Stefan,

I did not know about condition-case-unless-debug, it indeed does make
debugging possible with a simpler change and without any side effects. I
attach the patch.

Cheers,
Jarosław Rzeszótko

On Sat, Jan 13, 2018 at 10:22 AM, Jarosław Rzeszótko <jrzeszotko@gmail.com>
wrote:

> I attach a patch for ielm.el that makes it possible to enter the debugger
> when executing expressions in ielm, and that makes ielm respect the
> debug-on-error and debug-on-exit flags.
>
> I guess that possibly the reason that was not originally done is that it
> is not obvious how to restore ielm to a usable state after the debugger has
> been entered. My patch makes use of the fact that unwind-protect unwind
> forms are still executed when user resumes execution from the debugger
> after an error.
>
> Without the patch, ielm wraps the evaluation of the expression given by
> the user in a condition case, and in case of an error, or exit, displays an
> appropriate message in its buffer, right under the evaluated expression,
> regardless of debug-on-error and debug-on-exit.
>
> With the patch, the message that ielm displays in its buffer will be a
> generic error message  regardless if there was an error in the evaluated
> form, or a quit. However:
>
> - if debug-on-error is t, emacs will enter the debugger and show a stack
> trace, just like with almost any other evaluation method. When the user
> continues the execution from the debugger, ielm will correctly resume
> execution
>
> - if debug-on-error is nil, emacs will display the specific error in the
> minibuffer anyway
>
> Similar things are true for debug-on-quit.
>
> I think this is an improvement over the current state of affairs. Of
> course ideally I would like to preserve the "nice" message in ielm buffer
> and make the improvements I made, but emacs does not seem to make it
> possible to do some handling of an error and then to re-raise it while
> preserving the original stack trace.
>
> Cheers,
> Jarosław Rzeszótko
>
>
>