Existential Typing (was Multi-parameter OOP)

[Simon Peyton-Jones]

| If GHC had true existential typing, as opposed to just=20
| existential datatypes,=20
| you could reasonably code what I think you want like this:
|=20
| class A a where
|     basicA :: Bool
|     nextA  :: a -> (EX a'. A a' =3D> a')
|     basicA =3D True
|     nextA  =3D id

Curiously enough, this thread intersects with the "higher-ranked
types" thread on the Haskell list.  If I do implement the Odersky/Laufer
higher-ranked types inference mechanism, then I'll also allow
existentials
in arbitrary positions.  =20

The elimination of "stupid" existential wrapper constructors
is discussed in a bit more detail in the paper Mark and I wrote
recently:

	First class modules for Haskell
=09
http://research.microsoft.com/~simonpj/papers/first-class-modules/index.
htm

Simon