Mateusz Kowalczyk wrote:
Is there a way however to do something along the lines of:
class Eq a => Foo a where bar :: a -> a -> Bool bar = (==)
class Num a => Foo a where bar :: a -> a -> Bool bar _ _ = False This would allow us to make an instance of Num be an instance of Foo or an instance of Eq to be an instance of Foo.
GADTs are a particular good way to constraint disjunction, if you can live with the closed universe. (In the following example I took a liberty to replace Int with Ord, to make the example crispier.)
{-# LANGUAGE GADTs #-}
data OrdEq a where Ord :: Ord a => OrdEq a -- representation of Ord dict Eq :: Eq a => OrdEq a -- representation of Eq dict
bar :: OrdEq a -> a -> a -> Bool bar Ord x y = x > y bar Eq x y = x == y
The function bar has almost the desired signature, only (OrdEq a ->) has the ordinary arrow rather than the double arrow. We can fix that:
class Dict a where repr :: OrdEq a
-- We state that for Int, we prefer Ord instance Dict Int where repr = Ord
bar' :: Dict a => a -> a -> Bool bar' = bar repr
test = bar' (1::Int) 2
I can see the utility of this: something like C++ STL iterators and algorithms? An algorithm could test if a bidirectional iterator is available, or it has to do, less efficiently, with unidirectional. Of course we can use ordinary type classes, at the cost of the significant repetition. In the OrdEq example above, there are only two choices of the algorithm for Bar: either the type supports Ord, or the type supports Eq. So the choice depends on wide sets of types rather than on types themselves.