expl3 - Iterate over non overlapping couples and apply a macro on each item

Question

This question is similar to this one but not exactly the same.

The following "not doing what I need" MWE comes from this answer.

I have not found how to make it evoluate such as to iterate over non overlaping couples.

IMPORTANT ! The number of arguments separated by pipes will allways be odd.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
    \NewExpandableDocumentCommand \multiapplycouple { m +m }
      { \mbc_multiapply_pair:Nn #1 {#2} }
    \cs_new:Npn \mbc_multiapply_pair:Nn #1 #2
      {
        __mbc_multiapply_pair:nNw { 0 } #1
          | \prg_do_nothing: #2 | \q_recursion_tail | \q_recursion_stop
      }
    \cs_new:Npn __mbc_multiapply_pair:nNw #1 #2 | #3 |
      { __mbc_multiapply_pair:nNww {#1} #2 | #3 | \prg_do_nothing: }
    \cs_new:Npn __mbc_multiapply_pair:nNww #1 #2 | #3 | #4 |
      { __mbc_multiapply_pair:oofN {#3} {#4} { \int_eval:n {#1+1} } #2 }
    \cs_new:Npn __mbc_multiapply_pair:nnnN #1 #2 #3 #4
      {
        \quark_if_recursion_tail_stop:n {#2}
        \quark_if_recursion_tail_stop:n {#1}
        \exp_not:e { \exp_not:N #4 {#3} { \tl_trim_spaces:n {#1} } { \tl_trim_spaces:n {#2} } }
        __mbc_multiapply_pair:nNww {#3} #4 | \prg_do_nothing: #2 | \prg_do_nothing:
      }
    \cs_generate_variant:Nn __mbc_multiapply_pair:nnnN { oof }
\ExplSyntaxOff
\newcommand\deco{?}
\begin{document}
\multiapplycouple\deco{ 1 }                 % [[1]]
\multiapplycouple\deco{ 1 | 2 | 3 }         % [[1]] (2-3)
\multiapplycouple\deco{ 1 | 2 | 3 | 4 | 5 } % [[1]] (2-3) (4-5)
\end{document}

Wouldn't it be better if you explain *what* you expect the macros do? — egreg, Aug 23 '20 at 14:08
@egreg One concrete use is for typing continued fractions. The expected output inside the MWE is easy to turn to this concrete use case. — projetmbc, Aug 23 '20 at 14:09
And what's the reason for requiring expandability, since you just need to typeset? — egreg, Aug 23 '20 at 14:10
You are right. Here the expandability is not useful but this could be the case in other situations. — projetmbc, Aug 23 '20 at 14:11

score 1 · Accepted Answer · answered Aug 23 '20 at 14:48

xparse can define many command signatures that are generalisations of standard LaTeX2e forms but at some level you are essentially writing a parser and need to drop lower. You can probably do what you want by turning the | separated list into an l3 sequence but this just parses it directly

\documentclass{article}
\def\deco#1#2{(#1-#2)}
\def\multiapplycouple#1#2{$\xmultiapplycouple#1#2|\relax|\relax|$}
\def\xmultiapplycouple#1#2|{[[#2]]\xxmultiapplycouple#1}
\def\xxmultiapplycouple#1#2|#3|{%
\ifx\relax#2\else
#1{#2}{#3}%
\expandafter\xxmultiapplycouple\expandafter#1\fi}
\begin{document}
\multiapplycouple\deco{ 1 }                 % [[1]]
\multiapplycouple\deco{ 1 | 2 | 3 }         % [[1]] (2-3)
\multiapplycouple\deco{ 1 | 2 | 3 | 4 | 5 } % [[1]] (2-3) (4-5)
\end{document}

You're right. Thanks for sharing this clear code. – projetmbc Aug 23 '20 at 21:41 — projetmbc, Aug 23 '20 at 21:41

expl3 - Iterate over non overlapping couples and apply a macro on each item

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