fundeps question

[Ashley Yakeley]

At 2002-12-14 15:27, nalexand@gianther.hypbus.com 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