setf order of evaluation

[David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>]

    Date: 9 September 1987, 10:31:51 EDT
    From: Timothy Daly <DALY@ibm.com>

    Given

     (setq r '(a 1 b 2 c 3))
     (setq s r)
     (setf (getf r 'b) (progn (setq r nil) 6))

    what is the value of R and S?

    Note that P97 of CLtL states that SETF guarantees the
    left-to-right order of evaluation of its arguments.

    (yes, i know it is crufty to change R).

    Both Symbolics and Lucid seem to generate
     R = (B 8) and S = (A 1 B 2 C 3)

    which implies that the second argument of setf
    is evaluated first. What did I miss?

It's not a good idea to speak of "arguments" when dealing with
forms that aren't function calls.  It's less confusing to speak
of subforms.  The subforms of that setf that get evaluated are

  r
  'b
  (progn (setq r nil) 6)

so I think the behavior you describe is a bug (although the description
in CLtL is so ambiguous that the implementors of the respective systems
could easily disagree with me.)

I didn't look inside the Lucid implementation, but in the Symbolics 
implementation the source of the bug is that (get-setf-method 'r)
returns NIL, NIL, (#:G6411), (SETQ R #:G6411), R, whereas it should
return (#:G6410), (R), (#:G6411), (SETQ R #:G6411), #:G6410.  See
CLtL page 104 for the description of the meaning of these values.

Changing it to return the latter makes your SETF example behave as you
expected, evaluating R -before- bashing it, and setq'ing it to the updated
property list -after- setq'ing it to nil.