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From: Berke Durak <berke.durak@gmail.com>
To: caml-list <caml-list@inria.fr>
Subject: [Caml-list] Trying to define a functor combining polymorphic variants
Date: Thu, 1 May 2014 16:58:09 -0400	[thread overview]
Message-ID: <CAALTfKDwuX06h7BmnOoe30xoy2c_tBHM+3TW3L9pj_L8L4wZkg@mail.gmail.com> (raw)

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

             reply	other threads:[~2014-05-01 20:58 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-05-01 20:58 Berke Durak [this message]
2014-05-01 23:55 ` Leo White
2014-05-02  6:14 ` Jacques Garrigue
2014-05-02 15:50   ` Berke Durak
2014-05-03  1:43     ` Jacques Garrigue

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