[Tex/LaTex] How to make a macro require math mode

macrosmath-mode

I have defined a macro that I would like to be used exclusively in math mode. In order to enforce this, I would like the macro to throw an error message whenever it is used outside of math mode. What is the easiest way of making a macro math-mode-only?

Best Answer

You can use \ifmmode to check if you are in math mode, and trigger an error if not. An attempt to use it outside of math mode yields:

Attempt to use \MyMathModeMacro outside of math mode.

See my preamble documentation for explanation.

Type H for immediate help.

l.16 \MyMathModeMacro % <--- This will produce an error ?

Notes:

As David Carlisle pointed out, need to use \DeclareRobustCommand instead of \newcommand to avoid premature expansion. See \ifmmode doesn't seem to work correctly inside an array environment.
If you want to allow the macro to be used in or outside of math mode, you can use \ensuremath to typeset the content in math mode. But you should refer to When not to use \ensuremath for math macro? before doing this.

Code:

\documentclass{article}
\makeatletter
\DeclareRobustCommand{\MyMathModeMacro}{%
    \ifmmode
         E = m c^2
    \else
        \GenericError{\space\space\space\space}
            {Attempt to use \@backslashchar MyMathModeMacro outside of math mode}
            {See my preamble documentation for explanation.}
            {Need to use either use inline or display math.}%
    \fi
}
\makeatother

\begin{document}
$\MyMathModeMacro$

\MyMathModeMacro% <--- This will produce an error
\end{document}

Related Solutions

[Tex/LaTex] Proper way to use \ensuremath to define a macro useable in and out of math mode

Probably

\ensuremath{\textcolor{red}{#2}}

is what you need, since \textcolor can be used in text and in math. The complete definition is

\newcommand{\DefineNamedFunction}[2]{% {FunctionName}{FunctionExpression}
    \expandafter\providecommand\csname#1\endcsname
      {\ensuremath{\textcolor{red}{#2}}}%
}
...
\DefineNamedFunction{FunctionF}{y=2\sin x}
$\FunctionF$ and \FunctionF

Sample

I've also deleted the braces that require another \expandafter, but that's not the problem.

Of course, you can't call

\DefinedNamedFunction{FunctionFF}{$y=x$}

and I wouldn't know why you'd want it. But in any case there's a simple solution

\newcommand{\DefineNamedFunction}[2]{%
  \expandafter\providecommand\csname#1\endcsname
    {\ensuremath{\begingroup\color{red}\DNFnorm#2\endgroup}}}
\makeatletter
\def\DNFnorm{\@ifnextchar$\DNFnormi{}}
\def\DNFnormi$#1${#1}
\makeatother

The input is "normalized" by removing the $ tokens before and after, if present.

With \begingroup\color{red}...\endgroup the spaces in the subformula participate to the stretching and shrinking of the spaces in the line.

[Tex/LaTex] When not to use \ensuremath for math macro

Two objections come to mind:

If you don't know you will be entering math mode, then you may write something "semantically" correct that breaks.

For example:

\documentclass{article}
\newcommand\mathmacro[1][A]{\ensuremath{{#1}_1}}
\begin{document}
 \mathmacro[$x^2$] % The dollar signs *leave* math mode
\end{document}

What's going on here is that \mathmacro[$x^2$] expands to

\ensuremath{{$x^2$}_1}

which expands to (effectively)

\ifmmode
 {$x^2$}_1
\else
 ${$x^2$}_1$
\fi

and you can see that if you write it outside of math mode, the second branch is taken, so the first dollar sign brings you into math mode and the one at the front of $x^2$ takes you out of it, with the reverse operation happening afterwards. This gives an error.

Of course, you aren't supposed to do that, since \mathmacro is actually a "math macro usable in text mode", so you should think of the thing between the brackets of its argument as being in math mode. Alas, this confuses both the author and the text editor's syntax highlighting, since it is a nonstandard assumption.

Edit: I would define this macro as:

\newcommand\mathmacro[1][A]{{#1}_1}

and use it as:

$x^2$ sub one: $\mathmacro[x^2]$.

This way, the parts that are math are clearly math.

In the unlikely event that you or some package sets \everymath, you will be very surprised when your apparently text-mode macros start to look different.

On the subject of semantics, though, the issue is clear: \ensuremath breaks the separation between math and text, which are two very different things. TeX even has the distinction built in: different fonts, different spacing rules, different parsing rules. You can probably construct a lot more counterexamples by exploiting these.

What I mean by this is that in the following situation:

\documentclass{article}
\newcommand\mathmacro[1][A]{\ensuremath{{#1}_1}}
\begin{document}
 A sub one: \mathmacro

 \bfseries A sub one: \mathmacro
\end{document}

you may be surprised that the bold text does not extend to the contents of the apparently text-mode \mathmacro.

What I'm saying is not so much that \ensuremath actually breaks anything as that it violates your expectations to the point that it makes things harder rather than easier.