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Re: Issue: SETF-SUB-METHODS (Version 5)
- To: Jon L White <@sail.stanford.edu:firstname.lastname@example.org>, email@example.com
- Subject: Re: Issue: SETF-SUB-METHODS (Version 5)
- From: Jeff Dalton <jeff%aiai.edinburgh.ac.uk@NSS.Cs.Ucl.AC.UK>
- Date: Wed, 11 Jan 89 20:16:22 GMT
- Cc: firstname.lastname@example.org, email@example.com
- In-reply-to: Jon L White's message of Wed, 11 Jan 89 00:02:47 PST
I thought about this issue for a long time this morning and found
it very confusing. Eventualy, I decided that the proposal was
probably right, but it could have been better explained. What
finally convinced me was the thinking below (which I do not think
is necessarily the better explanation).
(setf (getf -variable- -expr1-) -expr2-)
expands into something like:
(let* ((#:temp0 -variable-) ;let's assume we do this binding for now.
(#:temp1 -expr1-) ;in the example, -expr2- is just 'B.
(#:temp2 -expr2-)) ;in the example, this does the setq.
(list-putprop [ ? ] #:temp1 #:temp2))
Let's assume LIST-PUTPROP returns the entire plist, which may be a new
list or a modification of the old.
Now the question is: what goes in [ ? ]? If it's #:temp0, then in
(setq s (setq r (list 'a 1 'b 2 'c 3)))
(setf (getf r 'b) (progn (setq r nil) 6))
LIST-PUTPROP would get the original value or R (before it was set to
NIL), and the results would be:
r ==> (a 1 b 6 c 3)
s ==> (a 1 b 6 c 3) or (a 1 b 2 c 3)
The other alternative is that [ ? ] is R. Then LIST-PUTPROP gets the
new value of R (i.e., nil), and the results are:
r ==> (b 6)
s ==> (a 1 b 2 c 3)
Now suppose we're doing
(setf (getf (car -variable-) -expr1-) -expr2-)
This expands into
(let* ((#:temp0 -variable-)
(setf (car #:temp0)
(list-putprop (car #:temp0) #:temp1 #:temp2))
Here, (car #:temp0) acts as a sort of pointer to what we want to
change. We can't point to a car (only to a whole cons), but we
want the car of that cons, not the cons that happens to be the
value of the variable later on. That's what (car ...) means as
a place. But a variable as a place wants a "pointer" to that
variable, not to it's value, because that's what a variable means
as a place. And such a "pointer" is just the variable itself.
This suggests that [ ? ] should be R. That is, we should dereference
the "pointer" when calling LIST-PUTPROP.