bug#40827: 28.0.50; Macroexpansion at runtime

bug#40827: 28.0.50; Macroexpansion at runtime

[Omar Antolín Camarena]

I'm not sure this is a bug, but I certainly think it's odd behavior.
Basically, incorrect use of (rx ... (eval ...) ...), which should raise
an error, can instead result in macro expansion happening at runtime
rather than at function definition time.

Starting from emacs -Q, put the following in the scratch buffer:

(defmacro macro1 (arg)
  `(list :macro1 ,arg ,(format-time-string "%s")))

(setq lexical-binding nil)

(defun fun1 (arg) (list (macro1 arg) (rx (eval arg))))

Now, if you evaluate all three forms with eval-last-sexp (C-x C-e), the
definition of fun1 correctly (IMO) raises an error.

Instead evaluate the first two with eval-sexp and the last function with
eval-defun (C-M-x). No error is raised!

Next examine the definition of fun1 and call it a couple of times:

(symbol-function 'fun1)
;; evaluates to (lambda (arg) (list (macro1 arg) (rx (eval arg))))
;; notices the macros are unexpanded

(fun1 "hey")
;; evaluated to ((:macro1 "hey" "1587754829") "hey")

(fun1 "hey")
;; now I got ((:macro1 "hey" "1587754834") "hey")
;; notice the timestamp changed!

It seems that now, most of the time both the intepreter and the
byte-compiler, whether using lexical binding or not, will expand macros
at function definition time. This trick of combining an illegal (rx ...
(eval ...) ...), with lexical binding, and with eval-defun is the only
way I've found to trigger macro expansion at runtime.

I discovered this during an interesting discussion on reddit, which you
can read here:

https://www.reddit.com/r/emacs/comments/g5bat3/weekly_tipstricketc_thread/fo362s8

In that discussion others pointed out that this does not happen in Emacs
26.3: there the recipe I gave above also results in an error being
raised.

The Emacs 28 package I installed unfortunately does not come with a
manual, so I don't know what the current version says, but the manual
for Emacs 26.3 says something seemingly incorrect most of the time.

(info "(elisp) Repeated Expansion") says:

> 14.5.5 How Many Times is the Macro Expanded?

> Occasionally problems result from the fact that a macro call is
> expanded each time it is evaluated in an interpreted function, but is
> expanded only once (during compilation) for a compiled function. If
> the macro definition has side effects, they will work differently
> depending on how many times the macro is expanded.


In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.5, cairo version 1.16.0)
 of 2020-04-13, unofficial emacs-snapshot build: http://emacs.ganneff.de/, git commit fc336a46553919206d9ac621d1ea5e9740477e18 built on runner-d40f5ff3-project-26-concurrent-1
Repository revision: b5bdd1ca1d3978fc7cfc61ee14c9945e868eff7a
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12001000
System Description: Debian GNU/Linux 10 (buster)

Recent messages:
Loading mb-depth...done
Loading minibuf-eldef...done
For information about GNU Emacs and the GNU system, type C-h C-a.

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/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/28.0.50/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/28.0.50/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/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/28.0.50/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/28.0.50/site-lisp:/usr/share/emacs/site-lisp
 --with-x=yes --with-x-toolkit=gtk3 --with-toolkit-scroll-bars
 'CFLAGS=-g -O2
 -fdebug-prefix-map=/builds/joerg/emacs/buster_amd64/emacs-snapshot-20200413+emacs-27.0.90-697-gfc336a4655=.
 -fstack-protector-strong -Wformat -Werror=format-security -Wall
 -fno-omit-frame-pointer' 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2'
 LDFLAGS=-Wl,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS PDUMPER LCMS2
GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  goto-address-prog-mode: t
  show-paren-mode: t
  electric-pair-mode: t
  beginend-global-mode: t
  beginend-prog-mode: t
  global-dot-mode: t
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  global-gobble-whitespace-mode: t
  icomplete-mode: t
  minibuffer-electric-default-mode: t
  minibuffer-depth-indicate-mode: t
  override-global-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-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
  line-number-mode: t
  transient-mark-mode: t

