A sequence was misused, but inlining the function works

Question

I have some code that works but stops working when I try to refactor it into a function. Here is a MWE:

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn

\seq_new:N \l_shepi_sub_seq:nnn
\cs_new_protected:Nn \shepi_seq:nnn {
  \seq_set_split:Nnn \l_shepi_sub_seq {,} {#3}
  \seq_set_map:NNn \l_shepi_sub_seq \l_shepi_sub_seq {\exp_not:n {#2}}
  \seq_use:Nn \l_shepi_sub_seq {#1}
}

\NewDocumentCommand \works { m } {
  \seq_set_split:Nnn \l_shepi_sub_seq {,} {#1}
  \seq_set_map:NNn \l_shepi_sub_seq \l_shepi_sub_seq {\exp_not:n {l}}
  \[\begin{array}{\seq_use:Nn \l_shepi_sub_seq {}}
  \end{array}\]
}

\NewDocumentCommand \fails { m } {
  \[\begin{array}{\shepi_seq:nnn {} {l} {#1}}
  \end{array}\]
}

\ExplSyntaxOff

\begin{document}

\works{A, B, C}

\ExplSyntaxOn
\shepi_seq:nnn{}{l}{A, B, C}
\ExplSyntaxOff

%\fails{A, B, C}

\end{document}

Uncommenting the \fails call interrupts the compilation with the following error:

! Undefined control sequence.
<argument> \LaTeX3 error: 
                           A sequence was misused.
l.55     \fails{A, B, C}

As my \shepi_seq:nnn function is used by other parts of my codebase, I'd prefer to leave it as it is, but if changes are required, so be it.

Can someone explain what I did wrong?

Side question: Is it good practice to call \seq_set_map:NNn like I did? (The two first parameters denote the same sequence).

Answer 1

The argument to \begin{array} should be fully expandable to a correct array preamble, but \shepi_seq:nnn isn't expandable.

Also, the argument to \begin{array} will not be expanded anyway if the array package is loaded, so you want a different approach.

You're apparently counting the number of items, so do it. ;-)

\documentclass{article}
\usepackage{xparse}
\usepackage{array} % just in case

\ExplSyntaxOn

\seq_new:N \l_shepi_sub_seq

\NewDocumentCommand{\fabianarray}{m}
 {
  \[
  \seq_set_split:Nnn \l_shepi_sub_seq { , } { #1 }
  \begin{array}{ *{ \clist_count:n { #1 } } { l } }
  \seq_use:Nn \l_shepi_sub_seq { & }
  \end{array}
  \]
 }

\ExplSyntaxOff

\begin{document}

\fabianarray{A, B, C}

\end{document}