Re: [Caml-list] Who was working on ocaml bindings for zeromq?

[Gabriel Scherer <gabriel.scherer@gmail.com>]

> 2) Note the _ there. The type is not polymorphic and gives an error
> that it can't be unified or something. Don't have the exact error in
> memory.
>
> I think there is something wrong with the way the socket types are
> specified that cause this. First time I ever got that kind of error.

This smells like value restriction kicking in because your avstract
type is not marked as covariant, as explained for example here:
  http://www.reddit.com/r/ocaml/comments/t8g2e/pdf_relaxing_the_value_restriction_by_jacques/c4kf68s

Note that there is a risk of bad interaction between:
- adding a covariance annotation to your phantom type
- using polymorphic variants for you phantom type parameter
- switching from a purely-phantom to an internally-safe GADT implementation

Covariance (or any other annotation) is always accepted for a
purely-phantom implementation, but would often be rejected by the
type-checker for a corresponding GADT implementation (and would be
unsound anymay with many modes of use of polymorphic variants for that
purpose). Switching to a GADT implementation is often an excellent
idea in the life of a library using phantom types (I have no idea
whether that is the case here), so it may be best to avoid variance
claims and just add a unit parameter to those values that hit typing
problems.


On Mon, Jan 20, 2014 at 10:12 AM, Goswin von Brederlow
<goswin-v-b@web.de> wrote:
> On Sun, Jan 19, 2014 at 06:16:29PM +0100, Anders Peter Fugmann wrote:
>> Hi Goswin,
>>
>> Please see my reply below. For completeness your original mail is
>> also included in full at the end of the mail.
>>
>> On 18/01/14 12:33, Goswin von Brederlow wrote:
>> >3) sockets have phantom types, use them
>> Agreed.
>>
>> >4) Polling is ugly
>> Indeed this could be made simpler.
>
> Yesterday I actually run into a problem with this. Creating a socket
> gave me a odd type (from memory):
>
> val stream: _[> `Push | `Pull | `Pub | `Sub | `Stream] ZMQ.Socket.t
> val dealer: _[> `Push | `Pull | `Pub | `Sub | `Dealer] ZMQ.Socket.t
>
> A few problems with that:
>
> 1) A stream socket should be just `Stream.
>
> 2) Note the _ there. The type is not polymorphic and gives an error
> that it can't be unified or something. Don't have the exact error in
> memory.
>
> I think there is something wrong with the way the socket types are
> specified that cause this. First time I ever got that kind of error.
>
> 3) Polling takes an ('a ZMQ.Socket.t, ZMQ.Socket.event) array. Trying
> to poll a stream and dealer socket causes type errors.
>
> I think phantom typed sockets are incompatible with putting them into
> an array outside the module. This needs an interface where you have a
> poller instance and can add sockets to it one at a time. Internally
> the phantom type is known to be "phantom" and the sockets can be put
> into an array, hashtbl or list.
>
> I didn't have time yet to look into those problems. Currently I used
> Obj.magic to make it work because I wanted to work on the application
> instead of on zmq. But that certainly isn't a solution.
>
>
> So in summary: We need a new polling interface (or phantom typing).
> Not just because I don't like it but because it doesn't work for
> combining different socket types.
>
> MfG
>         Goswin
>
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