bug#67116: byte-compile-let: reversing the order of evaluation of the clauses CAN make a difference.

[Alan Mackenzie]

Hello, Eli.

On Sun, Nov 12, 2023 at 08:13:39 +0200, Eli Zaretskii wrote:
> > Cc: 67116@debbugs.gnu.org
> > Date: Sat, 11 Nov 2023 23:52:38 -0500
> > From:  Stefan Monnier via "Bug reports for GNU Emacs,
> >  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>

> > > In lisp/emacs-lisp/bytecomp.el (byte-compile-let), when the following
> > > form (from jit-lock--debug-fontify):

> > >                           (let
> > >                               ((beg pos)
> > >                                 (end (setq pos
> > >                                                
> > > (next-single-property-change
> > >                                                 pos 'fontified
> > >                                                 nil (point-max)))))
> > >                             (put-text-property beg end 'fontified nil)
> > >                             (jit-lock-fontify-now beg end))

> > > gets byte compiled, the order of evaluating BEG and END gets reversed so
> > > that END gets evaluated first.

> > Sounds like a bug.

> It does?  I always thought that the order of evaluation in a let form
> is unspecified, and in practice I had several bugs of exactly this
> nature, which I fixed by using let*, as expected.

No.  The order of _evaluation_ is specified as top to bottom.  The order
of _binding_ is unspecified.  Quoting from the elisp.info page "Local
Variables":

     All of the VALUE-FORMs in BINDINGS are evaluated in the order they
     appear and _before_ binding any of the symbols to them.

and a little later on the same page:

     On the other hand, the order of _bindings_ is unspecified:

> Why on Earth should we require any particular order of evaluation in a
> let form??

To make the value of a form unambiguous?  In any case, we do require a
particular order.

-- 
Alan Mackenzie (Nuremberg, Germany).