Issue: EQUAL-STRUCTURE (Version 7)

[masinter.pa@Xerox.COM]

This is what I believe was voted in at the last meeting.

I'm not sure why we started discussing it again, but
I didn't see anything in the discussion that looked like
a proposal to revisit this issue.
!
Status:	Passed as amended, Jan 89 X3J13
Issue:        EQUAL-STRUCTURE
References:   EQUAL (p80), EQUALP (p81)
Category:     CLARIFICATION/CHANGE
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)
		15-Mar-89, Version 7 by Masinter (amended as per vote at Jan 89 X3J13)

Problem Description:

  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
  equivalence.

Proposal (EQUAL-STRUCTURE:MAYBE-STATUS-QUO):

  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                uses EQ
   other Array                uses EQ                descends
   Hash-Table                 uses EQ                (see below)
   Instance (Standard-Object) 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.

  EQUALP descends hash tables by first comparing the count of entries
  and the :TEST function; if those are the same, it compares the
  keys of the tables using the :TEST function and then the values
  of the matching keys using EQUALP recursively.


Rationale:

  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.

Current Practice:

  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:

  Same

Cost of Non-Adoption:

  Ongoing controversy about whether EQUAL and EQUALP "do the right thing".

Benefits:

  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.

Aesthetics:

  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
  application.

Discussion:

  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.''

!
Additional Comments to Version 6:

Version 6 attempts to fix some of the problems noted in Version 5.
There are still some open questions. Only the "Proposal"
part has been changed since Version 5; some of the costs,
benefits & other discussion is now incorrect.

Kent says:

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.


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