Tukukkk

The MWE_1 defines a document command which in turn calls a cs_ macro. Both are defined in the document's preamble section sandwiched between ExplSyntaxOn ExplSyntaxOff statements, as is the declaration of the associated local and global variables. There seems to be little reason to define document commands for the purpose of using them in a single document, and no reason whatsoever to develop cs_ macros with that scope in mind, hence I usually work with a number of .sty files. For the purpose of demonstration let the preamble of MWE_1 be packaged in the file myCommandsAndMacros.sty which is headed with the instruction ProvidesPackage{myCommandsAndMacros} and in MWE_2 loaded with the instruction usepackage{myCommandsAndMacros} - not sure where in the distant past I gleaned this process from, but it works. To my surprise I now learn (see Part II in the LaTeX3 Interfaces document) that ProvidesPackage is an instruction that belongs to LaTeX2 (although it may be used by LaTeX3), but all attempts to modify the syntax I have been using to ProvidesExplPackage{myCommandsAndMacros} fail with error !Paragraph ended before ProvidesExplPackage was complete.

MWE_1:

documentclass{article}

%-----------------------

usepackage[check-declarations,log-functions,enable-debug]{expl3}

%   or: RequirePackage[check-declarations,log-functions,enable-debug]{expl3}

usepackage{xparse}

%-----------------------

ExplSyntaxOn

%   Variable Declarations:

tl_new:N l_rn_auxOne_tl

int_new:N l_rn_auxOne_int

int_new:N g_rn_Result_int



cs_new:Npn rnUtils_Squared:n #1

  {

group_begin:

      int_set:Nn l_rn_auxOne_int {#1}

      int_gset:Nn g_rn_Result_int {int_eval:n {l_rn_auxOne_int     *l_rn_auxOne_int}}

group_end:

  }  %  rnUtils_Squared:n



NewDocumentCommandmyDocumentCommand{O{911}O{abc}}

  {

    int_set:Nn l_rn_auxOne_int {#1}

    rnUtils_Squared:n {l_rn_auxOne_int}

    tl_set:Nn l_rn_auxOne_tl {#2}

    int_use:N g_rn_Result_int,~tl_use:N l_rn_auxOne_tl

  }  %  myDocumentCommand



ExplSyntaxOff

%-----------------------

begin{document}

  myDocumentCommand[2][$alpha$]



  myDocumentCommand

end{document}

MWE_2:

documentclass{article}

%-----------------------

usepackage[check-declarations,log-functions,enable-debug]{expl3}

%   or: RequirePackage[check-declarations,log-functions,enable-debug]{expl3}

usepackage{xparse}

usepackage{myCommandsAndMacros}

%-----------------------

begin{document}

  myDocumentCommand[2][$alpha$]



  myDocumentCommand

end{document}

myCommandsAndMacros.sty

ProvidesPackage{myCommandsAndMacros}

%ProvidesExplPackage{myCommandsAndMacros}  ==> ERROR

% ===========================================================

ExplSyntaxOn



%   Variable Declarations:

tl_new:N l_rn_auxOne_tl

int_new:N l_rn_auxOne_int

int_new:N g_rn_Result_int



cs_new:Npn rnUtils_Squared:n #1

  {

group_begin:

      int_set:Nn l_rn_auxOne_int {#1}

      int_gset:Nn g_rn_Result_int {int_eval:n {l_rn_auxOne_int   *l_rn_auxOne_int}}

group_end:

  }  %  rnUtils_Squared:n



NewDocumentCommandmyDocumentCommand{O{911}O{abc}}

  {

    int_set:Nn l_rn_auxOne_int {#1}

    rnUtils_Squared:n {l_rn_auxOne_int}

    tl_set:Nn l_rn_auxOne_tl {#2}

    int_use:N g_rn_Result_int,~tl_use:N l_rn_auxOne_tl

  }  %  myDocumentCommand



ExplSyntaxOff

Unlike ProvidesPackage, which uses a single argument for several different pieces of data, ProvidesExplPackage has a separate argument for each. For example

RequirePackage{expl3}

ProvidesExplPackage {siunitx} {2018-07-21} {3.0.0-alpha}

  {A comprehensive (SI) units package}

% Here, `expl3` syntax is active