GNU bug report logs - #64127
30.0.50; mutate-constant warning with pure function

Previous Next

To add a comment to this bug, you must first unarchive it, by sending
a message to control AT debbugs.gnu.org, with unarchive 64127 in the body.
You can then email your comments to 64127 AT debbugs.gnu.org in the normal way.

Toggle the display of automated, internal messages from the tracker.

Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Basil Contovounesios <contovob <at> tcd.ie>
To: bug-gnu-emacs <at> gnu.org
Subject: 30.0.50; mutate-constant warning with pure function
Date: Sat, 17 Jun 2023 12:32:02 +0100

[Message part 1 (text/plain, inline)]

Given a file foo.el that defines a pure function foo:

[foo.el (application/emacs-lisp, inline)]

[Message part 3 (text/plain, inline)]

and another file bar.el that mutates the result of foo:

[bar.el (application/emacs-lisp, inline)]

[Message part 5 (text/plain, inline)]

byte-compiling bar.el emits a mutate-constant warning, even though the
result of foo is a fresh list:

$ for f in foo bar; do emacs -Q -batch -L . -f batch-byte-compile $f.el; done

In toplevel form:
bar.el:3:2: Warning: ‘setcar’ on constant list (arg 1)

Am I missing something?

Thanks,

-- 
Basil

In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw3d scroll bars) of 2023-06-17 built on blc
Repository revision: c279369a7a9e373bb4b88feff0a05f56f3c0fa3b
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
System Description: Ubuntu 22.04.2 LTS

Configured using:
 'configure CC=gcc-12 'CFLAGS=-Og -ggdb3' --prefix=/home/bic/.local
 --with-file-notification=yes --with-x --with-x-toolkit=lucid'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XAW3D XDBE XIM XINPUT2 XPM
LUCID ZLIB

Important settings:
  value of $LC_MONETARY: en_IE.UTF-8
  value of $LC_NUMERIC: en_IE.UTF-8
  value of $LC_TIME: en_IE.UTF-8
  value of $LANG: en_GB.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
  eldoc-mode: t
  show-paren-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
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search time-date subr-x mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
cl-loaddefs cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util
mail-prsvr mail-utils rmc iso-transl tooltip cconv eldoc paren electric
uniquify ediff-hook vc-hooks lisp-float-type elisp-mode mwheel
term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine 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
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo x-toolkit
xinput2 x multi-tty make-network-process emacs)

Memory information:
((conses 16 36748 9460)
 (symbols 48 5178 0)
 (strings 32 13894 1203)
 (string-bytes 1 379737)
 (vectors 16 9300)
 (vector-slots 8 148644 10010)
 (floats 8 23 25)
 (intervals 56 244 0)
 (buffers 984 10))

Message #8 received at 64127 <at> debbugs.gnu.org (full text, mbox):

From: Mattias Engdegård <mattias.engdegard <at> gmail.com>
To: Basil Contovounesios <contovob <at> tcd.ie>
Cc: 64127 <at> debbugs.gnu.org
Subject: Re: bug#64127: 30.0.50; mutate-constant warning with pure function
Date: Sat, 17 Jun 2023 13:58:08 +0200

17 juni 2023 kl. 13.32 skrev Basil Contovounesios <contovob <at> tcd.ie>:

> Given a file foo.el that defines a pure function foo:

> (defun foo (x) (declare (pure t)) (list x))

> and another file bar.el that mutates the result of foo:

> (setcar (foo nil) t)

> byte-compiling bar.el emits a mutate-constant warning, even though the
> result of foo is a fresh list:

No, the `pure` declaration means that the function can be evaluated at compile time which the compiler happily does, yielding a constant list, which your code then attempts to modify.

This is why the function `list` itself is not declared `pure` -- while it does look like a pure function when speaking informally, users relies on it returning a freshly allocated list that can be modified and that makes it non-pure. (If lists were immutable, then `list` would naturally be pure.)

Only the mutate-constant warning is new here; previously, the compiler would have let you make this mistake undisturbed.

Thus either you remove the pure-declaration from your function, or you don't mutate what it returns.

Message #11 received at 64127 <at> debbugs.gnu.org (full text, mbox):

From: Basil Contovounesios <contovob <at> tcd.ie>
To: Mattias Engdegård <mattias.engdegard <at> gmail.com>
Cc: 64127 <at> debbugs.gnu.org
Subject: Re: bug#64127: 30.0.50; mutate-constant warning with pure function
Date: Sat, 17 Jun 2023 17:28:01 +0100

Mattias Engdegård [2023-06-17 13:58 +0200] wrote:

