Is a command with an argument before and after the command possible?

Question

I'm trying to create a LaTeX3 command that works the same whether its argument comes before or after the command. For example, I'd like to define \squared to be used as either \m\squared or \squared\m where \m is defined as \mathrm{m}. Both should yield $\mathrm{m}^2$. Here's the markup I've been working with.

\documentclass[10pt]{article}
\usepackage{xparse}

\newcommand*{\unit}[2]{\ensuremath{#1\;#2}}
\newcommand*{\metre}{\ensuremath{\mathrm{m}}}
\newcommand*{\m}{\metre}

\DeclareDocumentCommand \square { m } {%

  \IfNoValueTF #1 {%
    $^2$
  }{%
    $#1^2$
  }%
}%

\begin{document}

The command used as a prefix gives \square\m.

The command used as a postfix gives \m\square\ .

\end{document}

The first example typesets correctly except for a line break that I can't explain, but the second example also has a line break between the \m and the exponent. I tried making the argument optional but that didn't work either.

It's okay if the construct I have in mind isn't possible since I can make slightly different commands for use before and after the argument.

Answer 1

In general, TeX does not allow use to 'look before' a command, only to 'look after' (exceptions are primitives such as \over and LuaTeX callbacks to alter the flow of processing). As such, the only way to deal with both cases is to make them 'aware' of what follows. Thus one could for example do

\documentclass{article}
\newcommand*{\metre}{%
  \ensuremath{\mathrm{m}}%
}
\newcommand*{\m}{\metre}
\makeatletter
\newcommand*{\square}{%
  \@ifnextchar\metre
    {\square@aux}
    {%
      \@ifnextchar\m
        {\square@aux}
        {\ensuremath{^{2}}}%
    }%
}
\newcommand{\square@aux}[1]{%
  \ensuremath{#1^{2}}%
}
\begin{document}
The command used as a prefix gives \square\m.

The command used as a postfix gives \m\square.
\end{document}

which looks ahead at the token following \square and makes a choice accordingly. You can see that this would get unwieldy for a long list of units: one possible way around that would be to use a 'marker token' at the start of each unit, do a partial expansion and then check for that token. That might look like

\documentclass{article}
\usepackage{pdftexcmds}
\makeatletter
\let\joeh@unit@marker\relax
\newcommand*{\metre}{%
  \joeh@unit@marker
  \@firstofone{\ensuremath{\mathrm{m}}}% \@firstofone needed to remove "{" and "}"
}
\newcommand*{\m}{\metre}
\newcommand*{\square}{%
  \expandafter\square@aux@i
  \romannumeral-`0%
}
\newcommand{\square@aux@i}{%
  \@ifnextchar\joeh@unit@marker % Tests by meaning
    {\square@aux@ii}
    {\ensuremath{^{2}}}%
}
\newcommand{\square@aux@ii}[1]{%
  \ifnum\pdf@strcmp{\unexpanded{#1}}{\unexpanded{\joeh@unit@marker}}=\z@
    \expandafter\square@aux@iii
  \else
    \expandafter\square@aux@iv
  \fi
    {#1}%
}
\newcommand{\square@aux@iii}[3]{%
  % #1 = \joeh@unit@marker
  % #2 = \@firstofone
  % #3 = Unit code
  \ensuremath{#3^{2}}%
}
\newcommand{\square@aux@iv}[1]{%
  #1%
  \ensuremath{^{2}}%
}
\begin{document}
The command used as a prefix gives \square\m.

The command used as a postfix gives \m\square.
\end{document}

You'll see this gets a bit tricky, particularly to cover many cases.

The bigger problem comes when you get to more complex units. For example, what would

\m\square\kg

actually mean? Getting the spacing and meaning right in more complex cases is what siunitx aims to do, and one part of that is differentiating between 'power to add to the next unit' and 'power to add to the previous unit'.