[cffi-devel] Re: unnamed callback (closures?)

[Jan Rychter]

[resending directly as it seems messages posted through Gmane don't
make it to the list :-/ ]

>>>>> "Luís" == Luís Oliveira <luismbo at gmail.com>:
 Luís> "Hoehle, Joerg-Cyril" <Joerg-Cyril.Hoehle at t-systems.com> writes:
 >> In CLISP, every closure can be turned into a callback, and this is
 >> valuable since it allows to retrieve context information out of the
 >> closure.  Is cffi's limitation caused by some implementations?

 Luís> To my knowledge, yes. AFAICT, only SBCL/x86 and CLISP support
 Luís> that.

 Luís> Regarding the behaviour of CFFI:DEFCALLBACK as a non-toplevel
 Luís> form, not only will it set it globally, the callback itself won't
 Luís> be generated at runtime on most Lisp, IIRC.

I've just ran into the same problem. I really really need closures as
callbacks. I've tried the naive approach:

(defmacro object-event-callback-add (obj type function)
  `(foreign-funcall "evas_object_event_callback_add"
    :pointer ,obj
    callback-type ,(foreign-enum-value 'callback-type type)
    :pointer (get-callback (defcallback ,(gensym "CB")
			       :void
			       ((data :pointer) (cb-e evas) (cb-obj object) (cb-event :pointer))
			     (funcall ,function cb-e cb-obj cb-event)))
    :pointer (null-pointer)))

... but that's a half-baked solution with too many limitations and only
works in a simple example that you run once.

What are the problems with unnamed callbacks? My guess was that they
will never get garbage-collected, but how about storing all of these in
a table somewhere and explicitly freeing them after they are no longer
needed? In my case I do have a way of finding out when a callback is no
longer needed, and I suspect this is the case for many other
applications.

I mostly develop on SBCL/x86 and ECL, so even if a solution worked on
these platforms only, I'd be really happy.

--J.