Not ignoring spaces before optional argument with xparse

Question

I am using xparse.sty to define a command that takes an optional argument in square brackets, and another one curly braces.

Problematically, it is not uncommon that the user will often want to begin the text after the command with square brackets. Since LaTex ignores spaces before the first optional argument, naively typing \myitem [not an argument] won't work, since "[not an argument]" will be considered an argument. MWE:

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{\myitem}{ o g }{
    \IfValueTF{#1}{%
        []-argument:~#1
    }{
        \IfValueTF{#2}{
            \{\}-argument:~#2
        }{
            no~argument:~
        }
    }
}

\ExplSyntaxOff

\begin{document}
\myitem [Not intended as argument] 
% Will output: []-argument: Not intended as argument

\end{document}

What's worse, because \myitem also takes a g argument, the standard technique for avoiding this issue (inserting a pair of braces before or around the text in square brackets) won't work eiter:

\myitem {}[Not intended as argument]
% Will output {}-argument: [Not intended as argument]

\myitem {[Not intended as argument]}
% Will output {}-argument: [Not intended as argument]

The only workaround right now is to use \relax before the square brackets, which I would like to avoid.

Is it possible to make LaTeX treat the spaces as terminating the argument list in this case? I.e., \myitem [foo] should output no argument: foo, while \myitem[foo] should (still) output []-argument: foo?

Answer 1

By the standard reading rules of TeX, there is no space even if you type

\myitem [abc]

because the space is ignored when the line is read in. A way out would be

\myitem\relax[abc]

because \relax will stop the scanning for optional arguments.

It has always been a problem when a command accepting an optional argument is followed by [. Note that xparse does its best in order to balance the ] of the optional argument, but it's a different problem.

Of course \myitem{[not optional]} would work as well if there wasn't a g optional argument.

In general, I wouldn't recommend this argument specifier: it has been added because some user interfaces (notably beamer) provide this convention. However, it is against the standard LaTeX interface and could be confusing: please, think well before adopting this kind of syntax.