Issue: REMF-MULTIPLE

[Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>]

Issue:        REMF-MULTIPLE
References:   REMPROP (p166), REMF (p167)
Category:     CLARIFICATION
Edit history: 26-Jan-88, Version 1 by Pitman
Status:	      For Internal Discussion

Problem Description:

  The descriptions of REMF and REMPROP are not explicit about what happens in
  the case that a duplicated indicator occurs on the plist. One or both indicators
  might be removed.

Proposal (REMF-MULITPLE:REMOVE-ONE):

  Clarify that REMF and REMPROP at most one indicator/value pair from the designated plist.

  Rationale:

    In a property list maintained by the normal property list operations, there will
    only be one property by each name. This approach won't waste time trying to remove
    other properties which are not present in the first place.

Proposal (REMF-MULTIPLE:REMOVE-ALL):

  Clarify that REMF and REMPROP remove all matching indicator/value pairs from the
  designated plist.

  Rationale:

    In a property list maintained by other operations than the standard ones,
    this might be useful. Also, since the return value of REMF and REMPROP is
    not well-defined, iterating to remove more than one property is expensive
    because you have to start over from the head of the list.

Test Case:

  ;; Case #1 - removing symbol properties,etc. using REMPROP

  (DEFUN REMF-MULTIPLE-TEST-1 ()
    (LET ((SYMBOL (GENSYM)))
      (SETF (SYMBOL-PLIST SYMBOL) (LIST 'A 'B 'C 'D 'A 'B 'C 'D))
      (FORMAT T "~&Before: ~S~%" (SYMBOL-PLIST SYMBOL))
      (REMPROP SYMBOL 'A)
      (FORMAT T "~&After:  ~S~%" (SYMBOL-PLIST SYMBOL))
      (FORMAT T "~&This implementation uses REMF-MULTIPLE:~:[REMOVE-ALL~;REMOVE-ONE~] ~
		   for REMPROP.~%"
	      (GET SYMBOL 'A))))

  ;; Case #2 - removing keywords,etc. using REMF

  (DEFUN REMF-MULTIPLE-TEST-2 ()
    (LABELS ((HELPER (&REST ARGUMENTS &KEY (A 1) (B 2))
	       (FORMAT T "~&Helper received: ~S~%" ARGUMENTS)
	       (LIST A B))
	     (DRIVER (&REST ARGUMENTS)
	       (FORMAT T "~&Helper received: ~S~%" ARGUMENTS)
	       (SETQ ARGUMENTS (COPY-LIST ARGUMENTS))
	       (REMF ARGUMENTS ':A)
	       (APPLY #'HELPER ARGUMENTS)))
      (LET ((RESULT (DRIVER :A 3 :B 4 :A 5 :B 6)))
	(FORMAT T "~&Returned: ~S~%" RESULT)
	(FORMAT T "~&This implementation uses REMF-MULTIPLE:~:[REMOVE-ALL~;REMOVE-ONE~] ~
		     for REMF.~%"
		(= (CAR RESULT) 5)))))

Current Practice:

  Symbolics implements REMF-MULTIPLE:REMOVE-ONE.

Cost to Implementors:

  For implementations needing to change, the cost of a change is probably very
  small in most cases. Implementations needing change which do REMPROP and/or REMF
  in microcode might have a slightly harder time, but any change should still be
  very localized.

Cost to Users:

  None. Users must tread lightly on this issue right now because it is not well-defined.

Cost of Non-Adoption:

  The language description would continue to be vague for no particularly good reason.

Benefits:

  Users who want to use REMPROP or REMF in situations which involve lists that might
  have duplicated elements would be able to do so more reliably.

Aesthetics:

  There is probably no particular aesthetic reason to think one of these solutions
  is better than the other, but having the issue nailed down is probably an aesthetic
  improvement.

Discussion:

  Pitman is agnostic on this for now. There are advantages to both.
  If everybody already implements REMOVE-ONE, that would seem the way to go.