CLHS: Issue EQUAL-STRUCTURE Writeup

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## Issue EQUAL-STRUCTURE Writeup

    Status:	Passed as amended, Jun 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)
    		3-Jul-89, Version 8, by Masinter (amended as per Jun89 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 here:

       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                (see below)
       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.

      EQUALP on two DEFSTRUCT objects 's1' and 's2', where one is a
      non-:TYPEed DEFSTRUCT and the other is typed, is false.

      EQUALP on two DEFSTRUCT objects 's1' and 's2', where both are
      non-:TYPEed DEFSTRUCTS is true iff:

      (1) The type of 's1' is the same as the type of 's2' (this is
       the same as saying that the defstruct name for 's1' is the same
       as that for 's2').

      (2) The value of each slot of 's1' is EQUALP to the value of the
      same slot of 's2' (where "same" means same name) (this is not the
      same as 'slots' for standard-objects in CLOS).

    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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