bug#14694: 24.3; format-mode-line failing with :eval form via a variable in a conditional construct

bug#14694: 24.3; format-mode-line failing with :eval form via a variable in a conditional construct

[Phil Sainty]

 From emacs -Q the following returns an empty string, which seems
like a bug:

ELISP> (setq indirect-eval '(:eval "indirect eval"))
ELISP> (format-mode-line '(t indirect-eval "else"))
""


Other tests have worked as expected:

ELISP> (format-mode-line '(t "then" "else"))
"then"

ELISP> (setq indirect-string "indirect string")
ELISP> (format-mode-line '(t indirect-string "else"))
"indirect string"

ELISP> (format-mode-line '(:eval "direct eval"))
"direct eval"

ELISP> (format-mode-line '(t (:eval "direct eval") "else"))
"direct eval"

ELISP> (setq indirect-eval '(:eval "indirect eval"))
ELISP> (format-mode-line indirect-eval)
"indirect eval"


It just seems to be the combination of the (:eval) form as the value
of a variable in a conditional form which is failing.



And a related question, if I may: Other than wrapping advice around
format-mode-line to let-bind a variable, is there any way for me to
ascertain whether a mode line construct is being processed for an
actual mode line redraw (in which case format-mode-line is not called),
as opposed to some other situation (e.g. describe-mode) in which
format-mode-line will be used?

My goal is to use the conditional construct to display the original
value when calling format-mode-line, but an alternative value in the
mode line itself, with the two values being stored in buffer local
variables. At the moment I'm using advice to supply the condition
(and of course I found the above bug when testing that my code worked
for the different types of construct).


thanks,
-Phil





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bug#14694: 24.3; format-mode-line failing with :eval form via a variable in a conditional construct

[Eli Zaretskii]

> Date: Sun, 23 Jun 2013 17:11:57 +1200
> From: Phil Sainty <psainty@orcon.net.nz>
> 
>  From emacs -Q the following returns an empty string, which seems
> like a bug:
> 
> ELISP> (setq indirect-eval '(:eval "indirect eval"))
> ELISP> (format-mode-line '(t indirect-eval "else"))
> ""

It's not a bug.  The Lisp expressions supported in mode-line format
are those documented in the manual, and nothing else.  In particular,
the ':eval' part must appear explicitly, and any symbol stands for its
value (which is not eval'ed).

IOW, you seem to expect a full-blown Lisp evaluation there, but that
was never implemented or intended.

> And a related question, if I may: Other than wrapping advice around
> format-mode-line to let-bind a variable, is there any way for me to
> ascertain whether a mode line construct is being processed for an
> actual mode line redraw (in which case format-mode-line is not called),
> as opposed to some other situation (e.g. describe-mode) in which
> format-mode-line will be used?
> 
> My goal is to use the conditional construct to display the original
> value when calling format-mode-line, but an alternative value in the
> mode line itself, with the two values being stored in buffer local
> variables. At the moment I'm using advice to supply the condition
> (and of course I found the above bug when testing that my code worked
> for the different types of construct).

I don't understand: format-mode-line is a function you cannot change
or hook, so where exactly do you want to display a different value?
Both format-mode-line and the internal code that processes the mode
line format in the display engine access the same variable.

And anyway, why would you need format-mode-line to produce a different
value?

bug#14694: 24.3; format-mode-line failing with :eval form via a variable in a conditional construct

[Stefan Monnier]

ELISP> (setq indirect-eval '(:eval "indirect eval"))
ELISP> (format-mode-line '(t indirect-eval "else"))
> ""

You need (put 'indirect-eval 'risky-local-variable t)


        Stefan

bug#14694: 24.3; format-mode-line failing with :eval form via a variable in a conditional construct

[Phil Sainty]

On 24/06/2013 04:45, Stefan Monnier wrote:
> ELISP> (setq indirect-eval '(:eval "indirect eval"))
> ELISP> (format-mode-line '(t indirect-eval "else"))
>> ""
>
> You need (put 'indirect-eval 'risky-local-variable t)

Argh, I see that in the documentation now. Thank you Stefan
(and Eli).

I'm afraid I made an embarrassing rookie mistake in checking
the result of (format-mode-line indirect-eval) instead of
(format-mode-line 'indirect-eval), and thinking that I was
seeing an inconsistency when there was none. Otherwise I'd no
doubt have spotted the information about that property.

My apologies; please close.


Eli asked:
 > And anyway, why would you need format-mode-line to produce a
 > different value?

The specific goal was to enable customisation of the display of
`mode-name' in the mode line (generally to abbreviate it and reduce
the space used) *without* also affecting what is displayed when that
variable is processed in other contexts (such as by `describe-mode'),
in which cases `format-mode-line' will be called.

Advising `format-mode-line' to provide an indicator variable to use
in a conditional construct does facilitate this. I just wanted check
that I wasn't missing another approach which didn't require the advice.


-Phil