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announce: callbacks-0.1
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Date: 2005-09-08 (23:33)
From: Bardur Arantsson <spam@s...>
Subject: Re: announce: callbacks-0.1
Christophe Raffalli wrote:
> Maas-Maarten Zeeman a écrit :
>> Hello,
>>> link: third item of:
>>> http://www.lama.univ-savoie.fr/~raffalli/?page=soft&lang=en
>>> comment and idea are welcome !
>> Sounds like a nice idea. As the author of ocaml-expat a binding I've 
>> also worked with c -> ocaml callbacks.
>> It is not really clear to me what problem you are trying to solve. It 
>> is not very difficult for somebody writing a Ocaml binding to come up 
>> with a solution which is easy to call an ocaml function value from c.
>> What I did was create one fixed c-callback functions which:
>> receive c-parameters
>> transform them to ocaml-values
>> lookup the ocaml callback "cb" in a tuple (set earlier from ocaml),
> This is that step I want to avoid ... not mainly for performance, 

Now, I'll freely admit that I haven't tested it specifically, but I 
suspect performance will be worse when using register_global_root () to 
register callback closures instead of just using a mapping from "int" 
(or whatever type your callback identifier would be on the C side) to 
closures "stored" on the OCaml side. There was a post on this list not 
too long ago which exposed efficiency issues with register_global_root 
when registering lots and lots of roots.

> because there are some cases where you do not know which callback you 
> should call (this was the case for one of the glut callback, I don't 
> recall which one, and it was a bit tricky for glutTimer). It happens 
> that in glut, the current window or current menu is properly set before 
> calling the callback ... but there may be other library, more purely 
> functional, with no concept of "current window" ...

For any C library using callbacks there will *always* be *some* way to 
distinguish which event/object caused the callback. The reason is 
simple: Creating callback functions/closures on the fly is impossible(*) 
in C, or, in other words the callback function itself can't possibly 
carry enough information to distinguish callbacks from different 
events/objects. If there isn't enough information to distinguish what 
caused the callback, the callback would be pointless.

(*) Well, you can generate machine code on the fly, but no sane library 
will force you to do this.

Bardur Arantsson

- In my weaker moments I almost pity them, but then I remind
myself: They want to teach.
                                      Bart Simpson, 'The Simpsons'