Reference: CLtL-II, p.429
Draft 10.156 (X3J13/91-103)
Draft 12.24 (X3J13/92-102)
Barrett's public review comment #24 (X3J13/92-2524)
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.
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
Affirm the wording from CLtL, prohibiting NINTERSECTION from modifying the
second (list2) argument.
Affirm the wording from REMF-DESTRUCTION-UNSPECIFIED, permitting NINTERSECTION
to modify either list argument.
None. The 12.24 draft assumes that REVERT was accepted.
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.
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.
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.
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
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.
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: email@example.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
(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
Date: Tue, 16 Jul 1991 17:33-0400
From: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>
Subject: possible issue NINTERSECTION-DESTRUCTION
Cc: KMP@STONY-BROOK.SCRC.Symbolics.COM, firstname.lastname@example.org, 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