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Issue: EQUAL-STRUCTURE (Version 4)
- To: cl-cleanup@Sail.stanford.edu
- Subject: Issue: EQUAL-STRUCTURE (Version 4)
- From: masinter.pa@Xerox.COM
- Date: 1 Oct 88 15:20 PDT
- Cc: masinter.pa@Xerox.COM
- Line-fold: NO
JonL was right.. I tried to fix it. Ready for release?
References: EQUAL (p80), EQUALP (p81)
Edit history: 18-Mar-88, Version 1 by Pitman
8-Jun-88, Version 2 by Masinter (add Benson's proposal)
23-Sep-88, Version 3 by Masinter (remove all but STATUS-QUO)
1-Oct-88, Version 4 by Masinter (fix 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 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, descends
the structure of arrays, defstruct and CLOS instances. It uses EQ
for all other types; for example, it does not descend hash tables.
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.
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.