: To make Haskell a better OO language then current OO
languages, we need either first-class typeclasses (to quantify over the
classes an existential wrapped value supports) or inference of
existential wrappers (to infer complicated wrappers automatically).
(Since it's not the goal of Haskell to be any OO language at all this
may not be a problem)
Tillmann
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