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From: =?UTF-8?Q?Milan_Stanojevi=C4=87?= <milanst@gmail.com>
Date: Tue, 7 Oct 2014 12:14:01 -0400
Message-ID: <CAKR7PS9DnFanOxLgiCusp4LPQY64B-5SU2Bwd_hLYqb81k5Q9A@mail.gmail.com>
To: Caml List <caml-list@inria.fr>
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Subject: [Caml-list] constructor disambiguation for gadts

I was wondering what are technical difficulties that prevent
constructor disambiguation for GADTs?

I got an interesting error today, where it is clear that compiler
knows the type but because it is GADT it is still asking me to qualify
the constructor.

Here is the simple example.
module A : sig
  type _ t =
    | I : int t
    | S : string t
end = struct
  type _ t =
    | I : int t
    | S : string t
end

let add : type a . a A.t -> a -> a -> a =
  fun w a1 a2 ->
    match w with
    | I -> a1 + a2
    | S -> String.concat "" [a1; a2]
;;

I get File "foo.ml", line 14, characters 6-7:
Error: The GADT constructor I of type A.t must be qualified in this pattern.

I'm not sure what extra information I'm giving to the type checker by
qualifying the constructor.