Re: Fwd: C-[ is undefined

[John Shahid]

Ben Bacarisse <ben.lists@bsb.me.uk> writes:


[...]

>>>> Short story: ~C-x @ c [~ results in "C-[ is undefined" but I expected
>>>> it to be the Meta prefix like in for example ~C-[ t~ for ~M-t~.
>>>
>>> I would expect C-x @ c [ to be the same as C-[
>>
>> Did you try? For me it is not the same which is what I wanted to say
>> in the first place.
>
> Yes, I tried it, but my interpretation of the result ("C-[" is
> undefined) was that C-x @ c [ was a way to generate a "key" that could
> not otherwise be typed.
>
> That's kind of what happens.  For example, you can bind C-[ this way and
> still use C-[ to get the ESC- prefix.
>
> However, that seems like an unlikely explanation!  It is probably
> something to do with translated key sequences.  ESC and C-[ are not
> exactly the same, but I am not expert enough to explain the way in which
> they differ.

I think there is an implicit assumption in this thread that Emacs is
receiving key press events for both the 'Control' and '[' keys.  I tried
this on my system using 'xev' and it receives a KeyPress event for
'Control' but as soon as I hit '[' the window receives a KeyPress event
for Escape (i.e. '0x1b').  My conclusion is that Emacs doesn't even know
that you pressed 'C-['.

You can probably achieve the same result by translating 'C-[' to
'Escape' in the 'key-translation-map'.  This map is used after
'local-function-key-map' where the 'event-apply-control-modifier'
applies the control modifier.  Another idea is to add an advice to
'event-apply-control-modifier'.

cheers,

-js