Using expl3 macros and xcolor

Question

I'm trying my hand at expl3 for the first time right now, and while tinkering I ran into an error that I cannot wrap my head around: Every time I try to use the result of a tl_use or a clist_item as the second argument to \colorlet from xcolor something breaks completely.

Concretely, here's a small example:

\documentclass{article}
\usepackage{xcolor}

\ExplSyntaxOn
\tl_new:N \l__color_dummy_tl

\cs_new:Npn \passcolorthrough #1 {
    \tl_set:Nn \l__color_dummy_tl {#1}
    \tl_use:N \l__color_dummy_tl
}
\ExplSyntaxOff

\colorlet{mycolor}{\passcolorthrough{black}} % <- Error's here

\begin{document}
:(
\end{document}

Compiling this with a recent TexLive version yields the following error:

Runaway argument?
\XC@func >\@nnil \edef \@@tmp {{\@@mod }{\@@clr }}\aftergroupdef \@@clr \ETC.
! Paragraph ended before \XC@split@iv was complete.
<to be read again> 
                   \par 
l.14 
     
I suspect you've forgotten a `}', causing me to apply this
control sequence to too much text.

I haven't been able to make heads or tails of this. Replacing \l__color_dummy_tl with a clist and \tl_use with \clist_item yields the same error.

What's the issue here, and how do I avoid it?

Answer 1

The second argument of \colorlet has to expand to a color. Any command using assignments such as \tl_set:Nn (which is \def, more or less) is not expandable.

So the canonical expandable passthrough would be

\documentclass{article}
\usepackage{xcolor}

\ExplSyntaxOn
\tl_new:N \l__color_dummy_tl

\cs_new:Npn \passcolorthrough #1 {
 #1
}
\ExplSyntaxOff

\colorlet{mycolor}{\passcolorthrough{black}} % <- Error's here

\begin{document}
:)
\end{document}

You could use more complicated definitions as long as you stick to expandable commands (texdoc interface3 marks all expandable commands with a *)

Answer 2

Do you want to do some processing on the second parameter black instead of using \colorlet{mycolor}{black} directly? Is it possible to use the following method to achieve your purpose?

\documentclass{article}
\usepackage{xcolor}

\ExplSyntaxOn
\tl_new:N \l__color_dummy_tl

\cs_new:Npn \mycolorlet #1#2 {
  \tl_set:Nn \l__color_dummy_tl {#2}
  % some process for \l__color_dummy_tl
  \tl_set:Nx \l__color_dummy_tl {\l__color_dummy_tl !50! red}
  \exp_args:NnV \colorlet{#1}\l__color_dummy_tl
}
\ExplSyntaxOff

\mycolorlet{mycolor}{black} % <- Error's here

\begin{document}
:(
\textcolor{mycolor}{xxxx}
\end{document}