From: Paul Eggert <eggert@cs.ucla.edu>
To: Philipp Stephani <p.stephani2@gmail.com>,
Daniel Colascione <dancol@dancol.org>,
emacs-devel@gnu.org, Stefan Monnier <monnier@iro.umontreal.ca>
Subject: Re: [Emacs-diffs] emacs-25 3eb93c0: Rely on conservative stack scanning to find "emacs_value"s
Date: Wed, 30 Mar 2016 11:42:26 -0700 [thread overview]
Message-ID: <56FC1E12.3070108@cs.ucla.edu> (raw)
In-Reply-To: <CAArVCkS81--_T12knm0ufeHEy==r5Fx6OuwkC4as6Y=GKitxCA@mail.gmail.com>
On 03/30/2016 11:26 AM, Philipp Stephani wrote:
>
> It's not clear that this feature is worth the hassle of supporting it.
>
>
> If emacs_value* were a real pointer, then the feature would come for free.
This is begging the question, no? There are good reasons to be
suspicious of null values, regardless of whether they are represented by
null pointers or by something else.
> As Stefan mentioned, one can easily use Qnil etc. as sentinels.
>
>
> These aren't sentinels because functions can return nil without
> existing non-locally.
If there is a need for C code to return a value that is either an Emacs
value or something else, then a cleaner way to specify this is as some
sort of discriminated union that is either an Emacs value, or something
else. Attempting to shoehorn an "I'm not an Emacs value!" value into the
emacs_value type leads to unnecessary confusion. At least, any such
union type should be called "emacs_value_option", or something like
that, to clearly warn the maintainer that the type is an Emacs value or
is something else.
>
> XCAR etc. certainly can't be avoided. We should try to reduce
> undefined behavior though, not add more.
>
Yes, there are good arguments against the XCAR etc. approach that is
used throughout the Emacs interpreter. However, there are also good
arguments for the approach, and the current Emacs design clearly uses
and assumes it and it works well enough in practice to overcome the
mostly-theoretical objections raised in this thread. Regardless of the
implementation strategy chosen by the Emacs core, it's better if the
module system lives in harmony with the Emacs interpreter instead of
attempting to fight against it.
next prev parent reply other threads:[~2016-03-30 18:42 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20151130193446.22218.14685@vcs.savannah.gnu.org>
[not found] ` <E1a3UDm-0005my-HI@vcs.savannah.gnu.org>
2016-01-03 13:59 ` [Emacs-diffs] emacs-25 3eb93c0: Rely on conservative stack scanning to find "emacs_value"s Daniel Colascione
2016-01-03 15:45 ` Paul Eggert
2016-03-29 10:07 ` Philipp Stephani
2016-03-29 12:59 ` Stefan Monnier
2016-03-30 1:35 ` Paul Eggert
2016-03-30 18:26 ` Philipp Stephani
2016-03-30 18:28 ` Daniel Colascione
2016-03-30 18:42 ` Paul Eggert [this message]
2016-03-30 18:48 ` Daniel Colascione
2016-03-30 21:13 ` Stefan Monnier
2016-03-30 21:18 ` Daniel Colascione
2016-03-30 21:30 ` Stefan Monnier
2016-03-31 18:46 ` Philipp Stephani
2016-03-31 19:24 ` Daniel Colascione
2016-03-31 21:29 ` Stefan Monnier
2016-04-01 18:34 ` Daniel Colascione
2016-04-01 19:05 ` Stefan Monnier
2016-04-01 19:15 ` Daniel Colascione
2016-04-01 22:46 ` Stefan Monnier
2016-04-03 1:11 ` Daniel Colascione
2016-04-03 5:49 ` Stefan Monnier
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