Type Pattern-Matching for Existential Types

At 2001-01-30 19:52, Fergus Henderson wrote:

On 30-Jan-2001, Ashley Yakeley wrote:

...

At 2001-01-30 02:37, Fergus Henderson wrote:

...

class BaseClass s where downcast_to_derived :: s -> Maybe Derived

Exactly what I was trying to avoid, since now every base class needs to know about every derived class. This isn't really a practical way to build an extensible type hierarchy.

Right.

I don't know of any way to do that in Hugs/ghc without the problem that you mention. Really it needs language support, I think. (I have no idea if you can do it in O'Haskell.)

It can't be done in O'Haskell either... Given that we have existential types, it would be nice to have a pattern-matching mechanism to get at the inside value. Something like... -- data Any = forall a. Any a get :: Any -> Maybe Char get (Any (c::Char)) = Just c -- bad get _ = Nothing -- ...but as it stands, this is not legal Haskell, according to Hugs: ERROR "test.hs" (line 4): Type error in application *** Expression : Any c *** Term : c *** Type : Char *** Does not match : _0 *** Because : cannot instantiate Skolem constant This, of course, is because the '::' syntax is for static typing. It can't be used as a dynamic pattern-test. Question: how big of a change would it be to add this kind of pattern matching? Is this a small issue, or does it have large and horrible implications? -- Ashley Yakeley, Seattle WA