Re: [Caml-list] GADT and local modules

[Gabriel Scherer <gabriel.scherer@gmail.com>]

You will be familiar with the solution from the Coq world.

  match t with
   | Pack l ->
     begin fun (type a') (type b') l ->
       let module C = struct
         type a = a'
         type b = b'
         let foo = l
      end in
      let module R = M(C) in
      R.result
    end l

On Fri, May 23, 2014 at 10:15 AM, Thomas Braibant
<thomas.braibant@gmail.com> wrote:
> Hi list,
>
> I would like to find a way to name (or capture) the variables that are
> existentially quantified in a GADT constructor, to put them inside a
> module and apply a functor on the said module. Attached below is a
> mockup of what I would like to have. (I can live without this coding
> pattern, but I would like to know whether or not it is possible to
> make it work, even if I suspect that the answer is no).
>
> Best,
> Thomas
>
>
> type t = Pack : 'a * 'b list -> t
>
> (* I want to unpack a t, and put its content in a module of signature S *)
> module type S = sig
>   type a
>   type b
>   val foo : (a * b) list
> end
>
> (* I want to apply the functor M on my module to produce a result,
> whose type does not depend on the types in S.  I am using a functor as
> a way to structure the computations that happen inside. I started with
> a version without modules, which requires to write quite a few type
> annotations... *)
> module M (C:S) : sig val result : int end= struct
>
>   include C
>
>  (* Here I want bar to have type (a * b) list. Of course, I could have
> bar s a function that takes two type variables as arguments, and a
> list, and returns a list of the right type. *)
>   let bar = List.rev foo
>   let result = List.length bar
>
> end
>
> (* Here, I do not know of a way to retrieve the type variables
> quantified in the GADT constructor Pack, to feed them in C *)
> let f (t : t) =
>   match t with
>   | Pack l -> let module C = .... in M(C)
>
> (* alternative with modules, which is not satisfying. *)
>
> module type S = sig
>   val t : t
> end
>
> module M (C:S) : sig val result : int end = struct
>
>   include C
>
>   (* Now, I can unpack the t locally in each function, but I cannot
>      open the existential in the whole module, which is painful. *)
>
> end
>
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