Why does `(window-line-height)' return nil?

Why does `(window-line-height)' return nil?

[Xue Fuqiao]

Hi,

In my *scratch* buffer, all of the following expressions return nil with
`eval-print-last-sexp' (also tried `eval-last-sexp'):

--8<---------------cut here---------------start------------->8---
(window-line-height 'header-line)
(window-line-height 'mode-line)
(window-line-height)
(window-line-height 0)
(window-line-height -1)
--8<---------------cut here---------------end--------------->8---

Why do they return nil?  After reading the doc string, I only found one
sentence that may be relavant: "Return nil if window display is not
up-to-date."  So, does that mean window display is not up-to-date when
evaluating the expression?  I've searched the archives/web, but found
little information about this function.

-- 
Best regards, Xue Fuqiao.
http://www.gnu.org/software/emacs/

Re: Why does `(window-line-height)' return nil?

[Eli Zaretskii]

> Date: Sat, 3 Aug 2013 16:26:50 +0800
> From: Xue Fuqiao <xfq.free@gmail.com>
> 
> In my *scratch* buffer, all of the following expressions return nil with
> `eval-print-last-sexp' (also tried `eval-last-sexp'):
> 
> --8<---------------cut here---------------start------------->8---
> (window-line-height 'header-line)
> (window-line-height 'mode-line)
> (window-line-height)
> (window-line-height 0)
> (window-line-height -1)
> --8<---------------cut here---------------end--------------->8---
> 
> Why do they return nil?  After reading the doc string, I only found one
> sentence that may be relavant: "Return nil if window display is not
> up-to-date."  So, does that mean window display is not up-to-date when
> evaluating the expression?

Yes, that's what it means.  This function will almost always return
nil if you evaluate an expression that calls it in the *scratch*
buffer, because that evaluation forces a new redisplay cycle (to
display the result).  It does return non-nil values when called from
Lisp code that does not trigger redisplay, though.

Re: Why does `(window-line-height)' return nil?

[Xue Fuqiao]

On Sat, Aug 3, 2013 at 5:01 PM, Eli Zaretskii <eliz@gnu.org> wrote:
>> Date: Sat, 3 Aug 2013 16:26:50 +0800
>> From: Xue Fuqiao <xfq.free@gmail.com>
>>
>> In my *scratch* buffer, all of the following expressions return nil with
>> `eval-print-last-sexp' (also tried `eval-last-sexp'):
>>
>> --8<---------------cut here---------------start------------->8---
>> (window-line-height 'header-line)
>> (window-line-height 'mode-line)
>> (window-line-height)
>> (window-line-height 0)
>> (window-line-height -1)
>> --8<---------------cut here---------------end--------------->8---
>>
>> Why do they return nil?  After reading the doc string, I only found one
>> sentence that may be relavant: "Return nil if window display is not
>> up-to-date."  So, does that mean window display is not up-to-date when
>> evaluating the expression?
>
> Yes, that's what it means.  This function will almost always return
> nil if you evaluate an expression that calls it in the *scratch*
> buffer, because that evaluation forces a new redisplay cycle (to
> display the result).  It does return non-nil values when called from
> Lisp code that does not trigger redisplay, though.

Ah, I see.  Thank you.

-- 
Best regards, Xue Fuqiao.
http://www.gnu.org/software/emacs/