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

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Leo <sdl.web <at> gmail.com>
Cc: 12517 <at> debbugs.gnu.org
Subject: 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.

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