GNU bug report logs - #36418
26.2.90; mapconcat "SEPARATOR must be a string." is incorrect

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Xu Chunyang <mail <at> xuchunyang.me>
Subject: bug#36418: closed (Re: bug#36418: 26.2.90; mapconcat "SEPARATOR
 must be a string." is incorrect)
Date: Sat, 06 Jul 2019 07:34:02 +0000

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Your bug report

#36418: 26.2.90; mapconcat "SEPARATOR must be a string." is incorrect

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 36418 <at> debbugs.gnu.org.

-- 
36418: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=36418
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Stefan Kangas <stefan <at> marxist.se>
Cc: 36418-done <at> debbugs.gnu.org, mail <at> xuchunyang.me
Subject: Re: bug#36418: 26.2.90;
 mapconcat "SEPARATOR must be a string." is incorrect
Date: Sat, 06 Jul 2019 10:32:51 +0300

> From: Stefan Kangas <stefan <at> marxist.se>
> Date: Sat, 6 Jul 2019 06:17:08 +0200
> Cc: 36418 <at> debbugs.gnu.org
> 
> > C-h S mapconcat says
> >
> >> SEPARATOR, which also must be a string, or a vector or list of
> >> characters.
> >
> > C-h f concat says
> >
> >> Each argument may be a string or a list or vector of characters (integers).
> 
> In other words, since SEPARATOR can be a "list of characters", it
> follows that it can also be nil (which is equivalent to the empty list).

Right, I fixed the doc string.

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From: Xu Chunyang <mail <at> xuchunyang.me>
To: bug-gnu-emacs <at> gnu.org
Subject: 26.2.90; mapconcat "SEPARATOR must be a string." is incorrect
Date: Fri, 28 Jun 2019 21:23:24 +0800

C-h f mapconcat says

> SEPARATOR must be a string.

but nil works fine

    (mapconcat #'identity '("a" "b") nil)
    ;; => "ab"

and `string-join` relies on this fact (by the way, string-join's
docstring doesn't explain the case when separator is omitted or nil)

    (defsubst string-join (strings &optional separator)
      "Join all STRINGS using SEPARATOR."
      (mapconcat 'identity strings separator))

C-h S mapconcat says

> SEPARATOR, which also must be a string, or a vector or list of
> characters.

C-h f concat says

> Each argument may be a string or a list or vector of characters (integers).

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