From: Paul Eggert <eggert@cs.ucla.edu>
To: Eli Zaretskii <eliz@gnu.org>
Cc: emacs-devel@gnu.org
Subject: Re: FACE_FROM_ID vs FACE_OPT_FROM_ID
Date: Fri, 24 Jun 2016 23:34:56 +0200 [thread overview]
Message-ID: <576DA780.4080801@cs.ucla.edu> (raw)
In-Reply-To: <83twgi1z4m.fsf@gnu.org>
On 06/24/2016 03:43 PM, Eli Zaretskii wrote:
> When ENABLE_CHECKING is not defined, which is what happens by default
> in Emacs builds, we attempt to "do the best we can without crashing",
No, assertions should be always true; if an assertion fails Emacs should
just abort, and should not try to patch around the bug. This is the
longstanding intent of assertions, both in Emacs and in traditional
computer science. If X is a side-effect-free expression, then after an
eassert (X) there is no point to testing whether X is true, because X is
already known to be true. The code 'eassert (X); if (!X) foo (X);' is
redundant and somewhat silly, just as the code 'while (X) if (!X) foo
(X);' is redundant and somewhat silly.
I suppose Emacs could have a different kind of check, for an expression
that must be true when debugging but that might not be true in
production. We could call it 'check_when_debugging (X)', say. Its
semantics would be that Emacs aborts when debugging is enabled and X is
false; but that X might be either true or false in production code.
However, I don't think check_when_debugging (X) would be helpful. Unlike
eassert (X), it won't help the reader or simplify later code, because
later code won't be able to assume that X is true. Worse, it would cause
Emacs behavior in debugging mode to diverge further from production
mode, and the production-only code paths would not be easily debuggable.
The whole thing would be considerably more confusing than what Emacs
does now.
It could be that the font and image caches are special cases that have a
lot of bugs, and that we therefore want some sort of belt-and-suspenders
treatment for them that goes beyond ordinary assertions. I don't see
offhand why that would be, though. Are we getting an unusual number of
bugs in that area, and if so what sort of bugs are they?
next prev parent reply other threads:[~2016-06-24 21:34 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-23 20:03 FACE_FROM_ID vs FACE_OPT_FROM_ID Eli Zaretskii
2016-06-24 0:23 ` Paul Eggert
2016-06-24 8:49 ` Eli Zaretskii
2016-06-24 9:27 ` Paul Eggert
2016-06-24 9:57 ` Eli Zaretskii
2016-06-24 10:00 ` Eli Zaretskii
2016-06-24 11:17 ` Paul Eggert
2016-06-24 13:43 ` Eli Zaretskii
2016-06-24 21:34 ` Paul Eggert [this message]
2016-06-25 7:48 ` Eli Zaretskii
2016-06-25 21:34 ` Paul Eggert
2016-07-02 9:50 ` Eli Zaretskii
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