Commands from xparse inside functions created by \cs_new:Npn?

Question

Is it okay to put commands from xparse inside functions created by \cs_new:Npn instead of \NewDocumentCommand? Would that be a confusion of programming levels?

The example below works but I was wondering if it conforms to LaTeX3 conventions.

It tests #1 for -NoValue-; but #1 is an optional argument to public \create not to private \__mypkg_create.

mypkg.sty:

\RequirePackage{expl3}
\ProvidesExplPackage{mypkg}{2013/01/20}{0.01}{My Package}
\RequirePackage{xparse}

\NewDocumentCommand \foo {om}
 {
    \__mypkg_foo:nn {#1} {#2}
 }

\cs_new:Npn \__mypkg_foo:nn #1#2
 {
    \IfNoValueTF {#1} {true} {false}
 }
\endinput

sample.tex:

\documentclass{article}
\usepackage{mypkg}
\begin{document}
\foo{bar}
\end{document}

Edit:

mypkg.sty:

\RequirePackage{expl3}
\ProvidesExplPackage{mypkg}{2013/01/20}{0.01}{My Package}
\RequirePackage{xparse}

\NewDocumentCommand \foo {om}
 {
    \mypkg_foo:nn {#1} {#2}
 }

\cs_new:Npn \mypkg_foo:nn #1#2
 {
    \IfNoValueTF {#1} {true} {false}
 }
\endinput

Answer 1

As it stands, you are making the error of using an internal command from a document command: that's not the approach the team is aiming for. One problem with LaTeX2e is that many document commands directly use internals, and that makes it very difficult to alter interfaces, internal commands, etc. safely. The LaTeX3 approach is therefore that each document command should be a simple 'translation' layer, taking a user syntax and turning into the input for a documented code-level interface

\NewDocumentCommand \foo {om}
  { \mypkg_foo:nn {#1} {#2} }

If you need to test for \NoValue, this would currently normally be done as part of this interface conversion

\NewDocumentCommand { \foo } {om}
  {
    \IfNoValueTF {#1} 
      { \mypkg_foo:nn { } {#2} }
      { \mypkg_foo:nn {#1} {#2} }
  }

(The exact detail of dealing with this 'special' token is still something that perhaps needs work.)

An alternative formulation would be

\NewDocumentCommand { \foo } { O { } m }
  { \mypkg_foo:nn {#1} {#2} }

where as we don't care if #1 is present or not we simply set an empty default.

The reason for taking this approach is that it's then possible to redefine \foo while retaining access to the functionality of \mypkg_foo:nn (as it's documented). If you use an internal function inside \foo then you effectively tie the two together. (Note that getting this right is tricky, and parts of for example my own code need revision to get this correct. It's very much a work in progress.)