bug#66750: Unhelpful text in C-h v for variables with a lambda form as value

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

Supposing a variable has as a value a lambda function.  On doing C-h v
for this variable, there ensues a useless lack of information about the
lambda.  All one gets told is that it is a lambda function, not where the
source code for it is, or anything useful like that.

A typical example is C-h v search-default-mode, which prints something
like the following:

    Its value is
    #<subr F616e6f6e796d6f75732d6c616d626461_anonymous_lambda_109>
    Original value was nil
    Local in buffer *info*<3>; global value is nil

..  When native compilation is not enabled, the printout is only slightly
better.

Merely amongst the customisable options (i.e. not ordinary defvars and
defconsts) there are approximately 63 places where these options get set
to lambda functions.  The following list lists the file and line number
where it's done, and the customisable option it gets done to:

../autorevert.el:471 before-save-hook
../emacs-lisp/pp.el:320 temp-buffer-show-function
../emulation/viper.el:950 eshell-mode-hook
../erc/erc-pcomplete.el:99 pcomplete-default-completion-function
../eshell/em-cmpl.el:191 eshell-command-completion-function
../eshell/em-cmpl.el:202 eshell-default-completion-function
../eshell/em-cmpl.el:280 eshell-mode-hook
../eshell/em-prompt.el:50 eshell-prompt-function
../eshell/em-smart.el:187 eshell-post-command-hook
../gnus/gnus-art.el:3077 gnus-exit-gnus-hook
../gnus/gnus-delay.el:150 message-send-hook
../gnus/gnus-diary.el:213 gnus-summary-menu-hook
../gnus/gnus-diary.el:370 nndiary-request-accept-article-functions
../gnus/gnus-draft.el:274 message-post-method
../gnus/gnus-msg.el:1145 message-reply-to-function
../gnus/gnus-msg.el:387 message-header-setup-hook
../gnus/gnus-msg.el:392 message-mode-hook
../gnus/gnus-registry.el:1000 gnus-summary-menu-hook
../gnus/gnus-sum.el:1321 gnus-alter-articles-to-read-function
../gnus/gnus-util.el:1205 iswitchb-make-buflist-hook
../gnus/message.el:3157 facemenu-add-face-function
../gnus/message.el:701 message-cite-level-function
../gnus/nnmairix.el:286 nnmairix-widget-select-window-function
../help.el:1946 temp-buffer-max-height
../help.el:1963 temp-buffer-max-width
../isearch.el:4685 search-default-mode
../locate.el:670 locate-make-command-line
../mail/emacsbug.el:466 message-sent-hook
../mail/emacsbug.el:540 message-send-hook
../mail/feedmail.el:1400 feedmail-queue-runner-mode-setter
../mail/feedmail.el:1436 feedmail-queue-runner-message-sender
../mail/feedmail.el:1457 feedmail-queue-runner-cleaner-upper
../net/mairix.el:176 mairix-widget-select-window-function
../obsolete/gulp.el:103 mail-setup-hook
../org/ob-core.el:1524 org-mode-hook
../org/ob-latex.el:101 org-babel-latex-end-env
../org/ob-latex.el:79 org-babel-latex-preamble
../org/ob-latex.el:90 org-babel-latex-begin-env
../org/ob-python.el:208 python-shell-first-prompt-hook
../org/ol-bibtex.el:268 org-bibtex-headline-format-function
../org/org-compat.el:1272 imenu-after-jump-hook
../org/org-compat.el:1276 org-mode-hook
../org/org-compat.el:1339 speedbar-visiting-tag-hook
../org/org-crypt.el:317 org-mode-hook
../org/org-ctags.el:196 org-mode-hook
../org/org.el:15220 after-save-hook
../org/org.el:18651 occur-mode-find-occurrence-hook
../org/org-mouse.el:866 org-mode-hook
../org/ox-ascii.el:378 org-ascii-format-drawer-function
../org/ox-html.el:657 org-html-format-drawer-function
../org/ox-latex.el:888 org-latex-format-drawer-function
../org/ox-odt.el:635 org-odt-format-drawer-function
../org/ox-texinfo.el:334 org-texinfo-format-drawer-function
../pcomplete.el:305 pcomplete-command-completion-function
../pcomplete.el:315 pcomplete-default-completion-function
../play/gametree.el:118 gametree-make-heading-function
../progmodes/cperl-mode.el:1848 add-log-current-defun-function
../progmodes/octave.el:607 fill-nobreak-predicate
../progmodes/ps-mode.el:108 ps-mode-print-function
../vc/add-log.el:1129 fill-nobreak-predicate
../vc/emerge.el:2086 temp-buffer-show-function
../vc/smerge-mode.el:1358 ediff-quit-hook
../window.el:6662 pop-up-frame-function

..  Additionally there are about 29 instances in the doc strings where
users are urged to use lambda functions (rather than defuns) for this
purpose.  They are at:

../calendar/calendar.el:395 calendar-move-hook
../calendar/cal-tex.el:154 cal-tex-hook
../comint.el:189 comint-mode-hook
../dired-loaddefs.el:646 dired-mode-hook
../dired-x.el:152 dired-mode-hook
../erc/erc-log.el:218 erc-enable-logging
../erc/erc-page.el:55 erc-page-function
../files.el:466 auto-save-visited-predicate
../font-lock.el:704 c-mode-hook
../gnus/gnus-sum.el:1044 gnus-select-group-hook
../gnus/nnmail.el:179 nnmail-expiry-wait-function
../gnus/nnmail.el:286 nnmail-read-incoming-hook
../gnus/nnmail.el:299 nnmail-read-incoming-hook
../icomplete.el:154 icomplete-minibuffer-setup-hook
../ido.el:890 ido-minibuffer-setup-hook
../ielm.el:63 ielm-mode-hook
../ldefs-boot.el:29820 sql-mode-hook
../ldefs-boot.el:5069 c-mode-hook
../loaddefs.el:29950 sql-mode-hook
../loaddefs.el:5069 c-mode-hook
../obsolete/nnir.el:415 nnir-notmuch-filter-group-names-function
../org/org-keys.el:688 org-use-speed-commands
../org/ox-html.el:968 org-html-table-row-open-tag
../progmodes/compile.el:885 c-mode-hook
../progmodes/prog-mode.el:296 emacs-lisp-mode-hook
../progmodes/sql.el:4134 sql-mode-hook
../progmodes/sql.el:4256 sql-interactive-mode-hook
../textmodes/tildify.el:156 tildify-foreach-region-function
../transient.el:278 transient-substitute-key-function

..  Furthermore there are, perhaps, around 150 such settings of ordinary
defvars or defconsts to lambda functions.

#########################################################################

I see two ways to fix this bug:
(i) Systematically to replace each of these lambda functions by a named
  defun.
(ii) To adopt git branch feature/named-lambdas, which attaches to every
lambda function the name of the defun (or defvar) in which it was
defined.  This extra information could then be printed out for C-h v,
something like this:

    Its value is
    {isearch-fold-quotes-mode} #<subr F616e6f6e796d6f75732d6c616d626461_anonymous_lambda_109>
    Original value was nil
    Local in buffer *info*<3>; global value is nil

, thus enabling the user quickly to locate the source of the lambda form
in isearch.el.

-- 
Alan Mackenzie (Nuremberg, Germany).