Face text property for `minor-mode-alist` item.

Face text property for `minor-mode-alist` item.

[Narendra Joshi]

Hi everyone,

I am trying to understand the following behaviour regarding
`mode-line-format' rendering the `:lighter` for `flycheck-mode'. I
wanted to have the flycheck lighter display different colors for the
error count and the warning count as `flymake` does.

The following doesn't work. The face property of the first character of
the string returned by `flycheck-mode-line-status-text' is used for the
whole string in this case.  

--8<---------------cut here---------------start------------->8---

(setq flycheck-mode-line '(:eval (flycheck-mode-line-status-text))
(defun flycheck-mode-line-status-text ()
    ... returns a single string with substrings propertized,
    e.g. #("some string" 1 2 ('face 'error) 2 3 ('face 'warning)))

--8<---------------cut here---------------end--------------->8---

If I change it to something like this: 

--8<---------------cut here---------------start------------->8---

(setq flycheck-mode-line '(:eval (flycheck-mode-line-status-text))
(defun flycheck-mode-line-status-text ()
    ... returns a (list ... part_1_with_face_1  part_2_with_face_2 ...))

--8<---------------cut here---------------end--------------->8---

This seems to work as expected and the parts have their respective faces
applied to them in the mode-line. 

Is this a bug in Emacs or is this expected behaviour?

Best regards,
-- 
Narendra Joshi

Re: Face text property for `minor-mode-alist` item.

[Lars Ingebrigtsen]

Narendra Joshi <narendraj9@gmail.com> writes:

> Is this a bug in Emacs or is this expected behaviour?

It's expected (but possibly under-documented?) behaviour -- the mode
line code tries to be super-efficient, and doesn't allow text to have
different text properties.  As you've discovered, it only uses the text
property from the first character, and uses that over the entire text
fragment.

You have to use the special (:propertize ...) mode line constructs (and
chop up the text) to get more fine-grained text properties in the mode
line.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

Re: Face text property for `minor-mode-alist` item.

[Narendra Joshi]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Narendra Joshi <narendraj9@gmail.com> writes:
>
>> Is this a bug in Emacs or is this expected behaviour?
>
> It's expected (but possibly under-documented?) behaviour -- the mode
> line code tries to be super-efficient, and doesn't allow text to have
> different text properties.  As you've discovered, it only uses the text
> property from the first character, and uses that over the entire text
> fragment.

Thanks for clarifying this. I did check the Emacs Lisp manual entries
for mode line constructs and couldn't find any documentation about this.

> You have to use the special (:propertize ...) mode line constructs (and
> chop up the text) to get more fine-grained text properties in the mode
> line.

Best regards,
-- 
Narendra Joshi

Re: Face text property for `minor-mode-alist` item.

[Lars Ingebrigtsen]

Narendra Joshi <narendraj9@gmail.com> writes:

> Thanks for clarifying this. I did check the Emacs Lisp manual entries
> for mode line constructs and couldn't find any documentation about this.

Thanks for checking; I've now documented this quirk in the Emacs Lisp
manual in Emacs 28 (in the "Mode Line Data" node).

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

Re: Face text property for `minor-mode-alist` item.

[Narendra Joshi]

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Great, thanks! :)

On Mon, 28 Dec 2020, 02:19 Lars Ingebrigtsen, <larsi@gnus.org> wrote:

> Narendra Joshi <narendraj9@gmail.com> writes:
>
> > Thanks for clarifying this. I did check the Emacs Lisp manual entries
> > for mode line constructs and couldn't find any documentation about this.
>
> Thanks for checking; I've now documented this quirk in the Emacs Lisp
> manual in Emacs 28 (in the "Mode Line Data" node).
>
> --
> (domestic pets only, the antidote for overdose, milk.)
>    bloggy blog: http://lars.ingebrigtsen.no
>

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