Hi Mateusz, It's not directly possible to write a class with a choice of superclasses; as you point out, it's not really clear what that would mean. One workaround, though it might not be sensible in practice, is the following.
{-# LANGUAGE ConstraintKinds, GADTs #-}
First, reify the constraints we are interested in as types that pack up the corresponding dictionary. Thanks to ConstraintKinds, it's possible to do this once and for all.
data Dict c where Dict :: c => Dict c
Now we can describe types with either Num or Eq dictionaries (or both) as a class. The proxy argument makes it easy to specify the type, in the absence of explicit type application.
class NumOrEq a where numOrEq :: proxy a -> Either (Dict (Num a)) (Dict (Eq a))
Something like your Foo class can then be defined like this:
class NumOrEq a => Foo a where bar :: a -> a -> Bool bar x y = case numOrEq [x] of Left Dict -> False Right Dict -> x == y
When giving an instance for NumOrEq, you must choose which dictionary to pack up if both are available.
instance NumOrEq Int where numOrEq _ = Left Dict
instance NumOrEq Bool where numOrEq _ = Right Dict
instance Foo Int instance Foo Bool
And with all that, we have:
bar 3 (3 :: Int) == False bar True True == True
Now I'm wondering why we would want that in the first place. Hope this helps, Adam On 10/05/13 14:58, Mateusz Kowalczyk wrote: | Greetings, | | We can currently do something like |> class (Num a, Eq a) => Foo a where bar :: a -> a -> Bool bar = |> (==) | | This means that our `a' has to be an instance of Num and Eq. Apologies | for a bit of an artificial example. | | 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. | | The compiler currently complains about multiple declarations. Is there | currently a way to achieve this? | | The main issue I can see with this is that given an instance of both, | Num and Eq, it wouldn't be possible to pick the correct default | implementation. | | Purely a theoretical question. -- The University of Strathclyde is a charitable body, registered in Scotland, with registration number SC015263.