ielm automatic saving of history -- bug 67000

[Madhu]

* commit 60cff1ac9d216e5abcb350ea5e623ab0b377c131
|Author:     Simen Heggestøyl <simenheg@runbox.com>
|AuthorDate: Tue Jan 16 08:21:41 2024 +0100
|Commit:     Simen Heggestøyl <simenheg@runbox.com>
|CommitDate: Thu Feb 15 08:46:28 2024 +0100
|
|    Add support for reading/writing IELM input history (bug#67000)
|
|    * lisp/ielm.el (inferior-emacs-lisp-mode): Add support for saving input
|    history to a file.
|    (ielm--history-file-name): New variable indicating IELM input history
|    file.
|    (ielm--exit): Holds a function to call when Emacs is killed to write
|    out the input history.
|    (ielm--input-history-writer): Helper function for writing the IELM
|    input history out to file.


I see several problems with this implementation (based on opaque
closures), the worst of which is that I am unable to quit emacs, because
a closure (which tries to save the contents of a killed-buffer, is in
kill-emacs-hook, and it cannot get the coding system right)


The implementation roughly does:
```
      (setq-local comint-input-ring-file-name ielm-history-file-name)
      (setq-local ielm--exit (ielm--input-history-writer (current-buffer)))
      (setq-local kill-buffer-hook
                  (lambda ()
                    (funcall ielm--exit)
                    (remove-hook 'kill-emacs-hook ielm--exit)))
      (unless noninteractive
        (add-hook 'kill-emacs-hook ielm--exit))
      (comint-read-input-ring t)
```

2. The way to opt out of this is to set ielm-history-file-name to nil in
user customization.  In that case this code path should be avoided
altogether.

ielm--exit is a buffer local variable which gets set to
    ```
      (lambda ()
        (with-current-buffer buf
          (comint-write-input-ring)))
    ```

and pushed onto kill-emacs-hook. Another closure gets pushed onto
kill-buffer-hook, which removes the elt in kill-emacs-hook when it
runs.

The "worst" problem happens when ielm buffer exits without running
kill-buffer-hook ( -- historically I've used this pattern to avoid
saving history for a particular buffer, this is the way to opt out of
saving the history) but in this case it runs from the kill-emacs-hook,
and that forces me to choose a coding system to save the buffer. There
is no option to opt out.

I don't have any suggestions on how to solve this with this design.

3. The default Comint functions for storing eliisp history is less than
acceptable. it stores lines, while ielm and elisp permit multi-line sexp
forms.

personally I've used code (derived from sly so not probably fsf license)
in ielm-mode-hook to set up merge and save multiline forms using comint,
I'm attaching this file here, for reference.  However the currenlty
installed implementation in (inferior-emacs-lisp-mode) does not play
well with the this: It unconditionally touches comint history variables,
my code also has to set the comint variables to use the comint history
mechanism, and these are picked up with exit hooks.

Maybe others using the facility will have some opinions on this. My own
takeaway is it is not desirable to 1) force history saving mechanisms 2)
use opaque closures for implementing hook functions.

my-ielm-history.el
Description: Text document