Use : colon character in weird argument — expl3

Question

Since \ExplSyntaxOn changes catcode of : it makes command with weird argument break when using the macro outside.

\documentclass{article}

\begin{document}

\ExplSyntaxOn
\cs_set:Npn \my_foo:w #1:#2\q_stop {
  (#1)
}
\newcommand{\foo}[1]{
  \my_foo:w #1:\q_stop
}

\foo{abc:def} % works
\ExplSyntaxOff

\foo{abc:def} % doesn't work

\end{document}

Is there constant like \c_math_subscript_token but for colon?

Answer 1

The problem is that in the scope of \ExplSyntaxOn the colon has category code 11, but in the document it has category code 12.

You can store the desired parameter text in a token list and then use its value:

\documentclass{article}

\ExplSyntaxOn

\tl_set:Nx \l_tmpa_tl { ##1 \c_colon_str ##2 }
\exp_last_unbraced:NNV \cs_set:Npn \my_foo:w \l_tmpa_tl \q_stop
 {
  (#1)
 }

\NewExpandableDocumentCommand{\foo}{m}
 {
  \my_foo:w #1:\q_stop
 }

\ExplSyntaxOff

\begin{document}

\foo{abc:def}

\end{document}

Answer 2

Instead of changing the colon in the definition you could also change the catcode before reading the argument, that has the advantage that your command will work inside and outside \ExplSyntax:

\documentclass{article}

\begin{document}

\ExplSyntaxOn
\cs_set:Npn \__antshar_foo:w #1:#2\q_stop {
  (#1)
}

\cs_new:Npn \foo 
 {
  \group_begin: \char_set_catcode_letter:N :
  \__anthshar_foo_aux:n  
 }
\cs_new:Npn \__anthshar_foo_aux:n  #1
 {
  \group_end: \__antshar_foo:w #1:\q_stop
 } 


\foo{abc:def} % works

\ExplSyntaxOff

\foo{abc:def} % works

\end{document}

Answer 3

Here is a workaround (but the definition is global).

\documentclass{article}

\begin{document}

\ExplSyntaxOn

{
\catcode`\: = 12
\expandafter \gdef \csname my_foo:w \endcsname #1 : #2 \q_stop  { (#1) }
}

\newcommand{\foo}[1]{ \my_foo:w #1:\q_stop }

\ExplSyntaxOff

\foo{abc:def} % does work

\end{document}

Answer 4

You will need to produce a string version of the colon. There are a few ways: one example:

\use:x
  {
    \cs_set:Npn \exp_not:N \my_foo:w
      ##1 \token_to_str:N : ##2 \exp_not:N \q_stop
  }
  { ( #1 ) }

Answer 5

You can use TeX primitive \def, no Expl3:

\def\foo#1{\fooA#1\end}
\def\fooA#1:#2\end{(#1)}

\foo{abc:def} % works