Issue: EQUAL-STRUCTURE (Version 5)

[Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>]

Per Masinter's request, here's a version with my comments edited in.
 - Changed wording of EQUALP portion of Proposal to change
   "CLOS instance" to "instance" and "defstruct instance" to "structure".
 - Added Cleanup committee endorsement for option STATUS-QUO in Discussion.
 - Added editorial advice in Discussion.

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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)

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:STATUS-QUO):

  Clarify that EQUAL and EQUALP do not descend any structures or
  data types other than the ones explicitly specified in CLtL. 
  EQUAL uses EQL for numbers and characters, descends structure for CONSes 
  bit-vectors, strings; has special behavior for pathnames as specified
  in CLtL,  and uses EQ for all other types. 

  EQUALP is similar, except that it ignores case in strings, and it
  descends arrays, structures, and instances. It uses EQ for
  all other types; for example, it does not descend hash tables.

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.''