SETF-MULTIPLE-STORE-VARIABLESGET-SETF-METHOD-MULTIPLE-VALUE on page 107 of CLtL states that there are no cases in Common Lisp that allow multiple values to be stored into a generalized variable. This is seen by some as an arbitrary decision in light of the fact that a very reasonable semantics exists for multiple values being assigned by several Common Lisp macros, including SETF. The rationale on page 103 of CLtL suggests that this decision might be changed in the future.SETF, PSETF, SHIFTF, ROTATEF, and ASSERT to allow "places" whose SETF methods have more than one "store variable". In such cases, the macros accept as many values from the newvalue form as there are store variables. As usual, extra values are ignored and missing values default to NIL.
Extend the long form of DEFSETF to allow the specification of more than one "store variable", with the obvious semantics.
Clarify that GET-SETF-METHOD signals an error if there would be more than one store-variable.
(defun region-size (region) (values (region-width region) (region-height region)))
(defsetf region-size (region) (width height) `(values (setf (region-width ,region) ,width) (setf (region-height ,region) ,height)))
(setf my-reg (make-region :width 10 :height 20)) => #S(REGION :WIDTH 10 :HEIGHT 20)
(region-size my-reg)
=> 10
20
(setf (region-size my-reg) (values 30 40))
=> 30
40
(region-size my-reg)
=> 30
40
SETF method for the VALUES function, yielding a powerful MULTIPLE-VALUE-SETF form:
(setf (values (car a) (gethash b 'c) (aref d 13))
(some-hairy-computation))
In the language as currently defined, this example would have to be written:
(multiple-value-bind (x y z)
(some-hairy-computation)
(setf (car a) x
(gethash b 'c) y
(aref d 13) z))
Many other (perhaps more compelling) examples of generalized variables holding more than one value can easily be imagined. Their use, however, is severely discouraged by Common Lisp as defined in CLtL, since none of the built-in macros will accept them.
The clarification of GET-SETF-METHOD makes explicit what is implied by CLtL (CLtL uses the word "guarantee", whose relationship to signalling of errors is unclear).
GET-SETF-METHOD. Lucid signals an error in GET-SETF-METHOD. Symbolics Genera supports the proposal in SETF and PSETF, but not in SHIFTF, ROTATEF, and ASSERT.SETF itself, one need only call GET-SETF-METHOD-MULTIPLE-VALUE instead of GET-SETF-METHOD and emit a MULTIPLE-VALUE-BIND instead of a LET for binding the store variables.GET-SETF-METHOD-MULTIPLE-VALUE).
Moon supports this proposal except he is not sure about the inclusion of ASSERT.
GSB suggests that this is a clarification rather than an addition, because the lack of any predefined setf-methods that use multiple store variables should not mean that SETF, etc. should not work with such a setf-method if the user defined one. The problem is that CLtL examples such as the ones for SHIFTF on p.98 and the simplified definition for SETF on p.107 contradict this proposal, and might have been taken as specifications, rather than simplified examples, by some readers.
Predefined SETF methods for such functions as VALUES, CONS, and VECTOR could have been proposed, but we refrained. This proposal is necessary to allow the user to write such methods for himself, but if this proposal is adopted those setf-methods are very easy to write in a portable fashion.