At 2002-12-14 15:27, [EMAIL PROTECTED] wrote:
>I define
>
>class Mul a b c | a b -> c, b a -> c where mul :: a -> b -> c
The two constraints are identical. Each one says "given a and b, you have
c".
What you want is essentially this:
class (Mul b a c) => Mul a b c where
mul :: a -> b -> c
mul a b = mul b a
...but that's not allowed as superclass contexts cannot be circular.
>I want
>
>instance (Mul a b c) => Mul b a c where mul x y = mul y x
This will overlap with any other "Mul" instance. Instance declarations
cannot be made overridable the way functions in OOP languages can be:
each instance declaration must have an entirely separate domain.
--
Ashley Yakeley, Seattle WA
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