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.