Am Mittwoch, 31. März 2004 14:36 schrieb MR K P SCHUPKE:

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This syntax would also have the advantage that you can apply different contexts to different constructors.

Er, you can already do that as I just pointed out:

data (Ord a,Num b) => Either a b = Left a | Right b

Note that to constrain the same type variable differently for different constructors would be a type error. In other words this:

data Either a = Num a => Left a Ord a => Right a

Is not valid, because (forall a . Num a) is not the same type as (forall a . Ord a). So infact the current way of writing it as

data (Num a) => Either a = Left a | Right a

Is better because it does not allow you to make that mistake.

Regards, Keean.

I cannot see what the mistake is. What's wrong with having one data constructor of type (Num a) => a -> Either a and another of type (Ord a) => a -> Either a? Both are completely independent. So the current way of writing would not prevent me from making a mistake but instead means the lack of a useful feature. Wolfgang