Re: [Caml-list] Referencing a functor type

[Nick Betteridge <lists.nick.betteridge@gmail.com>]

Hi Nicolas, everyone,

I'm having problems in reconstituting the variables - I'm pretty sure I
need to do some form of 'casting'. As a recap, I've re-written
everything below with a simple use-case - forming the GADTs works perfectly:


module type Cipher = sig

  type local_t
  type remote_t

  val local_create : unit -> local_t
  val local_sign : local_t -> Cstruct.t -> Cstruct.t
  val remote_create : Cstruct.t -> remote_t
  val remote_validate : remote_t -> Cstruct.t -> bool

end

module Make_cipher :

  functor (Cipher_impl : Cipher) ->
    sig
      type local_t = Cipher_impl.local_t
      type remote_t = Cipher_impl.remote_t
      val local_create : unit -> local_t
      val local_sign : local_t -> Cstruct.t -> Cstruct.t
      val remote_create : Cstruct.t -> remote_t
      val remote_validate : remote_t -> Cstruct.t -> bool
    end

type local_key = LK : 'a * (module Cipher with type local_t = 'a) ->
local_key
type cipher_module = CM : (module Cipher with type local_t = 'a) ->
cipher_module

type self_t =
  {
    mutable modules : cipher_module list;
    mutable locals : local_key list
  }



module C1x = Make_cipher (Cipher1)
module C2x = Make_cipher (Cipher2)

let () =
  let cs1 = ( module C1x : Cipher with type local_t = Cipher1.local_t) in
  let cs2 = ( module C2x : Cipher with type local_t = Cipher2.local_t) in
  let module C1 = (val cs1 : Cipher with type local_t = Cipher1.local_t) in
  let module C2 = (val cs2 : Cipher with type local_t = Cipher2.local_t) in
  let local1 = C1.local_create () in
  let local2 = C2.local_create () in
  let l_k1 = LK (local1, cs1) in
  let l_k2 = LK (local2, cs2) in
  let cm1 = CM (cs1) in
  let cm2 = CM (cs2) in
  let cslist = [cm1;cm2] in
  ignore (C1.local_sign local1 (Cstruct.create 23));
  let self = {
    modules = [cm1;cm2];
    locals = [l_k1;l_k2];
  } in
  let CM cs1' = cm1 in
  let module C1' = (val cs1' : Cipher with type local_t = 'a) in
  let LK (local1', cs1') = l_k1 in
  let signature = C1'.local_sign local1' (Cstruct.create 23) in
  Printf.printf "OK"


An error is given for 'let signature = ....'
Error: This expression has type a#1 but an expression was expected of type
         C1'.local_t = a#0

I've spent quite a while trying to figure this out and playing around
with various permutations and am simply getting nowhere!

Cheers
Nick