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Status:	Passed as amended, Jun 89 X3J13


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)

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



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).


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:


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.''

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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