Custom C-M-; key binding

Question

I have a custom comment-dwim-line command that I want to bind to the key combination C-M-; (in other words, control, meta and ; buttons pressed simultaneously). The built-in comment-dwim command is bound to M-; by default, and I want to keep this binding.

So far I have tried:

(global-set-key "\C-M-;" 'comment-dwim-line)

which throws global-set-key: Key sequence RET - ; starts with non-prefix key RET.

(global-set-key "\C-\M-;" 'comment-dwim-line)

which throws load-with-code-conversion: Invalid modifier in string.

(global-set-key "C-\M-;" 'comment-dwim-line)

which throws global-set-key: Key sequence C - M-; starts with non-prefix key C.

(global-set-key [?\C-M-;] 'next-line)

which throws read--expression: Invalid read syntax: "?".

(global-set-key [?\C-\M-;] 'next-line)

which throws Symbol's value as variable is void: M-.

Finally

(global-set-key (kbd "C-M-;") 'comment-dwim-line)

does not throw an error, but the key binding has no effect: when I actually press C-M-; it behaves the same as M-;(default comment-dwim is called).

I am using Emacs 24.4 on Mac OS X 10.10, running in iTerm2.

Answer 1

Your terminal likely translates C-M-; to M-;. Try testing it out in GUI Emacs.

If it works in the GUI, you'll know it's iTerm2 causing the issue.

Answer 2

Terminal emulators do not forward all possible key combinations to the shell, so Emacs likely does not see C-M-; when used in the terminal. You can visualize the command processed by your terminal emulator (iTerm2 in your case) by pressing C+v (CTRL+v) in the terminal, followed by a key combination. For C-v C-M-; I get with the XFCE terminal under Linux

$ ^[;

which is the same as M-;, so the CTRL key is basically ignored.

A possible workaround is to define an alternate key combination in Emacs, let us say C-c M-;, and to instruct iTerm2 to forward this combination when you press C-M-;. This way you can use the keys that you want in iTerm2, but Emacs will behave as if you pressed C-M-;. I have done this previously, but I do not have a Mac at hand right now to search for the iTerm2 options.

Edit: Just found this related question: Shift+Up isn't recognized by Emacs in a terminal

Answer 3

You forgot a backslash, \, before M.

(global-set-key [?\C-\M-;] 'next-line)

This one is OK (except you forgot the final )).

(global-set-key (kbd "C-M-;") 'comment-dwim-line)

This one does not work for ; (but it would work for "\C-\M-y", for instance).

(global-set-key "\C-\M-;" 'comment-dwim-line)

---

Finally, you say:

(global-set-key (kbd "C-M-;") 'comment-dwim-line) does not throw an error, but the key binding has no effect: when I actually press C-M-; it behaves the same as M-; (default comment-dwim is called).

I don't see that. Do you see this problem if you start Emacs using emacs -Q (no init file)? If so, provide a step-by-step recipe, starting with emacs -Q to reproduce it. If not, bisect your init file to find the culprit.

It works fine for me, on MS Windows, with all versions of Emacs (20 and later):

1. emacs -Q --debug-init -nw

2. Evaluate this:

   (defun comment-dwim-line () (interactive) (message "FOOOOOOOOO"))
   (global-set-key (kbd "C-M-;") 'comment-dwim-line)
   

3. C-h k C-M-;

   C-M-; runs the command comment-dwim-line (found in global-map), which is an interactive Lisp function.

   It is bound to C-M-;.

   (comment-dwim-line)

   Not documented.

4. C-M-; shows the message FOOOOOOOOO in the echo area.