Typing problem

["Daniel Bünzli" <daniel.buenzli@epfl.ch>]

Hello,

I would like to define the following types.

> type u = [ `U1 | `U2 ]
>
> type 'a t = [`A of 'a | `B of ([`U1 ] as 'a) ] constraint 'a = [< u ]

t's parameter is used to statically express constraints in other  
parts of the code. My problem is that
the constraint on 'a is downgraded to [`U1] while I would like to be  
able to write

> let param : 'a t -> 'a = function (`A v | `B v) -> v

and get the following typing behaviour.

> let v = param (`A `U1) (* should type *)
> let v = param (`A `U2) (* should type *)
> let v = param (`B `U1) (* should type *)
> let v = param (`B `U2) (* should not type *)

Is it possible to express that in ocaml's type system ?

Regards,

Daniel