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

[Markus Mottl <markus.mottl@gmail.com>]

On Wed, May 8, 2013 at 7:34 PM, Milan Stanojević <milanst@gmail.com> wrote:
> but this compiles fine
> let to_int a = (a : 'a Foo.t :> int)

This seems like the preferred solution and should always work, because
it enforces that "a" has a coercible type (rather than e.g. a mere
type variable) by the time type inference reaches the coercion
operator.  Otherwise it may depend on the order in which the type of
sub-terms is inferred whether coercion will work.

> 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.

I'm actually surprised that removing the phantom type variable makes a
difference in that respect.  I guess this just shows how unpredictable
the order of type inference can be, hence the need for proper type
annotations with coercions.

Regards,
Markus

--
Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com