\str_case not recognizing parameter if sent as an item of a \tl pseudo array argument

Question

Passing a menemonic as an optional argument in a \NewDocumentCommand to set colours for use in a \tikzpicture environment works fine if the argument is a straight \tl variable. However, if passed as an item of a \tl pseudo array it seems to translate correctly to the desired mnemonic which then fails to be recognized in the \str_case:NnTF statement. Any clues why?

\documentclass{article}
% RN. 20/10/2025
% MWE_115
%=======================
\usepackage{tikz} 

\ExplSyntaxOn
\NewDocumentCommand\myCommandOne{O{R}}
%  PARAMETERS:
%    #1 tl  
    {
\group_begin:
    
    \tl_set:Nn \l_rn_mnemColour_tl {#1}
    tl~variable~set~by~argument~\#1:~\l_rn_mnemColour_tl \\
    
    \str_case:NnTF {\l_rn_mnemColour_tl}
        { 
            {R}{\tl_set:Nn \l_rn_Colour_tl {red}}
            {B}{\tl_set:Nn \l_rn_Colour_tl {blue}}
            {G}{\tl_set:Nn \l_rn_Colour_tl {green}}
            {Y}{\tl_set:Nn \l_rn_Colour_tl {yellow}}
        }
        {TRUE\\}
        {FALSE\\}

    \begin{tikzpicture} 
        \draw [fill=\l_rn_Colour_tl] (0,0) rectangle (1,1);
    \end{tikzpicture}
\group_end:
  }  %  \myCommand  
\ExplSyntaxOff
  
\begin{document}
1. \verb+\myCommandOne+\\
\myCommandOne

2. \verb+\myCommandOne[B]+\\
\myCommandOne[B]

3. \verb+\myCommandOne[G]+\\
\myCommandOne[G]

4. \verb+\myCommandOne[Y]+\\
\myCommandOne[Y]
\end{document} 

\documentclass{article}
% RN. 20/10/2025
% MWE_115_tl_array_case
%=======================
\usepackage{tikz} 

\ExplSyntaxOn
\NewDocumentCommand\myCommandTwo{O{{R}{B}{G}{Y}}}
%  PARAMETERS:
%    #1 tl posing as an array
    {
\group_begin:
    
    \tl_set:Nn \l_rn_mnemColour_Array_tl {#1}
    \tl_set:Nn \l_rn_mnemColourOne_tl {\tl_item:Nn \l_rn_mnemColour_Array_tl{1}}
    tl~variable~set~by~argument:~\l_rn_mnemColourOne_tl \\
    
    \str_case:NnTF {\l_rn_mnemColourOne_tl}
        { 
            {R}{\tl_set:Nn \l_rn_ColourOne_tl {red}}
            {B}{\tl_set:Nn \l_rn_ColourOne_tl {blue}}
            {G}{\tl_set:Nn \l_rn_ColourOne_tl {green}}
            {Y}{\tl_set:Nn \l_rn_ColourOne_tl {yellow}}
        }
        {TRUE\\}
        {FALSE\\}

    \begin{tikzpicture} 
        \draw [fill=\l_rn_ColourOne_tl] (0,0) rectangle (1,1);
%       \draw [fill=red] (0,0) rectangle (1,1);
    \end{tikzpicture}
\group_end:
  }  %  \myCommand  
\ExplSyntaxOff  
    
\begin{document}
1. \verb+\myCommandTwo+\\
\myCommandTwo

2. \verb+\myCommandTwo[B]+\\
\myCommandTwo[B]
\end{document} 

Answer 1

If you do

\tl_set:Nn \l_rn_mnemColourOne_tl {\tl_item:Nn \l_rn_mnemColour_Array_tl{1}}

then the token list will not contain R as you hope, but

\tl_item:Nn \l_rn_mnemColour_Array_tl{1}

which is not the same. You want

\tl_set:Ne \l_rn_mnemColourOne_tl {\tl_item:Nn \l_rn_mnemColour_Array_tl{1}}

but, since you want to compare strings, use the str type.

