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I have some customisations in ~/.emacs.el. Today I refactored a piece that used dolist, directory-files, and find-file. I am now unable to quit Emacs in the normal way and I get this from the mini-buffer:

Wrong type argument: hash-table-p, nil

I only have one kill hook, which calls a function mu4e-mark-execute-all defined in mu4e:

(require 'mu4e)
(defun mu4e-execute-marks ()
   (interactive)
   (mu4e-mark-execute-all t)
)
(add-hook 'kill-emacs-hook 'mu4e-execute-marks)

I understand that the LISP debugger is for evaluating expressions or functions. Can I set a stacktrace to see the cause of the error?

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    Where is mu4e-execute-marks defined? – muffinmad May 28 '19 at 14:06
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    You get a backtrace if you switch on "Enter Debugger on Error" in the "Options" menu. You can disable all kill hooks with M-: (let (kill-emacs-hook) (kill-emacs)). – Tobias May 28 '19 at 15:01
  • @muffinmad It is defined in mu4e and I updated the question. – miguelmorin May 28 '19 at 15:03
  • @Tobias Your code to remove all hooks actually quits Emacs on macOS. You are right about "Enter Debugger on Error", that's what I wanted, and I found the culprit in mu4e throwing an error when it has no marks to execute, so I wrapped the call in (condition-case ...). Do you want to write an answer? – miguelmorin May 28 '19 at 15:22
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What is the actual problem and what can I do about it?

The hash is only initialized in mu4e-headers-mode. Therefore, you should correct mu4e-execute-marks in the following way if you want to call it in kill-emacs-hook:

(defun mu4e-execute-marks ()
   (interactive)
   (when (derived-mode-p 'mu4e-headers-mode)
     (mu4e-mark-execute-all t))
)

How to debug such errors?

Switch on "Enter Debugger on Error" in the "Options"-menu. That meu item runs the command toggle-debug-on-error that just toggles the value of the variable debug-on-error between nil and t.

First sentence of the doc of debug-on-error:

Non-nil means enter debugger if an error is signaled.

The debugger includes the backtrace.

Alternatively you can instrument mu4e-mark-execute-all with M-x edebug-on-entry RET mu4e-mark-execute-all RET.

edebug is an interactive debugger. You can step through the function with the space button and you can display the backtrace with the menu item "Edebug"->"Views"->"Show Backtrace".

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  • Well done. Your ELisp code executes marks when I quit Emacs from the mu4e mode but does not execute marks if I quit outside that mode. – miguelmorin May 28 '19 at 16:20
  • The command M-x edebug-on-entry is unavailable on my machine. Is edebug available on macOS or only on Linux? – miguelmorin May 28 '19 at 16:21
  • @mmorin Could you try M-x load-library edebug.el before calling edebug-on-entry? – Tobias May 28 '19 at 20:42
  • That works and M-x edebug-on-entry succeeds. I enter the function that throws the error and nothing happens. If I call that same function directly, I see the error Wrong type argument in the mini-buffer. – miguelmorin May 29 '19 at 7:38

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