Lennart Augustsson wrote:
Ashley Yakeley wrote:
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?
It has large and horrible implications. To do dynamic type tests you need to carry around the types at runtime. This is not something that Haskell does (at least you don't have to).
-- Lennart
It can also be questioned from a software engineering standpoint. Much of the purpose with existential types is to provide information hiding; that is, the user of an existentially quantified type is not supposed to know its concrete representation. The merits of an information hiding dicipline is probably no news to anybody on this list. However, this whole idea gets forfeited if it's possible to look behind the abstraction barrier by pattern-matching on the representation. Allowing that is a little like saying "this document is secret, but if you're able to guess its contents, I'll gladly confirm it to you!". The same argument also applies to information hiding achieved by coercing a record to a supertype. This doesn't mean that I can't see the benefit of dynamic type checking for certain problems. But it should be brought in mind that such a feature is a separate issue, not to be confused with existential types or subtyping. And as Lennart says, it's a feature with large (and horrible!) implications to the implementation of a language. -- Johan