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From: rixed@happyleptic.org
To: caml-list@inria.fr
Subject: [Caml-list] phantom types and coercion
Date: Mon, 28 Oct 2019 15:27:43 +0100
Message-ID: <b070e40a-3ff9-4ede-8c95-6219fc76f723@www.fastmail.com> (raw)

Hello list.

I'm trying to use several parameters in a phantom type ; everything works alright, but coercion.
For instance, in the code below, how come s1 can be coerced into a string by s2 cannot ?

# module M : sig type +'a t = private string val m1 : string -> [`C1] t val m2 : string -> ([`C2] * 'a) t end = struct type 'a t = string let m1 s = s let m2 s = s end;;   module M :
  sig
    type +'a t = private string
    val m1 : string -> [ `C1 ] t
    val m2 : string -> ([ `C2 ] * 'a) t
  end
# let s1 = M.m1 "foo";;
val s1 : [ `C1 ] M.t = "foo"
# let s2 = M.m2 "bar";;
val s2 : ([ `C2 ] * 'a) M.t = "bar"
# print_string (s1 :> string);;
foo- : unit = ()
# print_string (s2 :> string);;
Error: This expression cannot be coerced to type string; it has type
         ([ `C2 ] * 'a) M.t
       but is here used with type string


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Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-28 14:28 rixed [this message]
2019-10-28 15:11 ` Jacques Garrigue
2019-10-28 16:45   ` rixed

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