GNU bug report logs - #12517
24.2; (file-name-directory "~") returns nil

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Leo <sdl.web <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 24.2; (file-name-directory "~") returns nil
Date: Wed, 26 Sep 2012 10:35:10 +0800

I was bitten by this corner case that (file-name-directory "~") returns
nil. I wonder if this is a bug/oversight?

Leo

Message #8 received at 12517 <at> debbugs.gnu.org (full text, mbox):

From: Andreas Schwab <schwab <at> linux-m68k.org>
To: Leo <sdl.web <at> gmail.com>
Cc: 12517 <at> debbugs.gnu.org
Subject: Re: bug#12517: 24.2; (file-name-directory "~") returns nil
Date: Wed, 26 Sep 2012 09:14:43 +0200

Leo <sdl.web <at> gmail.com> writes:

> I was bitten by this corner case that (file-name-directory "~") returns
> nil. I wonder if this is a bug/oversight?

That's not different from (file-name-directory "a").  The function works
purely textual and does not expand the argument, you have to call
expand-file-name yourself.

Andreas.

-- 
Andreas Schwab, schwab <at> linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Message #11 received at 12517 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Leo <sdl.web <at> gmail.com>
Cc: 12517 <at> debbugs.gnu.org
Subject: Re: bug#12517: 24.2; (file-name-directory "~") returns nil
Date: Wed, 26 Sep 2012 09:24:29 +0200

> From: Leo <sdl.web <at> gmail.com>
> Date: Wed, 26 Sep 2012 10:35:10 +0800
> 
> I was bitten by this corner case that (file-name-directory "~") returns
> nil.

As does (file-name-directory "a").

> I wonder if this is a bug/oversight?

I think it is neither.  file-name-directory works on the syntax of its
argument, not on its semantics.  The ELisp manual says:

 -- Function: file-name-directory filename
     This function returns the directory part of FILENAME, as a
     directory name (*note Directory Names::), or `nil' if FILENAME
     does not include a directory part.

"FILENAME does not include a directory part" means literally that: the
string FILENAME must _include_ a directory part for
file-name-directory to return a non-nil result.

If you want the directory of its argument, even if the argument is
given as any kind of shorthand, run that argument through
expand-file-name first.

Message #14 received at 12517 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Andreas Schwab <schwab <at> linux-m68k.org>
Cc: 12517 <at> debbugs.gnu.org, Leo <sdl.web <at> gmail.com>
Subject: Re: bug#12517: 24.2; (file-name-directory "~") returns nil
Date: Wed, 26 Sep 2012 09:05:48 -0400

>> I was bitten by this corner case that (file-name-directory "~") returns
>> nil. I wonder if this is a bug/oversight?

I don't know if it should be treated as a misfeature, but at least I've
already relies on it in code such as locate-dominating-file, which
I quote:

  ;; Represent /home/luser/foo as ~/foo so that we don't try to look for
  ;; `name' in /home or in /.
  (setq file (abbreviate-file-name file))


-- Stefan

Message #19 received at 12517-done <at> debbugs.gnu.org (full text, mbox):

From: Leo <sdl.web <at> gmail.com>
To: 12517-done <at> debbugs.gnu.org
Subject: Re: bug#12517: 24.2; (file-name-directory "~") returns nil
Date: Thu, 27 Sep 2012 00:26:35 +0800

Thanks to all for the explanations. Closing.

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