From: Stefan Monnier <monnier@iro.umontreal.ca>
To: emacs-devel@gnu.org
Subject: Re: Slightly extending commit 16b0520a9
Date: Sun, 06 Aug 2017 12:18:21 -0400 [thread overview]
Message-ID: <jwvefso8yzb.fsf-monnier+gmane.emacs.devel@gnu.org> (raw)
In-Reply-To: 87h8xluxlf.fsf@lylat
>>> cond = eval_sub (XCAR (args));
>>> if (!NILP (cond))
>>> - return eval_sub (Fcar (XCDR (args)));
>>> + return eval_sub (XCAR (XCDR (args)));
>> I don't see anything in the preceding code that guarantees that `XCDR (args)`
>> holds a cons, so I think XCAR here is unsafe.
> The following line includes "XCDR (XCDR (args))",
Indeed, that looks like a bug.
> I believe the reason why we can assume that XCDR (args) is a cons cell
> is that `if' requires at least 2 (unevalled) arguments, so args must be
> a list of at least length 2.
Try (eval '(if nil . "hello"))
[ ... trying it himself ... ]
Hmm... it turns out that indeed it seems that XCDR and XCAR here are
safe because before calling those functions, eval_sub happens to call
Flength on the args, and that triggers an error if the form is not
a proper list, so `XCDR (args)` will indeed be a cons once we get
to Fif.
Arguably Fif could be called from elsewhere than eval_sub, and arguably
eval_sub's implementation could be changed in such a way that it doesn't
catch this error, so the safety of using XCDR is debatable.
The important thing to remember, tho, is that Fif should not be
performance sensitive: code whose performance matters should be
byte-compiled in which case it doesn't call Fif (as is the case for all
other special forms).
Stefan
next prev parent reply other threads:[~2017-08-06 16:18 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-06 3:58 Slightly extending commit 16b0520a9 Alex
2017-08-06 4:08 ` Stefan Monnier
2017-08-06 4:30 ` Alex
2017-08-06 16:18 ` Stefan Monnier [this message]
2017-08-07 0:10 ` Paul Eggert
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