Why can't this code successfully parse the date string?

Question

As an attempt to practice with expl3, I am trying to parse numerical date string to French. There is a \mymodule_display_date_year_month_day:n which receives a string of form yyyy-mm-dd (or yyyy-mm or mm-dd, which is not done yet), and tries to convert it to French text. However, it cannot successfully parse the yyyy-mm-dd string and pass the three integers to \mymodule_display_date_year_month_day_french:nnn.

Here is what I've tried so far. Where does it go wrong?

\documentclass{article}

\usepackage[french,english]{babel}

\begin{document}

\selectlanguage{french}

\ExplSyntaxOn

\int_new:N \l__mymodule_year_int
\int_new:N \l__mymodule_month_int
\int_new:N \l__mymodule_day_int

\cs_new:Nn \mymodule_text_superscript:n { \textsuperscript { #1 } }


\cs_new_protected:Nn \mymodule_display_date_year_month_day:n
  {
    \regex_match:nnTF { \A \d{4}-\d{1,2}-\d{1,2} \Z } { #1 }
      {
        \__mymodule_parse_yyyymmdd:www #1 \q_stop
        \exp_args:Nx \cs_if_exist_use:cTF { mymodule_display_date_year_month_day_ \languagename :nnn }
          {
            { \l__mymodule_year_int }
            { \l__mymodule_month_int }
            { \l__mymodule_day_int }
          }
          { #1 }
      }
      {
        \regex_match:nnTF { \A \d{4}-\d{1,2} \Z } { #1 }
          {
            %%
          }
          {
            \regex_match:nnTF { \A \d{4}-\d{1,2}-\d{1,2} \Z } { #1 }
              {
                %%
              }
              { ?? }
          }
      }
  }

\cs_new:Npn \__mymodule_parse_yyyymmdd:www #1-#2-#3 \q_stop
  {
    \int_set:Nn \l__mymodule_year_int  { #1 }
    \int_set:Nn \l__mymodule_month_int { #2 }
    \int_set:Nn \l__mymodule_day_int   { #3 }
  }

\cs_new:Nn \mymodule_display_date_year_month_day_french:nnn
  {
    \mymodule_display_date_month_day_french:nn { #2 } { #3 }
    \c_space_tl
    #1
  }

\cs_new:Nn \mymodule_display_date_year_month_french:nn
  {
    \mymodule_display_date_month_french:n { #2 }
    \c_space_tl
    #1
  }

\cs_new_protected:Nn \mymodule_display_date_month_day_french:nn
  {
    \mymodule_display_date_day_french:n { #2 }
    \c_space_tl
    \mymodule_display_date_month_french:n { #1 }
  }

\cs_new:Nn \mymodule_display_date_month_french:n
  {
    \int_case:nnF { #1 }
    {
      { 1 }  { janvier   }
      { 2 }  { février   }
      { 3 }  { mars      }
      { 4 }  { avril     }
      { 5 }  { mai       }
      { 6 }  { juin      }
      { 7 }  { juillet   }
      { 8 }  { août      }
      { 9 }  { septembre }
      { 10 } { octobre   }
      { 11 } { novembre  }
      { 12 } { décembre  }
    } { #1 }
  }

\cs_new:Nn \mymodule_display_date_day_french:n
  {
    \int_case:nnF { #1 }
      {
        { 1 } { 1 \mymodule_text_superscript:n { er } }
      } { #1 }
  }



\mymodule_display_date_year_month_day:n {2021-06-03}



\ExplSyntaxOff
\end{document}

Answer 1

You were getting an error:

! Missing number, treated as zero.
<to be read again> 
                   j

because your code was doing:

\mymodule_display_date_year_month_day_french:nnn
  \l__mymodule_year_int \l__mymodule_month_int \l__mymodule_day_int

with this definition of \mymodule_display_date_year_month_day_french:nnn:

\cs_new_protected:Nn \mymodule_display_date_year_month_day_french:nnn
  {
    \mymodule_display_date_month_day_french:nn { #2 } { #3 }
    \c_space_tl
    #1
  }

