Mix value and key/value with xparse

Question

Following an earlier question I switched to xparse. Now I am sitting in a compatibility trap and try to allow old and new stuff at the same time. The point of origin is the infamous macro with two optional arguments. My minimal example is:

\documentclass{minimal}

\usepackage{xparse}

\NewDocumentCommand \sidenotetext { o o m } {%
(#1), (#2), #3
}%

\begin{document}
\sidenotetext{arg} % case (1)

\sidenotetext[1]{arg} % \sidenotetext[mark=1]{arg}; case 2

\sidenotetext[][]{arg} % case 3

\sidenotetext[][2]{arg} %\sidenotetext[offset=2]{arg}; case 4

\sidenotetext[3][]{arg} %\sidenotetext[mark=3]{arg}; case 5

\sidenotetext[4][5]{arg} %\sidenotetext[mark=4,offset=5]{arg}; case 6 
\end{document}

Is it possible to make the left and the right macros (commented out) work at the same time? If not, Is it possible at all to make case 2 work, i.e. no key always means mark and the rest is key value? I assume, l3keys is the right choice to play nice with xparse?

Answer 1

If the two optional arguments deal with incompatible types of argument, the only feasible way is to have one before and one after the mandatory argument, so

\NewDocumentCommand{\sidenotetext} { o m o }
 {%
  \IfNoValueTF{#1}...
 }

or

\NewDocumentCommand{\sidenotetext} { O{default} m O{default} }
 {%
  ...
 }

In this way you can call

\sidenotetext{text}
\sidenotetext[1]{text}
\sidenotetext{text}[2]
\sidenotetext[1]{text}[2]

However this is clumsy. Better going with a key-value approach.

\ExplSyntaxOn
\NewDocumentCommand{\sidenotetext}{ O{} m }
 {
  \group_begin:
  \andy_sidenotetext:nn { #1 } { #2 }
  \group_end:
 }

\keys_define:nn { andy/sidenote }
 {
  mark .tl_set:N = \l_andy_sidenote_mark_tl,
  offset .tl_set:N = \l_andy_sidenote_offset_tl,
  offset .initial:n = 0,
 }

\cs_new_protected:Npn \andy_sidenotetext:nn #1 #2
 {
  \keys_set:nn { andy/sidenote } { #1 }
  <code using \l_andy_sidenote_mark_tl, \l_andy_sidenote_offset_tl
   and #2 (the actual text)>
 }
\ExplSyntaxOff

Making the two calls compatible with each other is something I wouldn't recommend: stick to one syntax. There are macros around that allow both approaches, but it's because they were defined when a key-value approach was unfeasible. An example is \section in Koma-Script classes, where the test for a key-value syntax is done on the basis whether a = appears in the optional argument, which is not very robust.

If what you want to do is redefining a legacy command, I would recommend using a different name.