Issue TEST-NOT-IF-NOT Writeup

status: Passed, as amended, Jan 89 X3J13

Note: some confusion in minutes as to whether COMPLEMENT was added

	as part of TEST-NOT-IF-NOT or whether FUNCTION-COMPOSITION was

	passed but with only COMPLEMENT preserved.

	In any case: TEST-NOT-IF-NOT:FLUSH-ALL as stated here

	passed with "Remove" -> "Deprecate". See issue

	FUNCTION-COMPOSITION for more details.


Forum: Cleanup

Issue:          TEST-NOT-IF-NOT

References:     Functions offering a :TEST-NOT keyword:

                 ADJOIN (p276), ASSOC (p280), COUNT (p257), DELETE (p254),

                 DELETE-DUPLICATES (p254), FIND (p257),

                 INTERSECTION (p277), MEMBER (p275), MISMATCH (p257),

                 NINTERSECTION (p277), NSET-DIFFERENCE (p278),

                 NSET-EXCLUSIVE-OR (p278), NSUBLIS (p275), NSUBST (p274),

                 NSUBSTITUTE (p256), NUNION (p276), POSITION (p257),

                 RASSOC (p281), REMOVE (p253), REMOVE-DUPLICATES (p254),

                 SEARCH (p258), SET-DIFFERENCE (p278), 

                 SET-EXCLUSIVE-OR (p278), SUBLIS (p274), SUBSETP (p279),

                 SUBST (p273), SUBSTITUTE (p255), TREE-EQUAL (p264),

                 UNION (p276);

                Functions with "-IF-NOT" in their name:

                 ASSOC-IF-NOT (p280), COUNT-IF-NOT (p257),

                 DELETE-IF-NOT (p254), FIND-IF-NOT (p257),

                 MEMBER-IF-NOT (p275), NSUBST-IF-NOT (p274),

                 NSUBSTITUTE-IF-NOT (p256), POSITION-IF-NOT (p257),

                 RASSOC-IF-NOT (p281), REMOVE-IF-NOT (p253),

                 SUBST-IF-NOT (p273), SUBSTITUTE-IF-NOT (p255);

Related Issue:  FUNCTION-COMPOSITION

Category:       CHANGE

Edit history:   02-Oct-88, Version 1 by Pitman (just FLUSH-ALL)

                05-Oct-88, Version 2 by Pitman (add option FLUSH-TEST-NOT)

                01-Dec-88, Version 3 by Masinter (add discussion)

                18-Mar-89, Version 4 by Masinter (as amended)


Problem Description:


  The -IF-NOT functions are functionally unnecessary.


  The :TEST-NOT keywords are not only functionally unnecessary but

  also problematic because it's not clear what to do when both :TEST

  and :TEST-NOT are provided.


  Many people think Common Lisp is more `bloated' than it needs

  to be and these aspects of the language are commonly cited

  specific examples.


Proposal (TEST-NOT-IF-NOT:FLUSH-ALL):


  Deprecate all -IF-NOT functions (named above) from Common Lisp.


  Deprecate the :TEST-NOT keyword from the Common Lisp functions which 

  currently provide them (named above).



  Rationale:

  

    This makes the language a bit simpler.

  

    The removal of :TEST-NOT also makes the language easier to explain.

  

  Cost to Implementors:

  

    Very slight.

  

    Some symbols would disappear from the LISP package but could

    still be offered in proprietary packages if deemed important

    enough.

  

    Implementations could compatibly retain the :TEST-NOT keywords

    for an interim period.

  

Current Practice:


  Presumably no one has done this yet.


Cost to Users:


  Some rewrites would be needed.


  Those rewrites, which are already fairly simple, would be even

  more simple if some form of the FUNCTION-COMPOSITION issue is

  voted in -- in particular, the COMPLEMENT function which it 

  proposes would help enormously in this regard.


Cost of Non-Adoption:


  Common Lisp would continue to be what some people feel is

  "bigger than it needs to be".


Benefits:


  The cost of non-adoption would be avoided.


Aesthetics:


  Presumably this makes the language easier to teach.


Performance impact:


  Very small; removing the :TEST-NOT keywords would

  make the simple implementation of the functions that

  used to have them slightly faster, but the resulting

  code of the inner loop is likely to be much slower.


Discussion:


  Many people objected strongly to this proposals --

  they might have been a nice idea five years ago, but are

  gratuitous incompatibilities now: incompatible changes with

  insufficient payback.


  Some of those objections might be tempered if some additional

  changes were made to Common Lisp: adding a COMPLEMENT

  function, or if there were a strategy to declare some parts of the

  language "obsolete". Since these conditions haven't been done,

  their objections stand.


  Steele noted that one main reservation to FLUSH-ALL is that

  he uses REMOVE-IF-NOT much more than REMOVE-IF.


  This issue is related to FUNCTION-COMPOSITION, but is not

  dependent on it.  Some support the combination of  FLUSH-ALL and 

  the NEW-FUNCTIONS part of FUNCTION-COMPOSITION in spite of

  the incompatible change because of the aesthetic appeal.


  Some people expressed their intention to vote for FLUSH-ALL

  only if FUNCTION-COMPOSITION:NEW-FUNCTIONS.


  It was noted that and

  adding a #~ readmacro such that 

      (FIND-IF-NOT #'ZEROP '(0 0 3))

   == (FIND-IF (COMPLEMENT #'ZEROP) '(0 0 3))

   == (FIND-IF #~ZEROP '(0 0 3))


  The modification of these functions is moot for those who

  prefer to use extended LOOP macro/iteration constructs

  in lieu of the sequence functions.


  Several alternative names for REMOVE-IF-NOT were

  suggested: KEEP-IF, ABSTRACT, FILTER. We did not

  pursue these suggestions.