[Haskell-cafe] Existentials and type var escaping

[Roberto Zunino]

In this function

data C = C Int
foo :: C -> C
foo ~(C x) = C x

foo is _not_ the identity: its result must be non bottom, i.e. the
constructor C is "forced" to its argument.

I wonder if a similar function is definable for existential types:

data E = forall a . E a
foo :: E -> E

foo, as defined above does not work (lazy patterns not allowed), and in

foo y = E (case y of E x -> x)

a variable escapes. I also tried with CPS with no success.

Is foo definable at all? I'm starting to think that it is not, and that
there must be a very good reason for that...

Thank you,
Zun.