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Message-ID: <508677C4.5040802@inria.fr>
Date: Tue, 23 Oct 2012 12:56:04 +0200
From: Romain Bardou <romain.bardou@inria.fr>
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Subject: [Caml-list] Constrained existential types

(This is the 4th time I send this e-mail because it does not seem to 
work. I'm trying with another SMTP server.)

Hello list,

I'm trying to use first-class modules to have existential types. But my 
existential type must be constrained. I have the following code:

type 'a t constraint 'a = [< `A | `B ]

module type SIG =
sig
   type a = private [< `A | `B ]
   val x: a t
end

let create (type u) (y: u t) =
   let module M: SIG =
     struct
       type a = u
       let x = y
     end
   in
   ()

It does not compile, because of the following error:

Error: This type u should be an instance of type [< `A | `B ]

In the manual I did not see any way to constrain type u. If I write 
something like this instead:

let create (y: 'a t) =
   let module M: SIG =
     struct
       type a = 'a
       let x = y
     end
   in
   ()

Then the definition "type a = 'a" is not correct, because 'a is not bound.

Is there any way to have constrained existential types?

Thanks,

-- 
Romain Bardou