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Date:         Fri, 11 Sep 2015 12:17:59 +0200
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From: Alexander Grahn <A.Grahn@HZDR.DE>
Subject: Re: non-intuitive behaviour of clist with empty items
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
In-Reply-To:  <55F2A790.9090709@morningstar2.co.uk>
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Envelope-To: <rainer.schoepf@GMX.NET>
Status: R

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