Adding class constraints to typeclass instance

Posted by BleuM937 on Stack Overflow See other posts from Stack Overflow or by BleuM937
Published on 2012-09-13T00:14:32Z Indexed on 2012/09/13 3:39 UTC
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I'm trying to implement the Cantor Pairing Function, as an instance of a generic Pair typeclass, as so:

module Pair (Pair, CantorPair) where

-- Pair interface
class Pair p where
    pi :: a -> a -> p a
    k :: p a -> a
    l :: p a -> a

-- Wrapper for typing
newtype CantorPair a = P { unP :: a }

-- Assume two functions with signatures:
cantorPair :: Integral a => a -> a -> CantorPair a
cantorUnpair :: Integral a => CantorPair a -> (a, a)

-- I need to somehow add an Integral a constraint to this instance,
-- but I can't work out how to do it.
instance Pair CantorPair where
    pi = cantorPair
    k = fst . cantorUnpair
    l = snd . cantorUnpair

How can I add the appropriate Integral constraint to the instance? I have a vague feeling I might need to modify the Pair interface itself, but not sure how to go about this.

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