M-P: M-p is undefined

Question

I noticed something odd. If I press M-P I get the message M-p is undefined rather than M-P etc.. Is there a reason for this?

Version 26.2. Using Arch Linux and the X window system.

Try using describe-key on both keys. It might be translated. — , Apr 22 '19 at 16:38
@DoMiNeLa10, that works as expected. just curious about this behaviour. — Toothrot, Apr 22 '19 at 16:39
Does it mention that keys are translated in some way? If they are, that's your answer - someone set them up to be translated, and you might even get a link to see where it's been done. — , Apr 22 '19 at 17:14
I can't reproduce that from emacs -Q. Is this in a terminal? OS may also affect key input. — npostavs, Apr 22 '19 at 17:42
Uh, unless this is just confusion over notation, by "press M-P" did you mean press Alt+Shift+p or Alt+p? — npostavs, Apr 22 '19 at 17:50
Hmm, dunno then. If you (progn (define-key global-map (kbd "M-P") 'foo) (define-key global-map (kbd "M-p") 'bar)) do you get different results? — npostavs, Apr 22 '19 at 17:55

phils · Accepted Answer · 2019-04-23T00:07:15.767

It's a feature. Refer to this duplicate Q&A on the main S.O. site:

https://stackoverflow.com/a/54591047/324105

It's still slightly misleading, given that in your case the lower-case sequence also has no binding (as the docs imply that the translation only occurs when there is a lower-case binding), but I think it's all part and parcel of the same mechanism.

read_key_sequence contains the following comments:

  /* If KEY is not defined in any of the keymaps,
 and cannot be part of a function key or translation,
 and is an upper case letter
 use the corresponding lower-case letter instead.  */
 [...]
  /* We have to do this unconditionally, regardless of whether
     the lower-case char is defined in the keymaps, because they
     might get translated through function-key-map.  */
  [...]
  shift_translated = true;

M-P: M-p is undefined

1 Answers1