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From: =?UTF-8?Q?Milan_Stanojevi=C4=87?= <milanst@gmail.com>
Date: Wed, 8 May 2013 19:34:35 -0400
Message-ID: <CAKR7PS9fBVyBOutzTBw2n=ten55RP4n6Fb_8WQSkGZg71Z1BZw@mail.gmail.com>
To: Caml List <caml-list@inria.fr>
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Subject: [Caml-list] an issue with coercing private types

I have this module

module Foo : sig
  type 'a t = private int
end = struct
  type 'a t = int
end

and then I want to coerce Foo.t into int
let to_int (a : 'a Foo.t) = (a :> int)

this fails to compile with
Error: This expression cannot be coerced to type int; it has type 'a Foo.t
       but is here used with type int

but this compiles fine
let to_int a = (a : 'a Foo.t :> int)

and gives me the right type.

What is wrong with the first version of [to_int]? Is this a bug in the
compiler? I'm using ocaml 4.0.1

I also tried this with Foo.t that doesn't have phantom type variable
and in that case both versions of [to_int] work.