On 2003-06-06 at 08:15BST "Simon Peyton-Jones" wrote: > > I forget whether I've aired this on the list, but I'm > seriously thinking that we should change 'forall' to > 'exists' in existential data constructors like this one.
You did mention it, and there were several replies. I'd characterise them as mainly falling into two classes: "Yes, the change is sensible" and "No, it's all right as it is so long as you stand on your head when reading programmes". It doesn't seem so difficult to me. It's a matter of thinking in terms of expressions for types and functions that return types. If you define type F a = forall t . (a, t) and subsequently write e:: F Int this is equivalent to writing e:: forall t . (Int, t) Now, although we don't have type expressions that correspond to the RHSs of data declarations, it seems perfectly reasonable to expect things to work as if we did -- the chief problem being that we can't see from the context which constructors are data and which type. So data D a = forall t . MkD a t leads us to interpret e:: D Int as e:: forall t . MkD a t I don't think that the problem of type and constructor namespaces detracts from this argument -- if anything, it points up a problem with data constructors, not quantifiers. >From there it's easy to decide that to get an existential type we need to write data D a = exists t . MkD a t (and type F a = exists t . (a, t) looks quite reasonable too). > One has to explain 'forall' every time. But we'd lose a > keyword. Seems like a small price to pay. As Christian Maeder points out it is a loss only in the type variable namespace. As to omitting the quantifier, I say no, since the omission of quantifiers elsewhere corresponds uniformly to universal quantification. Jón PS that's one heck of an email address you have there, Simon! -- Jón Fairbairn [EMAIL PROTECTED] 31 Chalmers Road [EMAIL PROTECTED] Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!) _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
