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From: Gallagher@GILGAMESH.CS.UMASS.EDU (Kevin Gallagher)
Newsgroups: comp.lang.clos
Subject: SLOT-VALUE and structures
Message-ID: <2880914762-3910514@Gilgamesh>
Date: 17 Apr 91 22:06:02 GMT
Sender: welch@tut.cis.ohio-state.edu
Distribution: inet
Organization: CommonLoops
Lines: 33


    From: Gregor Kiczales <Gregor@parc.xerox.com>  (via barmar@think.com)
    Date: Mon, 15 Apr 1991 09:37:22 -0700

    The MOP spec does not require SLOT-VALUE to work on structures.  This
    requirement was dropped in the final version because some people were
    concerned that it interfered with efficient structure implementations.

I am very disappointed to see this.  For several *years* the standard
answer to the question ``How do I access an arbitrary slot in a structure
instance?'' has been ``Use SLOT-VALUE (when CLOS is available).''  This
question is asked almost monthly on the various lisp mailing lists and
newsgroups.

Providing this capability does not prevent efficient structure
implementations.  All that is required is to maintain declarative
information about the implemenation of the structure.  Most lisps already
save this information anyway for the benefit of functions like describe
and inspect.  The defstruct generated slot access functions can still use
whatever implementation tricks you like.

In addition, it is not possible to implement this capability in Common
Lisp.  Many Common Lisp solutions have been proposed, however none of
them are completely general.

Kevin Gallagher
Gallagher@cs.umass.edu


P.S.  I have a set of implementation specific structure slot access
utilities for each lisp that I use, as I'm sure many other people do as
well.  I can certainly define SLOT-VALUE-USING-CLASS in terms of these,
but this is less than optimal.