Path: utzoo!attcan!uunet!nih-csl!lhc!mimsy!dftsrv!ames!pasteur!ucbvax!elmer-fudd.berkeley.edu!konstan
From: konstan@elmer-fudd.berkeley.edu (Joe Konstan)
Newsgroups: comp.lang.clos
Subject: Re: Is CLOS Object-Oriented ???
Message-ID: <39639@ucbvax.BERKELEY.EDU>
Date: 17 Nov 90 00:36:18 GMT
References: <1990Nov15.101847.26285@diku.dk> <3904@s3.ireq.hydro.qc.ca> <PAB.90Nov16151057@3-mile-island.lucid.com>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: konstan@elmer-fudd.berkeley.edu (Joe Konstan)
Distribution: comp
Lines: 45


In article <PAB.90Nov16151057@3-mile-island.lucid.com>, pab@lucid.com (Peter Benson)
writes:
|> 
|> In article <27443D49.2BE8@wilbur.coyote.trw.com> scott@wiley.uucp (Scott Simpson)
writes:
|>    I thought they were the same too until someone at work came up with
|>    this problem in Flavors:
|> 
|> 	   (setf (send self (send button :slot-to-change)) value)
|> 
|>    What is happening is that (send button :slot-to-change) returns the
|>    name of a slot to change and that slot is changed to VALUE on the
|>    object SELF. How do you do this is CLOS?
|> 
|> It's not a generic function doing it, but the designers of CLOS provided
|> slot-value for occasions like this.
|> 
|> (setf (slot-value self (slot-to-change button)) value)

Unfortunately, this is not an acceptable substitute in many situations.  Since
the setf method can have customized features (indeed, there does not even need
to be a slot defined to have a setf method) this hack won't work.

I agree that slot-value is a useful (if dangerous) feature, but it doesn't
take the place of the above case.  One bad (but functional) solution is:

	(eval `(setf (,(slot-to-change button) self) value))

Which forces run-time evaluation, but is semantically correct.  A simple macro
can hide this detail as follows:

	(defmacro setf-indirect (indirect object new-value)
             `(setf (,(eval indirect) ,object) ,new-value))

And this could be called as:

	(setf-indirect (slot-to-change button) self value)

I'm sure people could write this into nicer syntax, but this seems to be
the best approach to solving that problem for occasional use.  If
high-performance were an issue, I'd suggest storing a pointer to the
setf-method instead of the name of the slot.

Joe Konstan