[Haskell-cafe] Re: Polymorphic (typeclass) values in a list?

Brandon S. Allbery KF8NH wrote:

On Oct 19, 2007, at 12:11 , Sebastian Sylvan wrote:

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On 19/10/2007, Kalman Noel wrote:

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data ExistsNumber = forall a. Num a => Number a

I'm without a Haskell compiler, but shouldn't that be "exists a."?

The problem is that "exists" is not valid in either Haskell 98 or any current extension, whereas "forall" is a very common extension. But you can simulate "exists" via "forall", which is the thrust of these approaches.

When 'exists' is not a keyword, why 'forall' is needed at all? Isn't everything 'forall' qualified by default? ... or are type variables sometimes 'exists' qualified by default depending on context? That would be confusing though... I do not understand why 'forall' keyword is needed. Peter.