bug#24221: 25.1.50; binding anonymous functions to keys cause help buffer navigation errors

bug#24221: 25.1.50; binding anonymous functions to keys cause help buffer navigation errors

[Zachary Kanfer]

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This is a bug that happens when trying to navigate between two *Help*
buffers.

==Reproduction instructions==

emacs -Q

Evaluate this code:

(global-set-key (kbd "C-c l") (lambda () (interactive) (insert "This
messes up the help buffer somehow.")))

Then open up a help buffer for that function:

C-h k C-c l

And a help buffer for something else:

C-h k a

Then, in the help buffer, go to the bottom, and try to select the
"[back]" link. I get an error: "user-error: You didn’t specify a function
symbol". If I try again, I get a different error: "apply: Symbol’s
function definition is void: nil". These errors happen whether I've
select the link with pressing Enter on the keyboard or by clicking the
mouse.

==End of instructions==

In GNU Emacs 25.1.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.18.9)
 of 2016-07-10 built on zck-laptop
Repository revision: 466ee1b3ea76425d201b5d59950e88251870c836
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description:    Ubuntu 16.04.1 LTS

Recent messages:
Mark set
(lambda nil (interactive) (insert "This messes up the help buffer
somehow."))
Type C-x 1 to delete the help window.

Mark set
user-error: You didn’t specify a function symbol
apply: Symbol’s function definition is void: nil
scroll-up-command: End of buffer [2 times]
apply: Symbol’s function definition is void: nil [3 times]
p is undefined

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-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
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message puny seq byte-opt gv bytecomp
byte-compile cconv dired dired-loaddefs format-spec rfc822 mml mml-sec
password-cache epa derived epg epg-config gnus-util rmail rmail-loaddefs
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr
mail-utils cl-extra help-fns radix-tree help-mode easymenu edmacro
kmacro cl-loaddefs pcase cl-lib time-date mule-util tooltip eldoc
electric uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win
x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list newcomment elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev obarray minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote dbusbind inotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 98309 6988)
 (symbols 48 20970 0)
 (miscs 40 262 168)
 (strings 32 18601 4494)
 (string-bytes 1 595182)
 (vectors 16 13861)
 (vector-slots 8 449156 6768)
 (floats 8 188 43)
 (intervals 56 248 0)
 (buffers 976 12)
 (heap 1024 25332 985))

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bug#24221: 25.1.50; binding anonymous functions to keys cause help buffer navigation errors

[Peder O. Klingenberg]

Zachary Kanfer <zkanfer@gmail.com> writes:

> emacs -Q
>
> Evaluate this code:
>
> (global-set-key (kbd "C-c l") (lambda () (interactive) (insert "This
> messes up the help buffer somehow.")))
>
> Then open up a help buffer for that function:
>
> C-h k C-c l
>
> And a help buffer for something else:
>
> C-h k a
>
> Then, in the help buffer, go to the bottom, and try to select the
> "[back]" link. I get an error: "user-error: You didn’t specify a function
> symbol". If I try again, I get a different error: "apply: Symbol’s
> function definition is void: nil". These errors happen whether I've
> select the link with pressing Enter on the keyboard or by clicking the
> mouse.

I can confirm this.  Here's what I think is happening:

`describe-key' (C-h k) finds your lambda expression when it calls
`key-binding'.  It then pushes this lambda on the help-xref-stack as an
argument to `describe-function'.  But `describe-function' doesn't know
what to do with a lamda expression, and gives you that "You didn't
specify a function symbol" message when the xref stack is popped and
`describe-function' is called.

I don't know what the best fix would be - fix the xref setup in
`describe-key' or teach `describe-function' to do something useful with
a lambda expression.

bug#24221: 25.1.50; binding anonymous functions to keys cause help buffer navigation errors

[Drew Adams]

> > C-h k C-c l
> > And a help buffer for something else:
> > C-h k a
> >
> > Then, in the help buffer, go to the bottom, and try to select the
> > "[back]" link. I get an error: "user-error: You didn’t specify a
> > function symbol".

This is apparently a regression from Emacs 24.5.

I see the same regression in this older pre-Emacs 25 build:

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2015-05-29 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/mingw32 --host=i686-pc-mingw32
 --enable-checking=yes,glyphs'

So it seems to have been introduced on or before 2015-05-29.

And I do not see the regression in this older pre-25 build:

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2015-04-05 on LEG570
Repository revision: 16eec6fc55dcc05d1d819f18998e84a9580b2521
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --host=i686-pc-mingw32 --enable-checking=yes,glyphs'

So it seems to have been introduced after 2015-04-05, i.e., between
2015-04-05 and 2015-05-29.

> I don't know what the best fix would be - fix the xref setup in
> `describe-key' or teach `describe-function' to do something useful with
> a lambda expression.

Remove the regression.  Maybe restore some of the code from before
it was introduced.

bug#24221: 25.1.50; binding anonymous functions to keys cause help buffer navigation errors

[Drew Adams]

This is the problem behind this regression:

1. `describe-key' calls `describe-function', passing it the
   command definition.

2. Someone changed `describe-function' so that it now barfs
   if its arg is not a function symbol.

The doc string of `describe-function' says (and has long
said) that the arg is a symbol.  However, that should be
the case only for interactive use (the `completing-read'
call requires a match against a function symbol).

What should be done:

1. Remove the error for a non-symbol, except perhaps for the
interactive case.

2. Fix the doc, to say that it need not be a symbol when
called from Lisp.

Before the regression was introduced, both `describe-function'
and using `[back]' to get to it worked for a lambda form.

