Capturing a change in slot value.

[macrolyte]

Hey.
I've been pouring over the Hyperspec and some other resources but haven't found a reasonable solution. Initially I played hide & seek with slot-boundp:

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defmethod initialize-instance :after ((node b-node) &key) ;; DON'T EVEN THINK ABOUT TRYING THIS!!!!!
 (loop while (slot-boundp node 'stat) do ;; test to see if a change in slot value would trigger an unbound condition, (non working code...)
      (print "started"))
 (funcall (lambda()(print "done"))))


Please don't use the code above. I've looked at make-load-form-saving-slots as well but I would have to poll a parsed version of the value it returns which is just, ugh. My last option is to create an additional slot which I can use as a wayback machine... Thanks in advance.

[pjstirling]

I'm not entirely sure what you want to do here, do you want to:

• check if the instance is created using the default value for a slot
• fire some code when the slot is given a value the first/every time

[macrolyte]

I'm not entirely sure what you want to do here, do you want to:

• check if the instance is created using the default value for a slot
• fire some code when the slot is given a value the first/every time

Sorry about being unclear, the "fire some code ..." is what I'm trying to do. I just realized that a function object can be a slot value so I've been trying to piece this together from that direction. Thanks!

[Goheeca]

I don't know at which level do you want the capturing. That's my solution:

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(defclass foo ()
  ((bar :accessor bar-acc)))

(defmethod (setf bar-acc) :before (new-value (instance foo))
  (format t "~&Hey, you are setfing bar in this baz (~a) to ~a.~%" instance new-value))

and usage

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(defvar *obj* (make-instance 'foo))

(setf (bar-acc *obj*) t)

Just for higher meta level use MOP.

[macrolyte]

I need to read the Hyperspec, give me a few minutes. (It works BTW...). Thanks!

[macrolyte]

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(defmethod (setf bar-acc) :before (new-value (instance foo)) ;; <===== see below
  (format t "~&Hey, you are setfing bar in this baz (~a) to ~a.~%" instance new-value))



Is this about the specialize lambda list with parameters matching types/objects thing? And where the @$%^ is the method name?

Just for higher meta level use MOP.

Huh?

[Goheeca]

Yes it is. The method name is bar-acc, the notation is for getter:

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(defmethod accessor ...)

and for setter:

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(defmethod (setf accessor) ...)

MOP is API for the object metaprogramming, the wrapping library negotiating implementation-dependent details is closer-mop.

[macrolyte]

Thanks again, Goheeca. At least you've given me a starting point, so I'll get back to work now. Pax vobiscum.