bug#71429: Inconsistent y-or-n-p prompt behavior in Emacs Lisp

[Stephen Berman]

On Sat, 08 Jun 2024 11:24:50 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> Date: Sat, 8 Jun 2024 09:03:37 +0200
>> From: Gabriele Nicolardi <gabriele@medialab.sissa.it>
>> 
>> I have the following Emacs Lisp code:
>> 
>> (progn
>>   (y-or-n-p "Test: ")
>>   (let ((search-spaces-regexp "\\(?:\\n?[\s\t]+\\|\n\\)?"))
>>     (y-or-n-p "Test: ")))
>> 
>> The first prompt from the y-or-n-p function appears as expected:
>> 
>> Test: (y or n)
>> 
>> However, the second prompt appears differently:
>> 
>> Test: (‘y’ or ‘n’)
>> 
>> I’m trying to understand why the second prompt format changes. What causes 
>> this inconsistency in the
>> y-or-n-p prompt?
>> 
>> I suspect it might be related to the search-spaces-regexp variable or how 
>> Emacs handles interactive
>> prompts, but I’m not sure. Any insights or explanations would be greatly 
>> appreciated!
>
> Stefan, can you please look into this?  It sounds like some issue with
> substitute-command-keys:
>
>   (substitute-command-keys "(\\`y' or \\`n') ")
>    => #("(y or n) " 1 2 (font-lock-face help-key-binding face 
> help-key-binding) 6 7 (font-lock-face help-key-binding face help-key-binding))
>
> But
>
>   (let ((search-spaces-regexp "\\(?:\\n?[\s\t]+\\|\n\\)?"))
>     (substitute-command-keys "(\\`y' or \\`n') "))
>    => "(\\‘y’ or \\‘n’) "
>
> I actually don't understand why we use \\`y' and \\`n' in y-or-n-p.
> Why those backslashes, and not just `y' and `n'?  That's your change
> in commit a36ecc408a.  If I remove the backslashes, the results are
> identical whether or not search-spaces-regexp is let-bound.

Removing the final '?' in the regexp, i.e.

(let ((search-spaces-regexp "\\(?:\\n?[\s\t]+\\|\n\\)"))
  (y-or-n-p "Test: "))

results in the second prompt appearing like the first one.  Likewise
with '*', but not with '+':

(let ((search-spaces-regexp " ?"))
  (y-or-n-p "Test: "))
=> Test: (\‘y’ or \‘n’)

(let ((search-spaces-regexp " *"))
  (y-or-n-p "Test: "))
=> Test: (\‘y’ or \‘n’)

(let ((search-spaces-regexp " +"))
  (y-or-n-p "Test: "))
=> Test: (y or n)

(let ((search-spaces-regexp " "))
  (y-or-n-p "Test: "))
=> Test: (y or n)

Steve Berman