This would just “use” #1, which is \l__mymodule_year_int, which is equivalent to just typing:

using an \l__mymodule_year_int causes an error

You cannot use an integer variable by just writing it out. You must precede it with \int_use:N. So one solution would be to write \int_use:N #1 in that definition (and the one for the day). A better solution is to use the V-type expansion to pass the Value of the integer variables to the macro, so inside the macro you don't have to worry about what type of data you passed to it:

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage[french,english]{babel}

\begin{document}

\selectlanguage{french}

\ExplSyntaxOn

\int_new:N \l__mymodule_year_int
\int_new:N \l__mymodule_month_int
\int_new:N \l__mymodule_day_int

\cs_new_protected:Nn \mymodule_text_superscript:n { \textsuperscript { #1 } }


\cs_new_protected:Nn \mymodule_display_date_year_month_day:n
  {
    \regex_match:nnTF { \A \d{4}-\d{1,2}-\d{1,2} \Z } { #1 }
      {
        \__mymodule_parse_yyyymmdd:www #1 \s_stop
        \cs_if_exist_use:cTF { mymodule_display_date_year_month_day_ \languagename :VVV }
          {
            \l__mymodule_year_int
            \l__mymodule_month_int
            \l__mymodule_day_int
          }
          { #1 }
      }
      {
        \regex_match:nnTF { \A \d{4}-\d{1,2} \Z } { #1 }
          {
            %%
          }
          {
            \regex_match:nnTF { \A \d{4}-\d{1,2}-\d{1,2} \Z } { #1 }
              {
                %%
              }
              { ?? }
          }
      }
  }

\cs_new_protected:Npn \__mymodule_parse_yyyymmdd:www #1-#2-#3 \s_stop
  {
    \int_set:Nn \l__mymodule_year_int  { #1 }
    \int_set:Nn \l__mymodule_month_int { #2 }
    \int_set:Nn \l__mymodule_day_int   { #3 }
  }

\cs_new_protected:Nn \mymodule_display_date_year_month_day_french:nnn
  {
    \mymodule_display_date_month_day_french:nn { #2 } { #3 }
    \c_space_tl
    #1
  }
\cs_generate_variant:Nn \mymodule_display_date_year_month_day_french:nnn { VVV }

\cs_new_protected:Nn \mymodule_display_date_year_month_french:nn
  {
    \mymodule_display_date_month_french:n { #2 }
    \c_space_tl
    #1
  }

\cs_new_protected:Nn \mymodule_display_date_month_day_french:nn
  {
    \mymodule_display_date_day_french:n { #2 }
    \c_space_tl
    \mymodule_display_date_month_french:n { #1 }
  }

\cs_new_protected:Nn \mymodule_display_date_month_french:n
  {
    \int_case:nnF { #1 }
    {
      { 1 }  { janvier   }
      { 2 }  { février   }
      { 3 }  { mars      }
      { 4 }  { avril     }
      { 5 }  { mai       }
      { 6 }  { juin      }
      { 7 }  { juillet   }
      { 8 }  { août      }
      { 9 }  { septembre }
      { 10 } { octobre   }
      { 11 } { novembre  }
      { 12 } { décembre  }
    } { #1 }
  }

\cs_new_protected:Nn \mymodule_display_date_day_french:n
  {
    \int_case:nnF { #1 }
      {
        { 1 } { 1 \mymodule_text_superscript:n { er } }
      } { #1 }
  }



\mymodule_display_date_year_month_day:n {2021-06-03}



\ExplSyntaxOff
\end{document}

Answer 2

We should not forget that we have much more simple programming language: low level TeX:

\def\monthdef#1 #2 {\expandafter\def\csname m:#1\endcsname{#2}}

\monthdef 1   janvier   
\monthdef 2   février   
\monthdef 3   mars      
\monthdef 4   avril     
\monthdef 5   mai       
\monthdef 6   juin      
\monthdef 7   juillet   
\monthdef 8   août      
\monthdef 9   septembre 
\monthdef 10  octobre   
\monthdef 11  novembre  
\monthdef 12  décembre  

