bug#50781: 27.2; completing-read with a vector

bug#50781: 27.2; completing-read with a vector

[Al Haji-Ali]

I am not sure if `completing-read` is supposed to accept a vector but an error is not thrown on this usage (incidentally helpful uses `completing-read` in this way).

However, starting from `emacs -Q` and executing the following code

(completing-read "Test: " [A B C])

works but then pressing <TAB> for auto-completion shows objects beside A, B and C.

On the other hand, executing
(completing-read "Test: " [1 2 3])
and then pressing <TAB> shows the error `(error "Bad data in guts of obarray")`

with the following call stack

,----
| Debugger entered--Lisp error: (error "Bad data in guts of obarray")
|   try-completion("" [1 2 3] nil)
|   completion-basic-try-completion("" [1 2 3] nil 0)
|   #f(compiled-function (style) #<bytecode 0x7d5815>)(basic)
|   completion--some(#f(compiled-function (style) #<bytecode 0x7d5815>) (basic partial-completion emacs22))
|   completion--nth-completion(1 "" [1 2 3] nil 0 (metadata))
|   completion-try-completion("" [1 2 3] nil 0 (metadata))
|   completion--do-completion(7 7)
|   completion--in-region-1(7 7)
|   #f(compiled-function (start end collection predicate) #<bytecode 0x1ff60569ee1d>)(7 7 [1 2 3] nil)
|   apply(#f(compiled-function (start end collection predicate) #<bytecode 0x1ff60569ee1d>) (7 7 [1 2 3] nil))
|   #f(compiled-function (funs global args) #<bytecode 0x7d57ed>)(nil nil (7 7 [1 2 3] nil))
|   completion--in-region(7 7 [1 2 3] nil)
|   completion-in-region(7 7 [1 2 3] nil)
|   minibuffer-complete()
|   funcall-interactively(minibuffer-complete)
|   call-interactively(minibuffer-complete nil nil)
|   command-execute(minibuffer-complete)
|   read-from-minibuffer("Test: " nil (keymap (menu-bar keymap (minibuf "Minibuf" keymap (tab menu-item "Complete" minibuffer-complete :help "Complete as far as possible") (space menu-item "Complete Word" minibuffer-complete-word :help "Complete at most one word") (63 menu-item "List Completions" minibuffer-completion-help :help "Display all possible completions") "Minibuf")) (27 keymap (118 . switch-to-completions)) (prior . switch-to-completions) (63 . minibuffer-completion-help) (32 . minibuffer-complete-word) (9 . minibuffer-complete) keymap (menu-bar keymap (minibuf "Minibuf" keymap (previous menu-item "Previous History Item" previous-history-element :help "Put previous minibuffer history element in the min...") (next menu-item "Next History Item" next-history-element :help "Put next minibuffer history element in the minibuf...") (isearch-backward menu-item "Isearch History Backward" isearch-backward :help "Incrementally search minibuffer history backward") (isearch-forward menu-item "Isearch History Forward" isearch-forward :help "Incrementally search minibuffer history forward") (return menu-item "Enter" exit-minibuffer :key-sequence "\15" :help "Terminate input and exit minibuffer") (quit menu-item "Quit" abort-recursive-edit :help "Abort input and exit minibuffer") "Minibuf")) (10 . exit-minibuffer) (13 . exit-minibuffer) (7 . abort-recursive-edit) (C-tab . file-cache-minibuffer-complete) (9 . self-insert-command) (XF86Back . previous-history-element) (up . previous-line-or-history-element) (prior . previous-history-element) (XF86Forward . next-history-element) (down . next-line-or-history-element) (next . next-history-element) (27 keymap (60 . minibuffer-beginning-of-buffer) (114 . previous-matching-history-element) (115 . next-matching-history-element) (112 . previous-history-element) (110 . next-history-element))) nil nil nil nil)
|   completing-read-default("Test: " [1 2 3] nil nil nil nil nil nil)
|   completing-read("Test: " [1 2 3])
|   (progn (completing-read "Test: " [1 2 3]))
|   eval((progn (completing-read "Test: " [1 2 3])) t)
|   elisp--eval-last-sexp(nil)
|   eval-last-sexp(nil)
|   funcall-interactively(eval-last-sexp nil)
|   call-interactively(eval-last-sexp nil nil)
|   command-execute(eval-last-sexp)
`----

In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30, cairo version 1.15.12)
 of 2021-05-18 built on lxstaff02
Repository revision: deef5efafb70f4b171265b896505b92b6eef24e6
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: CentOS Linux 7 (Core)

Configured using:
 'configure --prefix=$HOME/.local/ --with-json --with-modules
 --with-harfbuzz --with-compress-install --with-threads
 --with-included-regex --with-zlib --with-sound=alsa --without-gconf
 --with-mailutils --with-xft --with-xml2 --with-cairo --with-gconf
 --with-xwidgets --without-toolkit-scroll-bars 'CFLAGS=-O3 -mtune=native
 -march=native -fomit-frame-pointer
 -I/usr/lib/gcc/x86_64-linux-gnu/9/include/
 -L/usr/lib/gcc/x86_64-linux-gnu/9'
 PKG_CONFIG_PATH=$HOME/.local/lib/pkgconfig:/usr/lib64/pkgconfig/:/usr/share/pkgconfig/
 CPPFLAGS=-I$HOME/.local/include LDFLAGS=-L$HOME/.local/lib'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ ZLIB GTK3 X11
XDBE XIM MODULES THREADS XWIDGETS PDUMPER GMP

Important settings:
  value of $LANG: en_GB.utf8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
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  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
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Load-path shadows:
None found.

Features:
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Memory information:
((conses 16 52982 7459)
 (symbols 48 6420 1)
 (strings 32 17020 2200)
 (string-bytes 1 561499)
 (vectors 16 10092)
 (vector-slots 8 133817 11898)
 (floats 8 22 43)
 (intervals 56 705 1)
 (buffers 1000 13)
 (heap 1024 11972 1101))

bug#50781: 27.2; completing-read with a vector

[Lars Ingebrigtsen]

Al Haji-Ali <abdo.haji.ali@gmail.com> writes:

> I am not sure if `completing-read` is supposed to accept a vector but
> an error is not thrown on this usage (incidentally helpful uses
> `completing-read` in this way).
>
> However, starting from `emacs -Q` and executing the following code
>
> (completing-read "Test: " [A B C])
>
> works but then pressing <TAB> for auto-completion shows objects beside
> A, B and C.

The documentation here says:

COLLECTION can be a list of strings, an alist, an obarray or a hash table.

So using a vector isn't really supported, but works in some instances by
chance.

> On the other hand, executing
> (completing-read "Test: " [1 2 3])
> and then pressing <TAB> shows the error `(error "Bad data in guts of obarray")`

It interprets all vectors as obarrays, and an obarray is just a vector
of symbols.  But numbers aren't symbols, which is what it's complaining
about.

We could make `completing-read' iterate over all the elements in the
vector to see whether it's valid, and signal an error earlier, but it
would make common usages slower.

So I don't think there's an error in Emacs here -- feeding a
(non-obarray) vector to `completing-read' is not supported, and is an
error in the code that does it.  So I'm closing this bug report.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no