"Simon Peyton-Jones"
Many of you have grown to love existential data types, which we current write like this: data T = forall a. Foo a (a -> Int)
or indeed more generally as data T = forall a. (Bar a, Baz a) => Foo a (a -> Int) I have certainly found the distinction between local universal and existential quantification a little confusing. For instance, data T = forall a. Enum a => Foo (a->a) or data T = Foo (forall a. Enum a => (a->a)) could indeed be local universal quantification. You can construct a Foo with any function of type (Enum=>a->a), e.g. (Foo succ) is ok, (Foo id) is not. When you pattern-match on the constructor, you get back the original universal type, e.g. the following is valid: f :: T -> (Int,Bool,Char) f (Foo g) = (g 0, g False, g 'a') It appears that ghc, Hugs, and nhc98 do not support local universals, although hbc does. On the other hand data T = forall a. Foo a (a -> Int) is actually existential quantification, even though the syntax suggests otherwise. The difference is that the existentially quantified variables are not permitted to escape from their constructor, whilst local universals may do so. So I agree that a change of syntax might be helpful.
I reckon that we should Allow 'exists' Deprecate 'forall' (for defining existentials, that is) Eventually allow only 'exists'
A reasonable suggestion.
It might even be possible to arrange that 'forall' and 'exists' were only keywords in types, and not in terms, but I'm not sure it's worth the bother.
I would want to insist that they are treated as special identifiers only in the context of types, rather than to turn them into full keywords. In nhc98 at least, it is just as easy to go the former route as the latter, and as a rule adding new keywords to the language breaks old programs. A different suggestion might be to adopt the hbc syntax for existentials, which bears a certain pleasing similarity to that for implicit parameters (a totally different type extension) at the value level: data T = C ?a (?a->Int) or data T = Bar ?a => C ?a (?a->Int)
I don't think it affects NHC, because it's not H98 anyway.
nhc98 has supported existentials since January 1999. Regards, Malcolm