\catcode`\_=11

\newcount\tmpnum
\def\display_date_year_month_day #1{\display_dateA #1;}
\def\display_dateA #1-#2-#3;{\tmpnum=#3\relax \the\tmpnum \
   \tmpnum=#2\relax \csname m:\the\tmpnum\endcsname \
   #1%
}

\display_date_year_month_day {2021-06-03}

\bye

Answer 3

You can speed up your code. You can check correctness of the input with

\regex_match:nnTF { \A \d{4}-\d{1,2}(|-\d{1,2}) \Z } { #1 }

so the last group might be missing.

After that you can split the input via \seq_set_split:Nnn and call a generic date function to which you also pass the current language (expanded).

The next step is to use the localized version. If the day is missing, the third argument will be 0 and you can check for it in the localized function.

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage[french,english]{babel}

\ExplSyntaxOn

% for testing
\NewDocumentCommand{\displaydate}{m}
  {
    \mymodule_display_date_year_month_day:n { #1 }
  }

\seq_new:N \l__mymodule_date_seq

\cs_new_protected:Nn \mymodule_text_superscript:n { \textsuperscript { #1 } }

\cs_new_protected:Nn \mymodule_display_date_year_month_day:n
  {
    \regex_match:nnTF { \A \d{4}-\d{1,2}(|-\d{1,2}) \Z } { #1 }
      {
        \seq_set_split:Nnn \l__mymodule_date_seq { - } { #1 }
        \mymodule_display_date_year_month_day:Veee
          \languagename
          { \seq_item:Nn \l__mymodule_date_seq { 1 } } % year
          { \seq_item:Nn \l__mymodule_date_seq { 2 } } % month
          { \seq_item:Nn \l__mymodule_date_seq { 3 } } % day
      }
      {
       ???#1???
      }
  }

\cs_new_protected:Nn \mymodule_display_date_year_month_day:nnnn
  {
    \use:c { __mymodule_display_date_#1:nnn } { #2 } { #3 } { \int_eval:n { 0#4 } }
  }
\cs_generate_variant:Nn \mymodule_display_date_year_month_day:nnnn { Veee }

\cs_new_protected:Nn \__mymodule_display_date_english:nnn
  {
    % month
    \int_case:nn { #2 }
      {
        { 1 }  { January   }
        { 2 }  { February  }
        { 3 }  { March     }
        { 4 }  { April     }
        { 5 }  { May       }
        { 6 }  { June      }
        { 7 }  { July      }
        { 8 }  { August    }
        { 9 }  { September }
        { 10 } { October   }
        { 11 } { November  }
        { 12 } { December  }
      }
    % day
    \int_compare:nF { #3 == 0 } { \nobreakspace #3 }
    % year
    ,\nobreakspace #1
  }

\cs_new_protected:Nn \__mymodule_display_date_french:nnn
  {
    % day
    \int_compare:nF { #3 == 0 } { \mymodule_display_date_day_french:n { #3 } \nobreakspace }
    % month
    \int_case:nn { #2 }
      {
        { 1 }  { janvier   }
        { 2 }  { février   }
        { 3 }  { mars      }
        { 4 }  { avril     }
        { 5 }  { mai       }
        { 6 }  { juin      }
        { 7 }  { juillet   }
        { 8 }  { août      }
        { 9 }  { septembre }
        { 10 } { octobre   }
        { 11 } { novembre  }
        { 12 } { décembre  }
      }
    \nobreakspace #1
  }

\cs_new_protected:Nn \mymodule_display_date_day_french:n
  {
    \int_case:nnF { #1 }
      {
        { 1 } { 1 \mymodule_text_superscript:n { er } }
      }
      { #1 }
  }

\ExplSyntaxOff


\begin{document}

\displaydate{abdarrr}

\displaydate{2021-06-03}

\displaydate{2021-06-01}

\displaydate{2021-06}

\selectlanguage{french}

\displaydate{2021-06-03}

\displaydate{2021-06-01}

\displaydate{2021-06}


\end{document}