[Caml-list] Trying to define a functor combining polymorphic variants

[Berke Durak <berke.durak@gmail.com>]

Hello all,

I have been using the following kind of construct:

  module A = struct
    type message = [`Alpha]
    let string_of_message = function `Alpha -> "alpha"
  end

  module B = struct
    type message = [`Beta]
    let string_of_message = function `Beta -> "beta"
  end

  module AB = struct
    type message = [ A.message | B.message ]
    let string_of_message = function
    | #A.message as msg -> A.string_of_message msg
    | #B.message as msg -> B.string_of_message msg
  end

So I naturally wanted to write a functor that does what the module AB does:

  module type S = sig
    type message
    val string_of_message : message -> string
  end

  module PROD(A : S)(B : S) = struct
    type message = [ A.message | B.message ]
    let string_of_message = function
    | #A.t as msg -> A.string_of_message msg
    | #B.t as msg -> B.string_of_message msg
  end

But we (me + people on #ocaml: mrvn, drup, ggole, whitequark...) couldn't find a
way to specify, in the signature S, that message is a polymorphic variant so
that [ A.message | B.message ] is legal.

We tried things like:

  module type S = sig type 'a t = ([> ] as 'a) end

  module PROD(X : S)(Y : S) = struct
    type ('a,'b) t = [ 'a X.t | 'b Y.t ]
  end

but all we get is:

  Error: The type [>  ] A.message is not a polymorphic variant type

Any suggestions?
-- 
Berke Durak