| Which is what we did in Hope+C (you have to say it out loud
| ;-)) as well.
Yes it was, and all credit to you for doing it then.
hbc followed suit some years ago. GHC and Hugs are latecomers.
| 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.)
It wasn't hard. The main tricky bit was making extending
our intermediate language, the polymorphic lambda calculus,
to support existentials. In particular, a case expression
over an existential type like
data T = forall a. C a (a->b)
needs to have a form like
case e of
C a (x::a) (y::a->b) -> ...right hand side...
... a is in scope here ...
The type variable b should already be instantiated by now,
but 'a', being existential, is bound by the case pattern
match. So case expressions needed a bit of extension.
Simon