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bug#40827: 28.0.50; Macroexpansion at runtime

[Noam Postavsky]

Omar Antolín Camarena <omar@matem.unam.mx> writes:

> It seems that now, most of the time both the intepreter and the
> byte-compiler, whether using lexical binding or not, will expand macros
> at function definition time. This trick of combining an illegal (rx ...
> (eval ...) ...), with lexical binding, and with eval-defun is the only
> way I've found to trigger macro expansion at runtime.
[...]
> I discovered this during an interesting discussion on reddit, which you
> can read here:
>
> https://www.reddit.com/r/emacs/comments/g5bat3/weekly_tipstricketc_thread/fo362s8
[...]
> (info "(elisp) Repeated Expansion") says:
>
>> 14.5.5 How Many Times is the Macro Expanded?
>
>> Occasionally problems result from the fact that a macro call is
>> expanded each time it is evaluated in an interpreted function, but is
>> expanded only once (during compilation) for a compiled function. If
>> the macro definition has side effects, they will work differently
>> depending on how many times the macro is expanded.

Also further down:

       [...] But in interpreted execution, the macro is expanded each
    time the macro call runs
    [...]
    If `initialize' is interpreted, a new list `(nil)' is constructed each
    time `initialize' is called.

(elisp "(elisp) Expansion") does mention that uncompiled macro calls
(usually) also get expanded just once:

       Note that Emacs tries to expand macros when loading an uncompiled
    Lisp file.  This is not always possible, but if it is, it speeds up
    subsequent execution.  *Note How Programs Do Loading::.

Ccing Stefan M, since you made the eager macroexpansion changes as far
as I recall.

bug#40827: 28.0.50; Macroexpansion at runtime

[Stefan Monnier]

>>> Occasionally problems result from the fact that a macro call is
>>> expanded each time it is evaluated in an interpreted function, but is
>>> expanded only once (during compilation) for a compiled function. If
>>> the macro definition has side effects, they will work differently
>>> depending on how many times the macro is expanded.

We could be more nuanced, indeed, since the "expanded once" can also
happen for interpreted code (and the "expand many times" can also happen
in code that's in a compiled file, if that code happened to be "hidden"
from the compiler, such as hidden within a `quote`).

But the basic idea remains the same: macro expansion may happen only
once as part of the preprocessing, or it may be performed anew each time
the code is run (or anything in-between, really), so the code should not
make assumptions about when macros are expanded or how many times
they're expanded.


        Stefan

bug#40827: 28.0.50; Macroexpansion at runtime

[Richard Stallman]

[[[ To any NSA and FBI agents reading my email: please consider    ]]]
[[[ whether defending the US Constitution against all enemies,     ]]]
[[[ foreign or domestic, requires you to follow Snowden's example. ]]]

      >    [...] But in interpreted execution, the macro is expanded each
      > time the macro call runs
      > [...]
      > If `initialize' is interpreted, a new list `(nil)' is constructed each
      > time `initialize' is called.

Aside from the issue you're discussing, that text has the drawback of
using the passive voice.  The passive voice tends to be less readable,
and it can obscure who or what is performing the action.  Would someone
please rewrite it to use the active voice?

Occasionally the passive voice is necessary, but please write
in the active voice whenever that works.

-- 
Dr Richard Stallman
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)

bug#40827: 28.0.50; Macroexpansion at runtime

[Lars Ingebrigtsen]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> We could be more nuanced, indeed, since the "expanded once" can also
> happen for interpreted code (and the "expand many times" can also happen
> in code that's in a compiled file, if that code happened to be "hidden"
> from the compiler, such as hidden within a `quote`).

---
  If the macro is expanded just once, in compilation, then the object is
constructed just once, during compilation.  But in interpreted
execution, the macro is expanded each time the macro call runs, and this
means a new object is constructed each time.
---

I think this is pretty clear -- it's guarded by "if", but it doesn't
claim that compilation will always expand the macro.  So I think this
should be sufficient, and I'm closing this bug report.

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