> 17 juni 2023 kl. 13.32 skrev Basil Contovounesios <contovob <at> tcd.ie>:
>
>> Given a file foo.el that defines a pure function foo:
>
>> (defun foo (x) (declare (pure t)) (list x))
>
>> and another file bar.el that mutates the result of foo:
>
>> (setcar (foo nil) t)
>
>> byte-compiling bar.el emits a mutate-constant warning, even though the
>> result of foo is a fresh list:
>
> No, the `pure` declaration means that the function can be evaluated at
> compile time which the compiler happily does, yielding a constant
> list, which your code then attempts to modify.
>
> This is why the function `list` itself is not declared `pure` -- while it does
> look like a pure function when speaking informally, users relies on it returning
> a freshly allocated list that can be modified and that makes it non-pure. (If
> lists were immutable, then `list` would naturally be pure.)
>
> Only the mutate-constant warning is new here; previously, the compiler
> would have let you make this mistake undisturbed.
>
> Thus either you remove the pure-declaration from your function, or you
> don't mutate what it returns.

Right.

Which approach do you think the dash.el library in GNU ELPA should
follow?  It generally defines nondestructive operations over lists, some
of which even claim in their docstring that they return a partial or
complete copy of their arguments (modulo the parts changed by the
operation).

Is it okay for a pure function to say it returns a copy in its
docstring, with the onus lying on the caller to realise that a pure
function call may be byte-compiled to a runtime constant?  Or should all
such functions be impurified?

Thanks,

-- 
Basil

Message #14 received at 64127 <at> debbugs.gnu.org (full text, mbox):

From: Mattias Engdegård <mattias.engdegard <at> gmail.com>
To: Basil Contovounesios <contovob <at> tcd.ie>
Cc: 64127 <at> debbugs.gnu.org
Subject: Re: bug#64127: 30.0.50; mutate-constant warning with pure function
Date: Sat, 17 Jun 2023 19:03:23 +0200

17 juni 2023 kl. 18.28 skrev Basil Contovounesios <contovob <at> tcd.ie>:

> Which approach do you think the dash.el library in GNU ELPA should
> follow?  It generally defines nondestructive operations over lists, some
> of which even claim in their docstring that they return a partial or
> complete copy of their arguments (modulo the parts changed by the
> operation).

Its doc strings seem to talk a lot about how the functions returns a new this or a copy of that. Maybe that provides the licence to return a constant when those words are absent, or perhaps the users will just assume mutability in absence of stern warnings. I don't know how dash.el is used in practice, so perhaps it's prudent to stay off the `pure` declarations.

> Is it okay for a pure function to say it returns a copy in its
> docstring, with the onus lying on the caller to realise that a pure
> function call may be byte-compiled to a runtime constant?  Or should all
> such functions be impurified?

A pure function cannot in general be guaranteed to return an eq-unique value. By definition it will, if all its arguments are constants, be called at compile-time to generate a constant used in the program.

There is nothing wrong with returning a newly created object from a `pure`-declared function, as long as reasonable steps are taken to prevent the returned value from being mutated. Depending on the context this can be as simple as not saying that it returns a new object.

Message #21 received at 64127-done <at> debbugs.gnu.org (full text, mbox):

From: Basil Contovounesios <contovob <at> tcd.ie>
To: Mattias Engdegård <mattias.engdegard <at> gmail.com>
Cc: 64127-done <at> debbugs.gnu.org
Subject: Re: bug#64127: 30.0.50; mutate-constant warning with pure function
Date: Sat, 17 Jun 2023 21:56:30 +0100

tags 64127 notabug wontfix
close 64127
quit

Mattias Engdegård [2023-06-17 19:03 +0200] wrote:

> 17 juni 2023 kl. 18.28 skrev Basil Contovounesios <contovob <at> tcd.ie>:
>
>> Which approach do you think the dash.el library in GNU ELPA should
>> follow?  It generally defines nondestructive operations over lists, some
>> of which even claim in their docstring that they return a partial or
>> complete copy of their arguments (modulo the parts changed by the
>> operation).
>
> Its doc strings seem to talk a lot about how the functions returns a new this or
> a copy of that. Maybe that provides the licence to return a constant when those
> words are absent, or perhaps the users will just assume mutability in absence of
> stern warnings. I don't know how dash.el is used in practice, so perhaps it's
> prudent to stay off the `pure` declarations.

Done for some of the likelier candidates in
https://github.com/magnars/dash.el/commit/d5182da04c.

>> Is it okay for a pure function to say it returns a copy in its
>> docstring, with the onus lying on the caller to realise that a pure
>> function call may be byte-compiled to a runtime constant?  Or should all
>> such functions be impurified?
>
> A pure function cannot in general be guaranteed to return an eq-unique
> value. By definition it will, if all its arguments are constants, be
> called at compile-time to generate a constant used in the program.
>
> There is nothing wrong with returning a newly created object from a
> `pure`-declared function, as long as reasonable steps are taken to prevent the
> returned value from being mutated. Depending on the context this can be as
> simple as not saying that it returns a new object.

Makes sense, thanks for elaborating.

-- 
Basil

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.