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            <55F2A790.9090709@morningstar2.co.uk>
            <20150911101759.GA30955@hzdr.de>
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Message-ID:  <55F2AB44.7010107@morningstar2.co.uk>
Date:         Fri, 11 Sep 2015 11:21:56 +0100
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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: non-intuitive behaviour of clist with empty items
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
In-Reply-To:  <20150911101759.GA30955@hzdr.de>
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Envelope-To: <rainer.schoepf@GMX.NET>
Status: R

On 11/09/2015 11:17, Alexander Grahn wrote:
> On Fri, Sep 11, 2015 at 11:06:08AM +0100, Joseph Wright wrote:
>> On 11/09/2015 10:46, Alexander Grahn wrote:
>>> consider the following example:
>>>
>>>   \documentclass{article}
>>>   \usepackage{expl3}
>>>   
>>>   \begin{document}
>>>     \ExplSyntaxOn
>>>       \clist_set:Nn\l_tmpa_clist{,,foo,bar,}
>>>       \typeout{\clist_count:N\l_tmpa_clist}    %expected:  5,    I get 2.
>>>       \typeout{\clist_item:Nn\l_tmpa_clist{3}} %expected: `foo`, I get `'.
>>>       \typeout{\clist_item:Nn\l_tmpa_clist{4}} %expected: `bar`, I get `'.
>>>     \ExplSyntaxOff
>>>   \end{document}
>>>
>>> Why does \clist_set:Nn ignore empty items when initialising a clist
>>> variable from a token list? Is this intended behaviour? Is there a
>>> work-around?
> 
>> This is by-design. Comma lists can't contain empty items, commas, etc.:
>> sequences can. The reason is comma lists are 'close' to the user level,
>> and there stray empty entries are normally best ignored.
> 
> This is very unfortunate, because I want to be able to correctly process
> user input of comma sparated items which /may/ contain empty items.
> 
> With \@for from LaTeX2e I can easily process such input properly:
> 
>   \documentclass{article}
>   
>   \begin{document}
>     \makeatletter
>      \@for\listitem:=,,foo,bar,\do{
>        \typeout{item:\listitem}
>      }
>     \makeatother
>   \end{document}
> 
> Now I am looking for an L3 equivalent of \@for.
> 
> Alexander

Can of course be done using a sequence

    \input expl3-generic %
    \ExplSyntaxOn
    \seq_set_split:Nnn \l_tmpa_seq { , } { ,,foo,bar, }
    \seq_show:N \l_tmpa_seq

Joseph