expl3 macro to expandably iterate on a list and apply a macro on each pair of items

Question

Can someone show me how to make the following macro (\applycouple) expandable using expl3, if it is possible?

This question is similar to this post of mine.

\documentclass{article}
\newcounter{applycoupleposition}
\newcommand\applycouplemacro{}
% #1 : macro to apply
% #2 : multi-args
\newcommand\applycouple[2]{
    \setcounter{applycoupleposition}{0}
    \renewcommand\applycouplemacro{#1}
    \applycouplerecu#2|\nil
    \renewcommand\applycouplemacro{}
}
\def\applycouplerecu#1|#2|#3\nil{
    \stepcounter{applycoupleposition}
    \applycouplemacro{#1}{#2}
    \if\relax\detokenize{#3}\relax\else
        \applycouplerecu#2|#3\nil
    \fi
}
\newcommand\decocouple[2]{%
    (#1)[#2]%
}
\newcommand\multiapplycouple[1]{%
    \applycouple{\decocouple}{#1}
}
\begin{document}
\multiapplycouple{1|12|123|1234}
\end{document}

Phelype Oleinik · Accepted Answer · 2020-08-07T19:52:37.967

1

Quite similar to the implementation in my answer to your other question, but this one takes two arguments, so we need to add two \q_recursion_tail. Also another intermediary macro is used to insert a \prg_do_nothing: in the second item to avoid losing spaces and braces.

\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\decocouple[3]{Item #1: (#2)[#3]\quad}
\begin{document}
\multiapplycouple\decocouple{ 1 | 12 | 123 | 1234 }
\end{document}

edited Aug 07 '20 at 19:52

answered Aug 06 '20 at 19:20

Phelype Oleinik

70,814

Thanks. The small changes are instructive. I will try the next time to start alone the expl3 version. – projetmbc Aug 06 '20 at 21:01
@projetmbc Added space trimming here as well :-) – Phelype Oleinik Aug 07 '20 at 19:52
Thanks a lot. I have done it by myself. ;-) – projetmbc Aug 07 '20 at 21:44
It seems that \quark_if_recursion_tail_stop:n {#1}. I have seen that by using \quark_if_recursion_tail_stop_do:Nn {#1} {--KO--} instead. – projetmbc Aug 23 '20 at 13:31
@projetmbc You want \quark_if_recursion_tail_stop_do:nn (note the :nn vs. :Nn): at that point in the code you don't know that #1 is a single token (and often it is not!), so you can't use an N argument there. – Phelype Oleinik Aug 23 '20 at 13:54
Thanks a lot for pointing that. – projetmbc Aug 23 '20 at 13:55

expl3 macro to expandably iterate on a list and apply a macro on each pair of items

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