Path: utzoo!attcan!uunet!samsung!sdd.hp.com!elroy.jpl.nasa.gov!forsight!gat
From: gat@robotics.Jpl.Nasa.Gov (Erann Gat)
Newsgroups: comp.lang.lisp
Subject: Re: hard to write (was Re: STRUCTUREP)
Message-ID: <761@forsight.Jpl.Nasa.Gov>
Date: 13 Oct 90 20:53:54 GMT
References: <1990Oct10.130925.18317@hellgate.utah.edu> <756@forsight.Jpl.Nasa.Gov> <3534@skye.ed.ac.uk>
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 18

In article <3534@skye.ed.ac.uk>, jeff@aiai.ed.ac.uk (Jeff Dalton) writes:
> >                          (and pity the poor programmer who spends hours
> >searching through the book saying to himself, "There's just got to be a
> >way to test for structures, there's just got to be! :->)
> 
> And the people who assume EQUAL must descend structures, notice
> that it works in one CL, and then discover that it's not portable.
> There are a number of little traps in CL, I agree.

EQUAL doesn't descend structures!  That was news that sent me scurrying to
CLTL2, and indeed it is so.  But then I found the following puzzling
"Clarification":  The predicate EQUALP, it says, never descends structures
either.  (Actually what it says is that it never recursively descends
any structure or data type other than conses, arrays and pathnames.)  It
then goes on (on p. 109) to describe how EQUALP compares structures!  So
can you compare structures or not?

E.