escape from existential quantification

[Keith Wansbrough]

> I understand that existentially bound types cannot escape.
> 
> For example, say we have
> data Foo = forall a. Foo Int a
> 
> Then we cannot define a function
> extract (Foo i a) = a
> 
> However,this limitation makes it extremly difficult to program with local
> quantifications.Is there any way to by pass this?

The idea is to use a type more like this:

data Foo = forall a. Foo Int a (a -> (Int,Bool)) (a -> Int) (a -> Foo)

where the functions are the operations you want to use on the data.  So now a list of Foos can contain data of many different types, as long as it is paired with the appropriate accessor functions for those types.  You can use it like this:

case x of
  Foo n x f g h -> if snd (f x) then g x else 0

for example.

--KW 8-)
-- 
Keith Wansbrough <kw217@cl.cam.ac.uk>
http://www.cl.cam.ac.uk/users/kw217/
University of Cambridge Computer Laboratory.