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

["Milan Stanojević" <milanst@gmail.com>]

Right, it seems (a : t1 :> t2) is actually a different operator that,
as you said, should I always work.
I was pointed to
http://caml.inria.fr/pub/docs/manual-ocaml/expr.html#toc53
which explains this is some detail (and I should have searched harder
it in the first place)

Thanks everyone!

On Wed, May 8, 2013 at 8:57 PM, Markus Mottl <markus.mottl@gmail.com> wrote:
> 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