[slime-devel] Slime support for metaclasses

[Ian Eslick]

Some Lisp implementations do not call the generic function
slot-boundp-using-class as part of executing the normal lisp slot-boundp
for performance reasons. 

The specific problem I ran into was inspecting objects in Elephant (a
persistent DB) where the metaobject protocol does not allocate storage
for slots but redirects (via metaclass override of
slot-boundp-using-class and slot-value-using-class) queries to disk
database lookups.  In this case slot-boundp returned nil, but
slot-boundp-using-slot was returning 't'. 

An alternative approach (safer?) rather than modifying swank-mop and
forcing always using the generic function, would be to write a
swank-slot-boundp defun that is selectively implemented by specific
backends with this problem.  i.e.

#-allegro
(defun swank-slot-boundp (object slot)
    (slot-boundp object slot))

#+allegro
(defun swank-slot-boundp (object slot)
    (slot-boundp-using-class (class-of object) object (<lookup-slot-def>
slot)))

Ian

Martin Simmons wrote:
>>>>>> On Mon, 30 Jan 2006 21:12:11 +0100, Helmut Eller <heller at common-lisp.net> said:
>>>>>>             
>> Delivered-To: slime-devel at common-lisp.net
>>
>> * Ian Eslick [2006-01-24 03:39+0100] writes:
>>
>>     
>>> It's a minor enough change I figured someone who checks in regularly
>>> can update this and
>>> provide some judgement on whether there are likely to be any problems.
>>>       
>> I committed that change and added the *-using-class functions to
>> swank-mop.  It seems to work for CMUCL, SBCL, and CLISP.  For the
>> rest, well, it may just work, or we will receive bug reports, or it
>> doesn't matter enough so that nobody sends a bug report.
>>     
>
> LispWorks's *-using-class methods work with the slot name, not the slot
> definition.  I suppose we can finess that in the swank-mop layer.
>
> BTW, what kind of object returns NIL from SLOT-BOUNDP but still has a slot
> value and how is a user supposed to access it?
>
> __Martin
>