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Date: Fri, 22 Aug 2014 04:17:33 +0100
From: =?utf-8?Q?Daniel_B=C3=BCnzli?= <daniel.buenzli@erratique.ch>
To: Caml-list <caml-list@inria.fr>
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Subject: Re: [Caml-list] Phantom types and variants

Sorry the coerce function type was wrong in my preceding message. Here is the example: 

-----
module M : sig
  type kind = [ `A | `B ]
  type +'a t constraint 'a = [< kind ]

  val a : unit -> [> `A] t
  val b : unit -> [> `B] t
  val coerce : ([< kind] as 'a) -> 'b t -> 'a t
end = struct
  type kind = [ `A | `B ]
  type +'a t =
    { kind : kind }
    constraint 'a = [< kind]

  let a () = { kind = `A }
  let b () = { kind = `B }
  let coerce k v = if v.kind <> k then invalid_arg "" else v
end

let (vl : [`A | `B ] M.t list) = [M.a (); M.b ()]
let (v : [`A] M.t) = M.coerce `A (List.hd vl)

------

and the error:

Values do not match:
val coerce : kind -> ([< kind ] as 'a) t -> 'a t
is not included in
val coerce : ([< kind ] as 'a) -> [< kind ] t -> 'a t