\documentclass{article}
% RN. 20/10/2025
% MWE_115_tl_array_case
%=======================
\usepackage{tikz} 

\ExplSyntaxOn

\tl_new:N \l_rn_ColourOne_tl
\str_new:N \l_rn_mnemColourOne_str

\NewDocumentCommand\myCommandTwo{O{{R}{B}{G}{Y}}}
%  PARAMETERS:
%    #1 tl posing as an array
 {
   \group_begin:
    
    \str_set:Ne \l_rn_mnemColourOne_str {\tl_item:nn {#1}{1}}
    tl~variable~set~by~argument:~\l_rn_mnemColourOne_str \\
    
    \str_case:NnTF \l_rn_mnemColourOne_str
        { 
            {R}{\tl_set:Nn \l_rn_ColourOne_tl {red}}
            {B}{\tl_set:Nn \l_rn_ColourOne_tl {blue}}
            {G}{\tl_set:Nn \l_rn_ColourOne_tl {green}}
            {Y}{\tl_set:Nn \l_rn_ColourOne_tl {yellow}}
        }
        {TRUE\\}
        {FALSE\\}

    \begin{tikzpicture} 
       \draw [fill=\l_rn_ColourOne_tl] (0,0) rectangle (1,1);
    \end{tikzpicture}
    \group_end:
  }  %  \myCommand  
\ExplSyntaxOff  
    
\begin{document}

1. \verb+\myCommandTwo+\\
\myCommandTwo

2. \verb+\myCommandTwo[B]+\\
\myCommandTwo[B]

\end{document} 

Less assignments, too, are recommended, besides declaring the needed variables. Remember that N type arguments should not be braced.

Answer 2

The following fixes your code. Your code doesn't set your variable to only R or B as you don't expand the \tl_item:Nn call. If you do that you get correct results. I also changed your "check" what the tl contains by using \tl_to_str:N to print its contents. Also, since you're not passing a string variable to \str_case:NnTF I changed that to \str_case:VnTF which is the correct way to achieve this (if you don't change the variable to a string variable, which would be another correct change). Lastly I changed your "check" what the tl contains by using \tl_to_str:N to print its contents. That way you can actually see the result of your \tl_set:Nn if you change it back:

\documentclass{article}
% RN. 20/10/2025
% MWE_115_tl_array_case
%=======================
\usepackage{tikz}

\ExplSyntaxOn
\NewDocumentCommand\myCommandTwo{O{{R}{B}{G}{Y}}}
%  PARAMETERS:
%    #1 tl posing as an array
  {
    \group_begin:

      \tl_set:Nn \l_rn_mnemColour_Array_tl {#1}
      \tl_set:Ne \l_rn_mnemColourOne_tl { \tl_item:Nn \l_rn_mnemColour_Array_tl { 1 } }
      tl~variable~set~by~argument:~\tl_to_str:N \l_rn_mnemColourOne_tl \\

      \str_case:VnTF \l_rn_mnemColourOne_tl
        {
          { R } { \tl_set:Nn \l_rn_ColourOne_tl { red } }
          { B } { \tl_set:Nn \l_rn_ColourOne_tl { blue } }
          { G } { \tl_set:Nn \l_rn_ColourOne_tl { green } }
          { Y } { \tl_set:Nn \l_rn_ColourOne_tl { yellow } }
        }
        {TRUE\\}
        {FALSE\\}

      \begin{tikzpicture}
          \draw [fill=\l_rn_ColourOne_tl] (0,0) rectangle (1,1);
  %       \draw [fill=red] (0,0) rectangle (1,1);
      \end{tikzpicture}
    \group_end:
  }  %  \myCommand
\ExplSyntaxOff

\begin{document}
1. \verb+\myCommandTwo+\\
\myCommandTwo

2. \verb+\myCommandTwo[B]+\\
\myCommandTwo[B]
\end{document}