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Re: master e315544: ; * src/fileio.c (Fdirectory_append): Doc fix.

From: Lars Ingebrigtsen
Subject: Re: master e315544: ; * src/fileio.c (Fdirectory_append): Doc fix.
Date: Sun, 25 Jul 2021 07:19:12 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

Michael Albinus <michael.albinus@gmx.de> writes:

> It's rather a nice to have. Sometimes, I have a list of file name
> fragments, and I hope to avoid the checks for being non-empty strings
> ...
> But I can do this in my code.

Thinking about it a bit more, I can see use cases where ignoring nil/""
would be useful (without making the function less difficult to reason
about).  (And it's analogous to `concat', besides.)  So I think I'll go
ahead and make that change...

>>> Furthermore, we have
>>> (directory-append "foo/" "/bar") => "foo//bar"
>>> I would expect "foo/bar", meaning that all leading slashes in COMPONENTS
>>> are removed.
>> Well, I think that's over into DWIM-land again (which is what
>> expand-file-name does, and this function deliberately doesn't do).
> expand-file-name and file-name-as-directory are functions which call a
> handler, for example Tramp. directory-append is much cheaper, and if it
> could do this string handling, I could avoid those functions. It would
> be more performant.

The reason `directory-append' is useful as a function at all (instead of
just using `concat') is that we're very vague about the difference
between a directory name and a file name in Emacs.  That is, we treat
"/tmp/" and "/tmp" as equivalent in more than 99% of cases when dealing
with directories.  This has naturally led to people setting
`foo-directory' variables etc to something ending with a slash or not
arbitrarily, which again means that you can't just use `concat' to
construct file names.

But "/bar.txt" does not happen naturally as a leaf file name, so I think
it'd be counter-productive to have `directory-append' do anything
particular about the start of file name components.

By the way -- I'm not sure that `directory-append' is the best function
name here...  `file-name-concat' would perhaps be more natural?  But
it's really about directories...  `directory-name-concat'?  Hm.

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

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