On Mon, 2008-01-21 at 22:36 +0100, Peter Verswyvelen wrote:
Hey, I knew about the forall (I use that to represent OO style collections, very handy), but not about the exists. Thanks. But GHC 6.8.2 (with -fglasgow-exts) does not seem to accept this "exists" keyword?
That's because it isn't GHC syntax, Stefan was just using for demonstration purposes. (However, I think HBC does accept free existentials.) GHC does support local existentials (with two different syntaxes) and that's what you are probably using to make your "OO style collections". When you write: data Foo = forall a.Foo (Num a => a) this is an -existential-. The type of (the data constructor) Foo is: Foo :: forall a.(Num a => a) -> Foo The rules of logic give: Foo :: (exists a.Num a => a) -> Foo note the change in scoping and compare this to local universal quantification: data Foo = Foo (forall a.Num a => a) Foo :: (forall a.Num a => a) -> Foo