Jean-Claude,

the basic point that the other, more knowledgeable responders made, was:  You cannot define a method on slot-value-using-class specializing on standard-class and expect the same behavior across implementations.  This is because standard-class may be used within the lisp implementation itself and/or be treated by the compiler differently to achieve better performance.  If you want to use slot-value-using-class, you need to define your own metaclass and specialize the methods on that.

-Hans


2014-08-03 9:40 GMT+02:00 Jean-Claude Beaudoin <jean.claude.beaudoin@gmail.com>:

On Sat, Aug 2, 2014 at 7:38 AM, Pascal Costanza <pc@p-cos.net> wrote:

On 2 Aug 2014, at 06:35, Jean-Claude Beaudoin <jean.claude.beaudoin@gmail.com> wrote:

> I think I found the subsection you refer to: AMOP  5.3.1 (pp. 142-144) "Implementation and User Specialization", which contains two sub-subsections with titles starting with "Restrictions on".
> I will try to decipher those and their implications now that I have renewed motivation to do so.


There are similar restrictions defined in http://www.lispworks.com/documentation/HyperSpec/Body/11_abab.htm - bullet 19.


I missed that one too. Thank you for pointing it out.

But after modifying my test code to take account of it I saw no difference in the results.
It turns out that it is the last bullet of the sub-subsection "Restrictions on Portable Programs" that is the key factor in the issue I am interested in here (last bullet of page 144 in AMOP, or here as pointed by Steve). It basically states that the defmethod on slot-value-using-class must happen before any call to make-instance on a class the method uses as a specializer. (And then goes on to motivate this definition order restriction).
If I modify my test code to comply with this rule (see slot_mop_strict.lsp here attached) then sbcl stops being the odd man out and behaves like clisp and ACL (which I think is the proper behavior).

Further, if I agree to stop tickling the system where it does not like to be tickled and define a class tracked_class as a subclass of standard-class to be used as a metaclass of the classes I want to track (see slot_mop_strict_tracked_class.lsp here attached), then all of sbcl, clisp, ccl and ACL show the same behavior, properly calling slot-value-using-class both from slot-value or from the slot accessor. Only LispWorks persists in its (erroneous, I think) behavior of calling s-v-u-c only from slot-value.
I think there is a consensus emerging here.

Thank you all for your help on this matter with a special emphasis toward Steve.





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