expl3 macro to expandably iterate over a list and apply a macro on each item

Question

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

\documentclass{article}
\newcounter{applyeachposition}
\newcommand\applyeachmacro{}
% #1 : macro to apply
% #2 : multi-args
\newcommand\applyeach[2]{
    \setcounter{applyeachposition}{0}
    \renewcommand\applyeachmacro{#1}
    \applyeachrecu#2|\nil
    \renewcommand\applyeachmacro{}
}
\def\applyeachrecu#1|#2\nil{
    \stepcounter{applyeachposition}
    \applyeachmacro{#1}
    \if\relax\detokenize{#2}\relax\else
        \applyeachrecu#2\nil
    \fi
}
\newcommand\decoone[1]{%
    [(\textbf{#1})] \quad%
}
\newcommand\multiapply[1]{%
    \applyeach{\decoone}{#1}
}
\begin{document}
\multiapply{1|12|123|1234}
\end{document}

Answer 1

Without the ability to change the delimiter, the definition is pretty straightforward. Though note that, as egreg says, setting a counter is an assignment, and assignments are not expandable, so you can't use them. You have to increase a number “on the fly” and pass that to the macro, but not as a counter.

The macro itself splits the argument at a | and applies a given function to that, until the end of the loop is reached. Adding a \prg_do_nothing: (and removing it later) takes care of braced items and spaces around the argument. If you need to change the delimiter then the code will get more complicated.

I defined \multiapply<cmd>{<list>} such that it iterates on the |-separated <list> and applies <cmd> to each item. The <cmd> needs to take two arguments: #1 is the number of the item in the list, and #2 is the item itself.

\documentclass{article}
\ExplSyntaxOn
\NewExpandableDocumentCommand \multiapply { m +m }
  { \mbc_multiapply:Nn #1 {#2} }
\cs_new:Npn \mbc_multiapply:Nn #1 #2
  {
    __mbc_multiapply:nNw { 0 } #1
      | \prg_do_nothing: #2 | \q_recursion_tail | \q_recursion_stop
  }
\cs_new:Npn __mbc_multiapply:nNw #1 #2 | #3 |
  { \exp_args:Nof __mbc_multiapply:nnN {#3} { \int_eval:n {#1+1} } #2 }
\cs_new:Npn __mbc_multiapply:nnN #1 #2 #3
  {
    \quark_if_recursion_tail_stop:n {#1}
    \exp_not:e { \exp_not:N #3 {#2} { \tl_trim_spaces:n {#1} } }
    __mbc_multiapply:nNw {#2} #3 | \prg_do_nothing:
  }
\ExplSyntaxOff
\newcommand\decoone[2]{%
  Item #1: [(\textbf{#2})]\quad}
\begin{document}
\multiapply\decoone{ 1 | 12 | 123 | 1234 }
\end{document}