In CLOS, instance remorphing considered useless in practice?

Robert Goldman rpgoldman at
Tue Dec 8 20:14:39 UTC 2020

Updating instances when classes are redefined is critical to interactive 
development with the REPL.

If you want to see why it's so critical, try working with Python and 
modifying a class definition.  You have to shut down the interpreter and 
start over, which is a huge pain if your system has a big, complex state 
you want to preserve.

On 6 Dec 2020, at 22:52, Jean-Claude Beaudoin wrote:

> 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
> usefulness?
> 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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