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Date: Tue, 15 Jul 2014 17:26:19 +0200
From: Alain Frisch <alain@frisch.fr>
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Subject: [Caml-list] Existential row types

Dear all,

GADTs allow to restrict existential type variables to being an instance 
of a row type, as in:

class type c = ...

type s =
   | EX: ((#c as 'a) -> unit) * (unit -> 'a) -> ex


I'm wondering if it is possible to simulate such restricted existential 
quantification with first-class module and private row types.  Something 
like:


module type S = sig
   type t = private #c
   val f: t -> unit
   val g: unit -> t
end

let foo (type t_) (f : (#c as t_) -> unit) (g : unit -> t_) =
   let module M = struct
     type t = t_
     let f = f
     let g = g
   end
   in
   (module M : S)


This does not work, of course, because of the "... as t_".  Is there a 
local work-around?  If not, I'm wondering if if would be easy (and make 
sense) to introduce a form for introducing locally private row types:

  let foo (type t_ = private #c) (f : t_ -> unit) (g : unit -> t_) = ...



-- Alain