[debbugs-tracker] bug#22212: closed (24.5; Odd return behavior of function lisp-indent-line (minor))

[GNU bug Tracking System]

Your message dated 21 Dec 2015 10:05:08 -0000
with message-id <address@hidden>
and subject line Re: bug#22212: 24.5; Odd return behavior of function 
lisp-indent-line (minor)
has caused the debbugs.gnu.org bug report #22212,
regarding 24.5; Odd return behavior of function lisp-indent-line (minor)
to be marked as done.

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22212: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=22212
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--- Begin Message --- Subject: 24.5; Odd return behavior of function lisp-indent-line (minor) Date: Sat, 19 Dec 2015 05:26:25 -0500

(lisp-indent-line) generally has an [undocumented] return value of

(point), but when embedded in structures with a number of commands that move

the point, it may return nil instead. I learned about the return value

of (point) from experimentatal evaluation, and then tried to use this to
write the following function:

(defun lisp-indent-lines (lnum1 lnum2)

       (let ((count 0)

    (pos (point)))

       (goto-char 0) (forward-line (- lnum1 1))

       (while (<= lnum1 lnum2)

(back-to-indentation)

(setq count (- count (- (point) (lisp-indent-line))))

(forward-line) (cl-incf lnum1))

       (goto-char (+ pos count))))

This exits with a type error because

     (lisp-indent-line)

returns nil. But if you execute

        (setq count 0)

        (setq count (- count (- (point) (lisp-indent-line))))

outside of a loop or recursion and outside of let/let* bindings,
it has no problem. The only problem with

        (lisp-indent-line)

is the inconsistent return value, so this isa very minor bug,
unless it is some kind of esoteric feature used

in emacs at some phase. The setq statement is not related to the

nil value, though it's the subtraction by nil in that statement that

causes the error -- it will also evaluate to nil in working functions that

avoid relying on its return value.

Similar behavior exists for every method I tested of placing

(lisp-indent-line) in a loop, including dolists and recursive functions,

named or anonymous. Furthermore, sometimes you *can*

evaluate

       (setq count (- count (- (point) (lisp-indent-line))))

            

inside a loop! You can evaluate it unless you also use one of a number

of commands that moves the cursor, such as (forward-line), (goto-line),

(back-to-indentation), or (goto-char).

I believe that the nil return value only occurs when 

I think the function comes from common-lisp-indent-line as implemented in

.../lisp/emacs-lisp/cl-indent.el?

In GNU Emacs 24.5.1 (x86_64-pc-linux-gnu, GTK+ Version 3.10.8)

 of 2015-08-20 on lgw01-16, modified by Debian

Windowing system distributor `The X.Org Foundation', version 11.0.11501000

System Description: Ubuntu 14.04.3 LTS

Configured using:

 `configure --build x86_64-linux-gnu --prefix=/usr

 --sharedstatedir=/var/lib --libexecdir=/usr/lib

 --localstatedir=/var/lib --infodir=/usr/share/info

 --mandir=/usr/share/man --with-pop=yes

 --enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.5/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.5/site-lisp:/usr/share/emacs/site-lisp

 --build x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib

 --libexecdir=/usr/lib --localstatedir=/var/lib

 --infodir=/usr/share/info --mandir=/usr/share/man --with-pop=yes

 --enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.5/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.5/site-lisp:/usr/share/emacs/site-lisp

 --with-x=yes --with-x-toolkit=gtk3 --with-toolkit-scroll-bars

 'CFLAGS=-g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat

 -Werror=format-security -Wall' CPPFLAGS=-D_FORTIFY_SOURCE=2

 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro''

Important settings:

  value of $LANG: en_US.UTF-8

  value of $XMODIFIERS: @im=ibus

  locale-coding-system: utf-8-unix

Major mode: Emacs-Lisp

Minor modes in effect:

  TeX-PDF-mode: t

  show-paren-mode: t

  pdf-occur-global-minor-mode: t

  image-diredx-async-mode: t

  image-diredx-adjust-mode: t

  tooltip-mode: t

  electric-indent-mode: t

  mouse-wheel-mode: t

  file-name-shadow-mode: t

  global-font-lock-mode: t

  font-lock-mode: t

  blink-cursor-mode: t

  auto-composition-mode: t

  auto-encryption-mode: t

  auto-compression-mode: t

  column-number-mode: t

  line-number-mode: t

  transient-mark-mode: t

Recent messages:

nil

eval: Symbol's function definition is void: current-minor-modes

Making completion list...

Quit [2 times]

Making completion list...

eval: Symbol's function definition is void: mode-local-active-mode

nil

<C-mouse-1> is undefined

Quit

Making completion list...

