In CLOS, instance remorphing considered useless in practice?

Jean-Claude Beaudoin jean.claude.beaudoin at
Mon Dec 7 04:52:57 UTC 2020

Hello Pros of Common Lisp,

Here is my attempt at starting a significant (and hopefully useful) debate
on a subject squarely about Common Lisp and its internals.

My main stance here is to state that I have yet to see, in a significant
application, any use of functions #'cl:make-instance-obsolete and
#'cl:update-instance-for-redefined-class and of the underlying machinery
that support the remorphing of instances following a class redefinition
(through a subsequent cl:defclass most likely). I can state the same thing
about #'cl:update-instance-for-different-class and #'cl:change-class.

The machinery required of any CL implementation to properly support those
functions (mentioned here above) is quite significant in its complexity and
usually imposes a sizeable performance penalty on the speed of any instance
slot access.

A different choice of class redefinition semantics can lead to an
implementation of CLOS with much reduced instance slot access overhead.

The necessity of supporting instance remorphing should therefore be well
motivated by very significant gains in application code performance (in
speed and/or size and/or expressiveness power).

Can anyone of you point me to some evidence of such application level
Is there any notorious usecase of this (instance remorphing) since its
inclusion into ANSI CL?

By the way, at the bottom of the entry about #'cl:change-class in the text
of the CL standard, one can find a "Notes:" section that starts with this
sentence: "The generic function change-class has several semantic
difficulties." And this was written in the context of a single-threaded
implementation, as ANSI-CL limited itself. I bet that almost all currently
significant CL implementations are now multi-threaded, therefore the
"several semantic difficulties" are greatly and gravely compounded. In my
opinion, this makes proper motivation of usefulness all the more imperious.
What do you think?
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