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Message-ID:  <51D121F7.9050308@morningstar2.co.uk>
Date:         Mon, 1 Jul 2013 07:30:15 +0100
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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: Defining \bool_case:nn
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
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Status: R

On 27/06/2013 17:42, Joel C. Salomon wrote:
> Building on the TeX.SE posts "Is there a \bool_case:Nnn?"
> (http://tex.stackexchange.com/q/121074/2966) and Joseph Wright's
> answer to "How do the \xx_case: functions work?"
> (http://tex.stackexchange.com/q/121390/2966), here is \bool_case:nn,
> proposed for addition:
> 
> \documentclass{minimal}
> \usepackage{expl3, xparse}
> 
> \ExplSyntaxOn
> 
> \cs_new:Npn \bool_case:nn #1#2
>   {
>     \tex_romannumeral:D
>       \__bool_case:w #1 \c_true_bool {#2} \q_recursion_stop
>   }
> \cs_new:Npn \__bool_case:w #1#2
>   {
>     \bool_if:nTF { #1 }
>       { \__bool_case_end:nw {#2} }
>       { \__bool_case:w }
>   }
> \cs_new_eq:NN \__bool_case_end:nw \__prg_case_end:nw
> 
> \NewDocumentCommand \testboolcase {}
>   {
>     \bool_set_false:N \l_tmpa_bool
>     \bool_set_true:N \l_tmpb_bool
> 
>     \bool_case:nn
>       {
>         \l_tmpa_bool { false;~ can't~ happen }
>         \l_tmpb_bool { true;~ should~ happen }
>       }
>       { else;~ can't~ happen }
>   }
> 
> \NewDocumentCommand \testboolcaseelse {}
>   {
>     \bool_set_false:N \l_tmpa_bool
>     \bool_set_true:N \l_tmpb_bool
> 
>     \bool_case:nn
>       {
>         \l_tmpa_bool { false;~ can't~ happen }
>         { !\l_tmpb_bool } { false;~ can't~ happen }
>       }
>       { else;~ should~ happen }
>   }
> 
> \ExplSyntaxOff
> 
> \begin{document}
> \testboolcase
> 
> \testboolcaseelse
> \end{document}

I'm not sure I see the need for this. In the other 'case' functions, at
most exactly one case can be true

  \int_case:nnn { #1 }
    {
      { 1 }  { stuff }
      { 2 } ...

However, in \bool_case:nn you stop at the first case that happens to be
true. That means that order matters for the result, which isn't what
happens in the other case functions.
-- 
Joseph Wright