SETF of APPLY

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

I agree with your proposed revised description of SETF of APPLY.  The specific
code in our implementation of APPLY's SETF-method follows; it may be useful
in constructing the English description.  This is not the whole SETF-method,
just the relevant code fragment.

(DEFINE-SETF-METHOD APPLY (FUNCTION &REST ARGS)
  ....
  (SETQ FUNCTION (SECOND FUNCTION))
  (MULTIPLE-VALUE-BIND (VARS VALS STORE-VARS STORE-FORM ACCESS-FORM)
      (GET-SETF-METHOD-MULTIPLE-VALUE (CONS FUNCTION ARGS))
    (LET ((LIST-VAR (LOOP FOR VAR IN VARS AND VAL IN VALS
			  WHEN (EQ VAL (CAR (LAST ARGS))) RETURN VAR)))
      (OR (AND LIST-VAR
	       (EQ (CAR (LAST ACCESS-FORM)) LIST-VAR)
	       (EQ (CAR (LAST STORE-FORM)) LIST-VAR))
	  (ERROR "APPLY of ~S not understood as a generalized variable" FUNCTION))


While SETF of APPLY of AREF could be implemented some other way, not involving
the last argument to the original form remaining last in the access-form and
store-form, I think it is necessary for SETF of APPLY of a user-defined function
to work this way.

This is all certainly ugly, but I still think the alternative (a separate
DEFSETF system for functions seen inside APPLY) would be a lot worse.