In article <[EMAIL PROTECTED]>,
Graham Klyne <[EMAIL PROTECTED]> wrote:
> class (Eq k, Show k) => Pair a k v where
> newPair :: (k,v) -> a k v
> getPair :: a k v -> (k,v)
I would consider one of these instead:
class Pair1 a where
newPair1 :: (Eq k, Show k) => (k,v) -> a k v
getPair1 :: (Eq k, Show k) => a k v -> (k,v)
class (Eq k, Show k) => Pair2 p k v | p -> k,v where
newPair2 :: (k,v) -> p
getPair2 :: p -> (k,v)
Pair2 is more general than Pair, for instance:
data CharBoolPair = MkCharBoolPair Char Bool
instance Pair2 CharBoolPair Char Bool where
newPair2 (c,b) = MkCharBoolPair c b
getPair2 (MkCharBoolPair c b) = (c,b)
CharBoolPair cannot be made an instance of your Pair (since it has kind
"*").
Pair is more general than my Pair1, there may be type constructors that
are instances of Pair but not Pair1; but I can't imagine them turning up
in well-written code.
--
Ashley Yakeley, Seattle WA
_______________________________________________
Haskell mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/haskell
