[Openmcl-devel] MOP initialize-instance question

Cyrus Harmon ch-openmcl at bobobeach.com
Mon Sep 13 09:02:08 PDT 2004

Wow. Lots of details to digest. I like the way this looks. A cursory 
glance suggests that the more-moppish-s-i would do what I want by 
calling slot-value-using-class.

Thanks for yet another great in-depth explanation of my (latest) 


On Sep 13, 2004, at 3:09 AM, Gary Byers wrote:

> On Mon, 13 Sep 2004, Cyrus Harmon wrote:
>> So I'm trying to make my own metaclasses and I have a class that has a
>> mix of some "special" slots and some standard slots. I can get
>> slot-value-using-class and setf s-v-u-c to work properly, but I can't
>> figure out how to hijack the initargs stuff so the MOP code Does What 
>> I
>> Mean. It would be nice if there were an initialize-slot that got 
>> called
>> by initialize-instance (or %shared-initialize). But there doesn't seem
>> to be. Any suggestions for how I can get slot-specific (or even
>> direct-slot-definition-specific) behavior so I can use the built-in
>> stuff for "normal" slots and do my own work to figure out what the
>> values (and where they should go are) for my non-normal slots?
>> Is there a better way to skin this cat?
>> Thanks,
>> Cyrus
> OpenMCL defines:
> (defmethod shared-initialize ((instance standard-object) slot-names 
> &rest initargs)
>   (declare (dynamic-extent initargs))
>   (%shared-initialize instance slot-names initargs))
> e.g., the function CCL::%SHARED-INITIALIZE happens to do all of the 
> heavy
> lifting.
> CCL::%SHARED-INITIALIZE does that heavy lifting under the assumption
> that it's dealing with standard objects that have standard slot 
> definitions,
> and it bypasses most of the MOP in doing what it does, for two reasons:
> 1) Because It Can
> 2) Because most of the MOP doesn't necessarily exist until the methods/
> classes/etc in question can be initialized with, and this function is
> used to initialize those objects.
> After CLOS has initialized itself (it's mostly all there by the time
> that "ccl:level-1;l1-clos" has finished loading during XLOAD-LEVEL-0),
> reason 2 becomes less compelling; it'd be possible to redefine
> CCL::%SHARED-INITIALIZE (mostly) in terms of MOP functions:
> (defun more-moppish-%shared-initialize (instance slot-names initargs)
>   (unless (or (listp slot-names) (eq slot-names t))
>     (report-bad-arg slot-names '(or list (eql t))))
>   ;; Check that initargs contains valid key/value pairs,
>   ;; signal a PROGRAM-ERROR otherwise.  (Yes, this is
>   ;; an obscure way to do so.)
>   (destructuring-bind (&key &allow-other-keys) initargs)
>   ;; I'm not sure if there's a more portable way of detecting
>   ;; obsolete instances.  This'll eventually call
>   (let* ((wrapper (instance-class-wrapper instance))
>          (class (%wrapper-class wrapper)))
>     (when (eql 0 (%wrapper-hash-index wrapper)) ; obsolete
>       (update-obsolete-instance instance))
>     ;; Now loop over all of the class's effective slot definitions.
>     (dolist (slotd (class-slots class))
>       ;; Anything that inherits from STANDARD-EFFECTIVE-SLOT-DEFINITION
>       ;; in OpenMCL will have a CCL::TYPE-PREDICATE slot, derived from
>       ;; the slot's specified TYPE.  This check for slot existence
>       ;; and -boundp-ness is probably overkill: it's not well-defined
>       ;; to try to inherit from EFFECTIVE-SLOT-DEFINITION without
>       ;; also inheriting from STANDARD-EFFECTIVE-SLOT-DEFINITION,
>       ;; and these checks might slow things down a bit.
>       (let* ((predicate
>               (if (and (slot-exists-p slotd 'type-predicate)
>                        (slot-boundp slotd 'type-predicate))
>                 (slot-value slotd 'type-predicate)
>                 #'true)))
>       (multiple-value-bind (ignore new-value foundp)
>           (get-properties initargs (slot-definition-initargs slotd))
>         (declare (ignore ignore))
>         (cond (foundp
>                ;; an initarg for the slot was passed to this function
>                ;; Typecheck the new-value, then call
>                ;; (SETF SLOT-VALUE-USING-CLASS)
>                 (unless (funcall predicate new-value)
>                   (error 'bad-slot-type-from-initarg
>                          :slot-definition slotd
>                          :instance instance
>                          :datum new-value
>                          :expected-type  (slot-definition-type slotd)
>                          :initarg-name (car foundp)))
>                 (setf (slot-value-using-class class instance slotd) 
> new-value))
>               ((and (or (eq slot-names t)
>                         (member (slot-definition-name slotd)
>                                 slot-names
>                                 :test #'eq))
>                     (not (slot-boundp-using-class class instance 
> slotd)))
>                ;; If the slot name is among the specified slot names, 
> or
>                ;; we're reinitializing all slots, and the slot is 
> currently
>                ;; unbound in the instance, set the slot's value based
>                ;; on the initfunction (which captures the :INITFORM).
>                (let* ((initfunction (slot-definition-initfunction 
> slotd)))
>                  (if initfunction
>                    (let* ((newval (funcall initfunction)))
>                      (unless (funcall predicate newval)
>                        (error 'bad-slot-type-from-initform
>                               :slot-definition slotd
>                               :expected-type (slot-definition-type 
> slotd)
>                               :datum newval
>                               :instance instance))
>                      (setf (slot-value-using-class class instance 
> slotd)
>                            newval))))))))))
>   instance)
> This is basically a slightly re-written version of
> CCL::%SHARED-INITIALIZE, with a few more comments and with lower-level
> accessors that know lots about the representation of STANDARD-OBJECTs
> replaced with more general MOP functions where possible (and quite
> possibly with a few bugs introduced; save important work before trying
> to use this, and don't be shocked if args are backwards or parens are
> misplaced ...)
> Although this -could- be used as a replacement for
> CCL::%SHARED-INITIALIZE after things are bootstrapped, there are some
> tradeoffs involved.  If we're dealing with things that are
> semantically far-removed from STANDARD-OBJECT, some concepts may not
> apply (the SHARED-INITIALIZE method that Randall Beer wrote for the
> ObjC bridge had to deal with the fact that ObjC has no real concept of
> "boundp-ness" and that ObjC may not even like the idea of instance
> reinitialization too much.)  Likewise, the typechecking that happens
> on standard objects may not apply (or be desirable) in some cases.
> If the objects in question are "standard" CLOS objects with standard
> semantics, skipping the MOP's generality makes instance initialization
> a little faster, which is probably A Good Thing.   (Perhaps the right
> thing is to use the current version of CCL::%SHARED-INITIALIZE if
> the metaclass of the instance is STANDARD-CLASS or 
> CLASS exactly, and fall back on the more general case if it's a 
> subclass
> of one of those classes.)

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