bug#29220: 26.0.90; eieio-persistent-read fail to restore saved object.

[Noam Postavsky <npostavs@users.sourceforge.net>]

pierre.techoueyres@free.fr (Pierre Téchoueyres) writes:

> I try to understand how, from print_object, objects in the hash table
> are print.
> I expected something like
>
> prin1 -> print -> print_object -> something in lisp or C

The sequence is

prin1 -> print -> print_object -> print_vectorlike

print_vectorlike calls print_object on all of the keys and values of the
hashtable (see 'case PVEC_HASH_TABLE').

> I'm trying to make pcache work correctly with emacs 26.
> Now I understand that eieio--class and eieio--class-tag have changed,
> but I don't know if I can or how to correct the printing of objects in
> order to read them back with eieio-persistent-read.

I see 3 possibilities:

1. Make eieio persistent save stuff use cl-print to print the whole
thing readably (this requires major additions to cl-print, currently it
doesn't preserve readability except by just punting to prin1).

2. Change the class tag to something that will be easily readable, and
round-trip as eq (i.e., some kind of symbol, like in Emacs 25 and
earlier).

3. Change child-of-class-p to use `equal'ness instead of `eq'ness for
classes.  I think would require an `equal' that can handle circular
objects.

Looks like #2 would be the easiest to implement.