From: "Davis Herring" <herring@lanl.gov>
To: "Stephen J. Turnbull" <stephen@xemacs.org>
Cc: Leo <sdl.web@gmail.com>, emacs-devel@gnu.org
Subject: Re: Is (provide 'foo) at the start good or bad?
Date: Sun, 14 Jun 2009 12:30:16 -0700 (PDT) [thread overview]
Message-ID: <51511.130.55.118.19.1245007816.squirrel@webmail.lanl.gov> (raw)
In-Reply-To: <87eitowck5.fsf@uwakimon.sk.tsukuba.ac.jp>
> > If you have two files which require each other, why do they each have a
> > feature symbol?
>
> Because external packages don't know about the mutual dependency, and
> shouldn't have to. Multiple versions of the files may support the
> same interfaces, some with and some without mutual dependency. Etc.
That was case #3, wasn't it? Once you have such a dependency, you might
as well (internally, for development) treat the two files as one package.
I didn't mean to imply that you never needed two symbols -- that's why I
gave solutions, for each case that I could think of, that didn't involve
putting `provide' at the top.
> > Put differently, `provide' is supposed to "Announce that FEATURE is a
> > feature of the current Emacs.". If you put it at the beginning of a
> > package, you're lying (until the end of it).
>
> Sure. There are other standard techniques that involve such "lying",
> like `(defvar foo)', which does exactly the same kind of thing that a
> provide at the top does. In both cases, there may be a path through
> the code leaves something uninitialized.
But the only thing which can even tell that (defvar foo) was present is
the byte-compiler; we know all about how to lie to it safely. `provide'
has a globally-visible effect (that's its whole purpose!), so arbitrary
code may react badly if we lie when we use it.
Davis
--
This product is sold by volume, not by mass. If it appears too dense or
too sparse, it is because mass-energy conversion has occurred during
shipping.
next prev parent reply other threads:[~2009-06-14 19:30 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-06-11 12:56 Is (provide 'foo) at the start good or bad? William Xu
2009-06-11 17:01 ` Leo
2009-06-12 4:09 ` Stephen J. Turnbull
2009-06-12 5:01 ` William Xu
2009-06-12 10:02 ` Thien-Thi Nguyen
2009-06-12 10:26 ` Stephen J. Turnbull
2009-06-12 15:15 ` William Xu
2009-06-12 8:36 ` Alan Mackenzie
2009-06-12 10:10 ` Stephen J. Turnbull
2009-06-12 23:00 ` Davis Herring
2009-06-13 12:19 ` Stephen J. Turnbull
2009-06-14 19:30 ` Davis Herring [this message]
2009-06-15 3:04 ` Stephen J. Turnbull
2009-06-15 18:20 ` Davis Herring
2009-06-16 3:47 ` Stephen J. Turnbull
2009-06-12 21:16 ` Stefan Monnier
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