[Sbcl-help] Re: [Sbcl-devel] [Fwd: problems with funcallable instances]

[Christophe R. <cs...@ca...>]

Matthew D Swank <ak...@co...> writes:

> I cross posted since this list seems to have more activity.

It would probably have been redirected here, because at least I am not
about to give you a straightforward answer. :-/

> "You do not really understand something unless you can explain it to
> your grandmother." =E2=80=94 Albert Einstein.

While I think I probably could explain General Relativity to my
grandmother, unfortunately I can't explain the slot access of
funcallable-standard-instances.

Last time I was in the area, I confess that I cargo-culted enough so
that it seemed to work.  We were implementing a hash value slot for
standard-instances and funcallable-standard-instances, and I think I
expressed my confusion thusly:

;;; FIXME: This seems to bear no relation at all to the CLOS-SLOTS
;;; slot in the FUNCALLABLE-INSTANCE structure, above, which
;;; (bizarrely) seems to be set to the NAME of the
;;; FUNCALLABLE-INSTANCE. At least, the index 1 seems to return the
;;; NAME, and the index 2 NIL.  Weird.  -- CSR, 2002-11-07
(defmacro fsc-instance-slots (fin)
  `(%funcallable-instance-info ,fin 0))

I have also observed in the past that there's something odd going on
with CLOS/PCL discriminating functions, which don't allow themselves
to be overridden: see bug #343.  This is likely related to some
weirdness happening in this area.

However, there _is_ a solution to your particular case.  I don't
really recommend it; it's pretty horrible, but it comes about because
the lisp calling convention is different for
funcallable-standard-instances than it is for regular functions.  If
you replace

> (defmacro promise (expr)
>   (let ((instance (gensym)))
>     `(let ((,instance (make-instance (quote promise)
>                                      :expr (quote ,expr))))
>        (sb-pcl:set-funcallable-instance-function
>         ,instance
>         (delay ,expr))
>        ,instance)))

with

(defmacro promise (expr)
  (let ((instance (gensym)))
    `(let ((,instance (make-instance 'promise :expr ',expr)))
       (sb-pcl:set-funcallable-instance-function
        ,instance
        #'(sb-kernel:instance-lambda () ,expr))
       ,instance)))

then I believe things start working.

Since I don't really know why things start working, I can't exactly
guarantee that these things will continue to work; I would love to be
told what is going on in this area.  However, I hope this gets you
going, at least.

Cheers,

Christophe