Re: [Caml-list] an issue with coercing private types

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2013/05/09, at 8:34, Milan Stanojević <milanst@gmail.com> wrote:

> 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.


The reason one works and not the other is explained here:
http://caml.inria.fr/pub/docs/manual-ocaml/manual021.html#toc76
Namely, when using single coercions from private type abbreviations,
the type of the expression should not contain type variables.

An interesting consequence is that the following is fine:

let to_int (type a) (a : a Foo.t) = (a :> int);;

	Jacques Garrigue