And a doc string in the lambda form was shown correctly as
the doc, in *Help* - just as if the function had a name.

And a byte-compiled lambda form also worked perfectly well
prior to the introduction of the regression:

(global-set-key
  (kbd "C-c l")
  (byte-compile
    (lambda () "DOC" (interactive) (message "FOO"))))

This regression removed a bunch of useful behavior.

`describe-function' should describe a function - any
function.  The argument should be limited to a function
symbol only when called interactively.

(An argument could even be made that it should not even
be so limited for interactive use, that is, that
`completing-read' should be lax, letting you type a
lambda form if you want.  I'm not making that argument
here, however.)

bug#24221: 25.1.50; binding anonymous functions to keys cause help buffer navigation errors

[Drew Adams]

I suggest using code such as that below - allow FUNCTION to be
`functionp', not only a symbol that is `fboundp' (and test for
the former first).

Also, the function that raises an error should be `user-error'
only when called interactively.  Otherwise, it should be `error'.

[I would even argue that for interactive use it should be
`message', not `user-error', as that allows `describe-function'
to be used, e.g., in the minibuffer to get help on different
function-name candidates.]

(let* ((interactivep  (called-interactively-p 'interactive))
       (err-fn        (if interactivep #user-error #'error)))
  (if (and interactivep  (not function))
      (funcall err-fn "You did not specify a function symbol")
    (if (not (or (functionp function) ; Allow anonymous functions.
                 (and function   ; Allow macros and special forms.
                      (fboundp (intern-soft function))))))
        (funcall err-fn "Not a defined function: `%S'" function)
    (help-setup-xref ...)
    ...)

bug#24221: 25.1.50; binding anonymous functions to keys cause help buffer navigation errors

[npostavs]

[-- Attachment #1: Type: text/plain, Size: 489 bytes --]

tags 24221 patch
quit

Drew Adams <drew.adams@oracle.com> writes:
>
> 2. Someone changed `describe-function' so that it now barfs
>    if its arg is not a function symbol.

Yes, seems to have been 82d0f42a "* lisp/help-fns.el
(describe-function): More type checking[...]".  I think it was an
accident: the non-interactive case wasn't considered, probably because
the docstring doesn't mention it.

Here's a patch that moves the checks into the interactive form and
updates the docstring:


[-- Attachment #2: patch --]
[-- Type: text/plain, Size: 2804 bytes --]

From 7dffe1868222f33c8845cac7b5e46836149c72f8 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Mon, 3 Oct 2016 18:49:56 -0400
Subject: [PATCH v1] Let describe-function work for lambda again

Since commit "* lisp/help-fns.el (describe-function): More type
checking[...]", `describe-function' throws a user-error when given a
non-symbol.  This prevents the [back] button in a *Help* buffer from
working when the page it goes back to describes an anonymous
function (e.g., the result of `describe-key' on a key which is bound to
a lambda form).

* lisp/help-fns.el (describe-function): Move the checks on FUNCTION
being an fbound symbol into the `interactive' form.  This allows
non-interactive calls to pass an anonymous function (Bug #24221).  Note
that passing a non-bound symbol non-interactively will still trigger a
`void-function' error from `describe-function-1'.
---
 lisp/help-fns.el | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/lisp/help-fns.el b/lisp/help-fns.el
index 26d8839..7dfa670 100644
--- a/lisp/help-fns.el
+++ b/lisp/help-fns.el
@@ -50,23 +50,24 @@ describe-function-orig-buffer
 
 ;;;###autoload
 (defun describe-function (function)
-  "Display the full documentation of FUNCTION (a symbol)."
+  "Display the full documentation of FUNCTION (a symbol).
+When called from lisp, FUNCTION may also be a function object."
   (interactive
-   (let ((fn (function-called-at-point))
-	 (enable-recursive-minibuffers t)
-	 val)
-     (setq val (completing-read (if fn
-				    (format "Describe function (default %s): " fn)
-				  "Describe function: ")
-				obarray 'fboundp t nil nil
-				(and fn (symbol-name fn))))
-     (list (if (equal val "")
-	       fn (intern val)))))
-  (or (and function (symbolp function))
-      (user-error "You didn't specify a function symbol"))
-  (or (fboundp function)
-      (user-error "Symbol's function definition is void: %s" function))
-
+   (let* ((fn (function-called-at-point))
+          (enable-recursive-minibuffers t)
+          (val (completing-read
+                (if fn
+                    (format "Describe function (default %s): " fn)
+                  "Describe function: ")
+                obarray 'fboundp t nil nil
+                (and fn (symbol-name fn)))))
+     (unless (equal val "")
+       (setq fn (intern val)))
+     (unless (and fn (symbolp fn))
+       (user-error "You didn't specify a function symbol"))
+     (unless (fboundp fn)
+       (user-error "Symbol's function definition is void: %s" fn))
+     (list fn)))
   ;; We save describe-function-orig-buffer on the help xref stack, so
   ;; it is restored by the back/forward buttons.  'help-buffer'
   ;; expects (current-buffer) to be a help buffer when processing
-- 
2.9.3

bug#24221: 25.1.50; binding anonymous functions to keys cause help buffer navigation errors

[npostavs]

tags 24221 fixed
close 24221 25.2
quit

npostavs@users.sourceforge.net writes:

>
> Here's a patch that moves the checks into the interactive form and
> updates the docstring:

Pushed as 24e4e4e8