GNU bug report logs - #36490
26.1; directory-files-recursively breaks when it encounters a directory named "~"

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Message #49 received at 36490 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 36490 <at> debbugs.gnu.org, erik_hahn <at> gmx.de
Subject: Re: bug#36490: 26.1;
 directory-files-recursively breaks when it encounters a directory
 named "~"
Date: Tue, 09 Jul 2019 19:00:13 +0200

Eli Zaretskii <eliz <at> gnu.org> writes:

> Sorry, that's not what I meant to say.  I meant to say how would a
> Lisp program know whether (expand-file-name "~/") means the home
> directory or a directory whose name is literally "~"?

Well, we have documented that in expand-file-name "~/" means the home
directory, and I have no problems with that.

"~/" isn't something you'll ever get from functions like
directory-files, so it's not something you'd feed to expand-file-name in
these situations...

> Btw, stuff like (expand-file-name "foo/~/") already does what you
> want, so the problem is only with the leading '~', and can be avoided
> if we avoid that situation.  IOW, why should this example:
>
>   (expand-file-name "~" "/tmp/")
>      => "/home/larsi"
>
> determine how directory-files-recursively behaves?

expand-file-name's use case is to (basically) concatenate a directory
name and a file name, but it's used instead of concat because nobody
wants to care about whether the directory name has a trailing slash or
not.

(concat "/tmp/" "foo")
=> "/tmp/foo" ; Good

(concat "/tmp" "foo")
=> "/tmpfoo" ; Bad.

(expand-file-name "foo" "/tmp")
=> "/tmp/foo" ; Yay

That's basically the use case for expand-file-name, and using it has
avoided a lot of basic concatenation problems over the years (because
Emacs allows sloppy handling of directory file names in most
situations).

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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