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## Issue NINTERSECTION-DESTRUCTION Writeup

    Issue:        NINTERSECTION-DESTRUCTION
    Document:     X3J13/91-412
    Reference:    CLtL-II, p.429
                  Issue REMF-DESTRUCTION-UNSPECIFIED
                  Draft 10.156 (X3J13/91-103)
                  Draft 12.24 (X3J13/92-102)
                  Barrett's public review comment #24 (X3J13/92-2524)
    Category:     CLARIFICATION
    Edit History: Version 1, 22-Dec-91, Kim Barrett
                  Version 2, 19-Apr-92, Kim Barrett
                    (Two proposals, note confused status)
                  Version 3, 20-Apr-92, Kim Barrett (KMP's comments)
                  Version 4, 7-Jun-92, Kim Barrett
    Status:       Confusion!  One of the proposals (REVERT or PERMIT) was passed at
                  the 12/91 meeting, but there is disagrement as to which.
                  Draft 12.24 (X3J13/92-102) was written assuming that REVERT was
                  the passed proposal.
                  Barrett now asks us to reconsider proposal PERMIT.
    	      Proposal REVERT passed 11-0 on letter ballot 93-302;
                  on same ballot, proposal PERMIT failed 2-6.

    Problem Description:

     Issue REMF-DESTRUCTION-UNSPECIFIED permits NINTERSECTION to modify either list
     argument, contrary to the description of NINTERSECTION in CLtL, which
     explicitly states that the second argument (list2) is not destroyed.  It is
     questionable whether this change was intentional or not, especially since
     REMF-DESTRUCTION-UNSPECIFIED was categorized as a clarification rather than a
     change.

    Proposal (NINTERSECTION-DESTRUCTION:REVERT):

     Affirm the wording from CLtL, prohibiting NINTERSECTION from modifying the
     second (list2) argument.

    Proposal (NINTERSECTION-DESTRUCTION:PERMIT):

     Affirm the wording from REMF-DESTRUCTION-UNSPECIFIED, permitting NINTERSECTION
     to modify either list argument.

    Editorial Impact:

     For REVERT:
       None.  The 12.24 draft assumes that REVERT was accepted.

     For PERMIT:
       Small.  The two-line paragraph on p.14-51 of X3J13/92-102 describing the
       difference between NINTERSECTION and INTERSECTION would have to be changed
       slightly, specifying that both the LIST-1 and LIST-2 arguments may be
       destroyed, rather than only permitting LIST-1 to be destroyed and requiring
       that LIST-2 be preserved.

    Rationale:

     For REVERT:
       The intent of REMF-DESTRUCTION-UNSPECIFIED was not to change side-effect
       behavior, but to tack down places where we had all pretty much agreed on
       side-effect behavior but never spelled it out.  This was a case where CLtL
       had spelled things out, and which was probably changed unintentionally.
       Some older programs may depend on this argument not getting bashed, and it
       would be a shame to have some implementation decide to bash it.

       Don't make an apparently unintentional incompatible change.

     For PERMIT:
       Affirm the more recent decision, which went through more than two years of
       discussion before finally being accepted.  The intent of
       REMF-DESTRUCTION-UNSPECIFIED was to permit greater implementation freedom
       in order to achieve better performance; prohibiting the destruction of one
       of the lists may have a negative performance impact.

       A ``destructive'' function that is only permitted to destroy one of it's two
       arguments (with no rationale for which argument may be destroyed and which
       may not) is confusing; this is the only case of such.

    Examples:

    Current Practice:

     Apple Macintosh Common Lisp 2.0 obeys REVERT.

     Symbolics Genera 8.2 (and before) obeys REVERT.

    Cost to Implementors:

     None for PERMIT.

     If REVERT is adopted, implementations that have been tracking the passed
     cleanups and have taken advantage of the permission granted by
     REMF-DESTRUCTION-UNSPECIFIED in this case will have to change.  It may require
     a significant amount of work to achieve similar performance.  Implementations
     that pre-existed the acceptance of that proposal may be able to revert to an
     older source that obeyed the CLtL1 restriction.

    Cost to Users:

     For REVERT, no semantic cost for users; there may be a performance cost.

     For PERMIT, an incompatible change.  In some cases it may be quite difficult
     to determine whether a call to NINTERSECTION can safely destroy the second
     argument.

    Performance Impact:

     There may be a negative performance impact in choosing REVERT over PERMIT.

     The ``obvious'' implementation is to iterate over the elements of one of the
     lists and if that element appears in the other list then collect it, reusing
     storage from the first list in the collection process.  If the length of the
     first list is less than the length of the second, this algorithm is
     asymptotically as much as a factor of two slower than if the lists were
     exchanged.  Thus, by preventing such a choice of which list to destroy, REVERT
     might result in slower performance for NINTERSECTION.

    Discussion:

     The status of this issue is unclear.

        Date: Wed, 22 Jan 92 13:25:51 EST
        From: kab (Kim Barrett)
        To: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>
        Subject: Re: Issue: NINTERSECTION-DESTRUCTION (Version 1)

        >     Date: Sun, 22 Dec 1991 14:48 EST
        >     From: kab@cambridge.apple.com (Kim Barrett)
        >
        >     Does this look ok to you?
        >
        > No! My meeting notes say:
        >
        >  "Move to say list-2 not modified.  PASS 8-2"
        >
        > Lest you think I mis-noted this, I certainly remember lobbying for a
        > position which said that REMF-DESTRUCTION-UNSPECIFIED had made the
        > change unintentionally, and that we should revert the wording.  Had I
        > wanted to retain the wording, I would not have raised the issue.

        Looks like we have a problem here.  My notes just say

          "NINTERSECTION-DESTRUCTION passed 8-2"

        which is not very helpful.  My memory says that we voted to affirm the
        words in the draft (as stated in the draft writeup I sent you), but
        obviously I could be mistaken.  Unfortunately, the minutes are also
        ambiguous:

          REMF-DESTRUCTION-UNSPECIFIED
          (In document 91-003b.)
          Vote: 7 - 2. Passed.

        There isn't any such thing in the specified document.  (I pointed this out
        to Jan after the last version of minutes was mailed out).

     The reference to REMF-DESTRUCTION-UNSPECIFIED in 91-003b was to the following
     message:

        Date: Tue, 16 Jul 1991 17:33-0400
        From: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>
        Subject: possible issue NINTERSECTION-DESTRUCTION
        To: kab@chestnut.com
        Cc: KMP@STONY-BROOK.SCRC.Symbolics.COM, barmar@think.com, GLS@Think.COM

        [Addressed to Kim because he's been keeping a list of my little nitpicks
         and figuring out which shoudl be written up and distributed.  Other
         relevant individuals cc'd in case they have comments.]

        One of Barmar's reviews turned up the following...

        CLtL said plainly that list-2 was not destroyed by NINTERSECTION.

        REMF-DESTRUCTION-UNSPECIFIED says quite clearly that it might be, even
        though I think the intent of REMF-DESTRUCTION-UNSPECIFIED was mainly to
        clarify points left vague by CLtL, not to introduce new things that were
        possible to destroy.

        The current draft is inconsistent in that in different places, it says
        things that are consistent either with CLtL or with the cleanup issue.

        The status quo is well-defined, so if no vote is taken I will believe
        REMF-DESTRUCTION-UNSPECIFIED.   But I don't think that people realized
        they were making an incompatible change in this particular case, so I
        wonder if the issue should be raised at X3J13 for confirmation.

        As an aside, I note that CLtL2's wording also implies that this was a
        "clarification" without making it apparent that an actual change had
        occurred.

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