In CLOS, instance remorphing considered useless in practice?

[Robert Goldman]

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?