Load-path shadows:

/usr/share/emacs/24.5/site-lisp/debian-startup hides /usr/share/emacs/site-lisp/debian-startup

Features:

(god-mode eieio-opt speedbar sb-image ezimage dframe find-func shadow

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mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils

mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr

mail-utils term disp-table ehelp misearch multi-isearch help-mode debug

warnings easy-mmode smartparens-latex smartparens thingatpt dash cl-macs

gv tex dbus xml crm my-theme-theme paren key-chord pdf-occur ibuf-ext

ibuffer tablist tablist-filter semantic/wisent/comp semantic/wisent

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semantic/tag semantic/lex semantic/fw eieio byte-opt bytecomp

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let-alist pdf-misc imenu pdf-tools compile comint ansi-color ring

cus-edit cus-start cus-load wid-edit pdf-view mule-util bookmark pp

jka-compr pdf-cache pdf-info tq pdf-util image-mode edmacro kmacro

tex-site info easymenu image-dired+ image-dired format-spec dired advice

help-fns cl-loaddefs cl-lib package epg-config time-date tooltip

electric uniquify ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd

tool-bar dnd fontset image regexp-opt fringe tabulated-list newcomment

lisp-mode prog-mode register page menu-bar rfn-eshadow timer select

scroll-bar mouse jit-lock font-lock syntax facemenu font-core frame cham

georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao

korean japanese hebrew greek romanian slovak czech european ethiopic

indian cyrillic chinese case-table epa-hook jka-cmpr-hook help simple

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custom widget hashtable-print-readable backquote make-network-process

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Memory information:

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--- End Message ---

> --- Begin Message ---
>
>
>
> Subject: 
>
> Re: bug#22212: 24.5; Odd return behavior of function lisp-indent-line (minor)
>
>
>
>
> Date: 
>
> 21 Dec 2015 10:05:08 -0000
>
>
>
>
> User-agent: 
>
> tin/2.3.1-20141224 ("Tallant") (UNIX) (FreeBSD/10.2-RELEASE-p7 (amd64))
>
>
>
>
> Hello, Jonathan.
>
> In article <address@hidden> you wrote:
> > [-- text/plain, encoding 7bit, charset: UTF-8, 160 lines --]
>
> > (lisp-indent-line) generally has an [undocumented] return value of
> > (point), but when embedded in structures with a number of commands that move
> > the point, it may return nil instead. I learned about the return value
> > of (point) from experimentatal evaluation, and then tried to use this to
> > write the following function:
>
> > (defun lisp-indent-lines (lnum1 lnum2)
> >        (let ((count 0)
> >     (pos (point)))
> >        (goto-char 0) (forward-line (- lnum1 1))
> >        (while (<= lnum1 lnum2)
> > (back-to-indentation)
> > (setq count (- count (- (point) (lisp-indent-line))))
> > (forward-line) (cl-incf lnum1))
> >        (goto-char (+ pos count))))
>
> > This exits with a type error because
> >      (lisp-indent-line)
> > returns nil. But if you execute
>
> >         (setq count 0)
> >         (setq count (- count (- (point) (lisp-indent-line))))
>
> > outside of a loop or recursion and outside of let/let* bindings,
> > it has no problem. The only problem with
>
> >         (lisp-indent-line)
>
> > is the inconsistent return value, so this isa very minor bug,
> > unless it is some kind of esoteric feature used
> > in emacs at some phase.
>
> lisp-indent-line's return value is random: it has no significance.  The
> function performs its actions, and the value returned is just that of the
> last form in lisp-indent-line which happened to be executed.
>
> > The setq statement is not related to the nil value, though it's the
> > subtraction by nil in that statement that causes the error -- it will
> > also evaluate to nil in working functions that avoid relying on its
> > return value.
>
> Can I suggest you actually look at the source code for lisp-indent-line?
> It's not difficult to read.  In Emacs 24.5, it's in
> emacs/lisp/emacs-lisp/lisp-mode.el at line 1253.  The very last two lines
> contain an `if' form: this `if' triggers when point was originally to the
> right of the indentation whitespace, and it restores the original
> position of point with a `goto-char' form, hence the apparent return
> value of point.  If the `if' doesn't trigger (when point started in the
> indentation), nil is returned.
>
> Just to emphasise, you cannot rely on the return value of this function.
> Instead, you must explicitly use `(point)'.
>
> [ .... ]
>
> Thanks for taking the trouble to report this.  But it isn't really a bug
> at all, so I'm closing it.
>
>
>
> > In GNU Emacs 24.5.1 (x86_64-pc-linux-gnu, GTK+ Version 3.10.8)
> >  of 2015-08-20 on lgw01-16, modified by Debian
> > Windowing system distributor `The X.Org Foundation', version 11.0.11501000
> > System Description: Ubuntu 14.04.3 LTS
>
> -- 
> Alan Mackenzie (Nuremberg, Germany).
>
>
>
> --- End Message ---