How to analyze the value of `mode-line-misc-info'?

How to analyze the value of `mode-line-misc-info'?

[Kevin Vigouroux via Users list for the GNU Emacs text editor]

#+begin_src emacs-lisp
ELISP> mode-line-misc-info
((global-mode-string
  ("" global-mode-string " ")))
#+end_src

I don’t understand why there are two nested lists. I thought the second
list was used as a conditional statement (e.g. `global-mode-string'
returns non-`nil') to finally concatenate several strings.

• Emacs Lisp Ref. manual — 23.4.2 “The Data Structure of the Mode Line”
- https://www.gnu.org/software/emacs/manual/html_node/elisp/Mode-Line-Data.html
-- 
Kevin Vigouroux,
Best regards

Re: How to analyze the value of `mode-line-misc-info'?

[Kevin Vigouroux via Users list for the GNU Emacs text editor]

I have some difficulties with recursion, and especially with the
development of the line mode construct. Below, I have reasoned in terms
of strings (the final result), comparing two potential recursive
approaches. The mark ‘𐄂’ indicates a potential issue in my process. The
arrows indicates some kind of recursion.

#+begin_src emacs-lisp
ELISP> global-mode-string
("" display-time-string)
ELISP> display-time-string
"12:00"
#+end_src

#+begin_quote
((global-mode-string ("" global-mode-string " ")))
✓ ⤷ (global-mode-string ("" global-mode-string " "))
✓  ⤷ ("" global-mode-string " ")
𐄂   ⤷ "" ("" display-time-string) " "
✓ ⤷ void
𐄂 ⤴ "" ("" display-time-string) " "
𐄂 → ("" ("" display-time-string) " ")
𐄂 ("" ("" display-time-string) " ")
𐄂  ⤷ "" "12:00" " "
𐄂  ⤷ "12:00 "
#+end_quote

#+begin_quote
((global-mode-string ("" global-mode-string " ")))
✓ ⤷ (global-mode-string ("" global-mode-string " "))
✓  ⤷ ("" global-mode-string " ")
𐄂   ⤷ "" ("" display-time-string) " "
𐄂    ⤷ "" "12:00" " "
𐄂    ⤷ "12:00 "
✓ ⤷ void
#+end_quote

-- 
Kevin Vigouroux,
Best regards

Re: How to analyze the value of `mode-line-misc-info'?

[Emanuel Berg via Users list for the GNU Emacs text editor]

Kevin Vigouroux via Users list for the GNU Emacs text editor wrote:

> I have some difficulties with recursion [...]
>
> #+begin_quote
> ((global-mode-string ("" global-mode-string " ")))
> ✓ ⤷ (global-mode-string ("" global-mode-string " "))
> ✓  ⤷ ("" global-mode-string " ")
> 𐄂   ⤷ "" ("" display-time-string) " "
> ✓ ⤷ void
> 𐄂 ⤴ "" ("" display-time-string) " "
> 𐄂 → ("" ("" display-time-string) " ")
> 𐄂 ("" ("" display-time-string) " ")
> 𐄂  ⤷ "" "12:00" " "
> 𐄂  ⤷ "12:00 "
> #+end_quote
>
> #+begin_quote
> ((global-mode-string ("" global-mode-string " ")))
> ✓ ⤷ (global-mode-string ("" global-mode-string " "))
> ✓  ⤷ ("" global-mode-string " ")
> 𐄂   ⤷ "" ("" display-time-string) " "
> 𐄂    ⤷ "" "12:00" " "
> 𐄂    ⤷ "12:00 "
> ✓ ⤷ void
> #+end_quote

I see the problem, maybe write on help-gnu-emacs and ask?

-- 
underground experts united
https://dataswamp.org/~incal

Re: How to analyze the value of `mode-line-misc-info'?

[Kevin Vigouroux via Users list for the GNU Emacs text editor]

My problem is that there may be a flaw in my reasoning. I may not
perceive something, I am not very comfortable with recursion.

First issue: How to process (STRING REST...)?

1. ("" global-mode-string " ")
    ⤷ "" ("" display-time-string) " "
     ⤷ "" ("" "12:00") " "
     ⤷ "" "12:00" " "
     ⤷ "12:00 "

2. ("" global-mode-string " ")
    ⤷ ("" ("" display-time-string) " ")

Second issue: How to process (LIST REST...)?

1. (("" global-mode-string " "))
   ⤷ "" ("" display-time-string) " "
   ⤴ ("" ("" display-time-string) " ")

   → ("" ("" display-time-string) " ")
    ⤷ "" "12:00" " "
    ⤷ "12:00 "

2. ??
-- 
Kevin Vigouroux,
Best regards

Re: How to analyze the value of `mode-line-misc-info'?

[Yuri Khan]

On Mon, 14 Feb 2022 at 23:46, Kevin Vigouroux via Users list for the
GNU Emacs text editor <help-gnu-emacs@gnu.org> wrote:
>
> #+begin_src emacs-lisp
> ELISP> mode-line-misc-info
> ((global-mode-string
>   ("" global-mode-string " ")))
> #+end_src
>
> I don’t understand why there are two nested lists. I thought the second
> list was used as a conditional statement (e.g. `global-mode-string'
> returns non-`nil') to finally concatenate several strings.
>
> • Emacs Lisp Ref. manual — 23.4.2 “The Data Structure of the Mode Line”
> - https://www.gnu.org/software/emacs/manual/html_node/elisp/Mode-Line-Data.html

Let’s apply the documented behavior to the data structure you have.

> ((global-mode-string
>   ("" global-mode-string " ")))

This is a list and its first (and only) element is a list, so we
process it recursively and concatenate the only result.

> (global-mode-string
>  ("" global-mode-string " "))

This is a list whose first element is a symbol, so it’s a conditional.
We take the value of ‘global-mode-string’ as a variable. If it’s nil,
the result is nil; if non-nil, we interpret the ‘then’ part:

>  ("" global-mode-string " ")

This is a list whose first element is an empty string, so we
concatenate the results of processing each element.


As for your question:

> I don’t understand why there are two nested lists.

Probably so that modes could plug their own mode line constructs into
the outer list. For me, ‘mode-line-misc-info’ has this value:

    ((which-function-mode
      (which-func-mode
       (#1="" which-func-format " ")))
     (global-mode-string
      (#1# global-mode-string " ")))

so, two independent conditionals, whose results are to be
concatenated. (The first conditional has another conditional inside
it, so the display in fact depends on non-nilness of two variables.)

Re: How to analyze the value of `mode-line-misc-info'?

[Kevin Vigouroux via Users list for the GNU Emacs text editor]

Okay, I get it. Thank you!
-- 
Kevin Vigouroux,
Best regards