At 10:11 pm +0000 5/11/99, Marcin 'Qrczak' Kowalczyk wrote:
>Fri, 5 Nov 1999 22:26:17 +1300, Nigel Perry <[EMAIL PROTECTED]> pisze:
>
> > I missed the start of this and am a bit confused - I keep seeing
> > "forall", which is universal, not "there exists", which is
> > existential - at least as I know them. Are the existentials talked
> > about in relation to Haskell something different, or has the notation
> > just got confused?
>
>A value of type
> data Action = forall b. Act (IORef b) (b -> IO ())
>is something like "there exists some type b, such that this is a pair
>of IORef b and b -> IO ()".
OK, we are talking about the same thing.
>Instead of providing separate pack+open constructs, GHC/Hugs extend
>the syntax of declarations of datatypes, so that building and using
>values of quantified types simply looks like using constructors and
>pattern matching, modulo those non-obvious rules about what is actually
Which is what we did in Hope+C (you have to say it out loud ;-)) as well.
So that's good, I was concerned my earlier answer and reference to a paper on
implementation was talking about the wrong thing when I kept seeing the "forall".
Implementing existentials in Hope+C was not hard, I wonder why it was hard in GHC?
(Rhetorical musing, I'll go find the answer myself.)
