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From: Markus Mottl <markus.mottl@gmail.com>
To: OCaml List <caml-list@yquem.inria.fr>
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Subject: [Caml-list] Late adding of type variable constraints

Hi,

I wonder whether there is a way to add constraints to type variables
in signatures after the signature was defined.  E.g.:

---
module type S = sig
  type 'a t
  type ('a, 'b) mappers

  val map : ('a, 'b) mappers -> 'a t -> 'b t
end

module type T = sig
  type 'a t constraint 'a = unit  (* whatever *)
  include S with type 'a t := 'a t
end
---

The above will fail, because 'a has additional constraints for type
"t" in signature "T".  If I write instead e.g. "type 'a t = 'a list",
this will work and also constrain the signature to something narrower.
What makes constraints on polymorphic variables special here?

Regards,
Markus

-- 
Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com