[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Issue: EQUAL-STRUCTURE (Version 6)
- To: CL-Cleanup@SAIL.Stanford.EDU
- Subject: Issue: EQUAL-STRUCTURE (Version 6)
- From: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>
- Date: Wed, 11 Jan 89 14:39 EST
- Cc: KMP@STONY-BROOK.SCRC.Symbolics.COM
Here's another draft. I changed only the proposal part. It tries
to correct the EQUALP problem we and others mentioned in mail.
Please read this very carefully before voting in favor of it.
There were a lot of Yes votes for the last version, which I think
had some serious bugs in it. This would be a very bad issue for
us to screw up.
Things that might need special attention:
- Moon contends that standard practice in Symbolics Lisp is
for instances to be compared using EQ under EQUALP, not by
descending. There may be performance issues involved here.
Some agreement needs to be reached.
- Neither the previous version of the proposal nor CLtL was
clear on what happens to pathnames under EQUALP. This showed
up when I converted the presentation below. That issue should
be addressed as well.
Hopefully if this version of the proposal isn't something you want to
vote yes for, at least it's in a suitable form for easy line-item
changes interactively in the meeting.
References: EQUAL (p80), EQUALP (p81)
Edit history: 18-Mar-88, Version 1 by Pitman
08-Jun-88, Version 2 by Masinter (add Benson's proposal)
23-Sep-88, Version 3 by Masinter (remove all but STATUS-QUO)
01-Oct-88, Version 4 by Masinter (fix description)
01-Oct-88, Version 5 by Pitman (correct wording, add discussion)
11-Jan-89, Version 6 by Pitman (attempt EQUALP correction)
The behavior of EQUAL and EQUALP on structures is a subject of controversy.
At issue are whether these functions should descend the slots of structures
or use simply the structure's primitive identity (i.e., EQ) to test for
Clarify that EQUAL and EQUALP do not descend any structures or data types
other than the ones explicitly specified in CLtL.
Type EQUAL Behavior EQUALP Behavior
Number uses EQL uses =
Character uses EQL uses CHAR-EQUAL
Cons descends descends
Bit-Vector descends descends
String descends descends
Pathname magic per CLtL same as EQUAL
Structure uses EQ descends
other Array uses EQ descends
Instance (Standard-Object) uses EQ descends
Hash-Table uses EQ uses EQ
Other uses EQ uses EQ
Note that the order of this table is in some cases important, with upper
entries taking priority over lower ones.
There seem to be as many different equality primitives as there
are applications for them. None of the possible ways of changing
EQUAL or EQUALP are flawless. Given the inability to "fix" them,
it is better to leave them alone.
We are unaware of any extensions to CLtL's set of operations,
although frequently users request them.
Cost to Implementors:
Since this seems to be compatible with the status quo, none.
Cost to Users:
Cost of Non-Adoption:
Ongoing controversy about whether EQUAL and EQUALP "do the right thing".
A feeling that EQUAL and EQUALP exist and/or do what they do because serious
consideration was given and we consciously decided on a particular resolution
to the numerous questions that have come up about them.
There seems to be wide debate about what the proper aesthetics for
how equality should work in Common Lisp. While the status quo is not
aesthetically more pleasing than the various alternatives, aesthetic
considerations vary widely. Different people model structures
differently. Sometimes the same person models structures differently in
different situations. The question of which should be descended and which
should not is a very personal one, and the aesthetic attractiveness of any
of these options will vary from person to person or application to
An earlier version of this issue with various alternatives was distributed
at the June 1988 X3J13 meeting. Since
this is a frequently raised issue, we thought we should submit it
as a clarification although there is no change to CLtL.
Options for which we considered proposals were:
- removing EQUAL and EQUALP from the standard.
- changing EQUALP to descend structures.
- changing EQUALP to be case sensitive.
- adding a :TEST keyword to EQUAL.
- making EQUAL a generic function
All of these had some serious problems.
The cleanup committee supports option STATUS-QUO.
It would be useful if descriptions of EQUAL and EQUALP contained some sort
of additional commentary alluding to the complex issues discussed here.
The following is offered to the Editorial staff as a starting point:
Object equality is not a concept for which there is a uniquely
determined correct algorithm. The appropriateness of an equality
predicate can be judged only in the context of the needs of some
particular program. Although these functions take any type of
argument and their names sound very generic, EQUAL and EQUALP are
not appropriate for every application. Any decision to use or not
use them should be determined by what they are documented to do
rather than any abstract characterization of their function. If
neither EQUAL nor EQUALP is found to be appropriate in a particular
situation, programmers are encouraged to create another operator
that is appropriate rather than blame EQUAL or EQUALP for ``doing
the wrong thing.''