Macro for a math map f:A\to B

Question

I want to write a macro for a math map f:A\to B when it is in inline math mode and f:A\longrightarrow B for displayed math. But I don't know how to check that it is in inline mode or displayed style.

Here is my try:

\documentclass{article}
\newcommand{\map}[3]{#1\mathpunct{:}#2\longrightarrow #3}
\begin{document}
$\map{f}{A}{B}$
[\map{f}{A}{B}]
\end{document}

I have tried the following but it produces Undefined control sequence. $\map

\makeatletter
\newcommand[3]{\map}{
    #1\mathpunct{:}#2
    \if@display
    \longrightarrow #3
    \else
    \rightarrow #3
    \fi}
\makeatother

Use the command \mathchoice. https://tex.stackexchange.com/questions/148740/what-is-mathchoice — Alan Xiang, May 30 '21 at 16:23

Marcel Krüger · Accepted Answer · 2021-05-30T17:40:38.317

7

You can't differentiate displaystyle and text style using a if test since use of the plain TeX \over can change the mathstyle after the fact. But you can use \mathchoice to provide four expressions (one for each math size (display/text/script/scriptscript)) and only the one for the right size will actually appear:

\documentclass{article}
\newcommand{\map}[3]{#1\colon #2\mathchoice{\longrightarrow}{\to}{\to}{\to}#3}
\begin{document}
$\map{f}{A}{B}$
[\map{f}{A}{B}]
\end{document}

edited May 30 '21 at 17:40

answered May 30 '21 at 16:23

Marcel Krüger

34,151

Maybe you should replace \mathpunct{:} by \colon? – Gaussler May 30 '21 at 16:45
@Gaussler Thanks, edited. – Marcel Krüger May 30 '21 at 17:40
@Gaussler: Why? \colon is more suitable and recurrent? I use \mathpunct{:} always. – C.F.G May 31 '21 at 05:49
@C.F.G Without amsmath, \colon is the same as \mathpunct{:} but it's shorter and IMO more readable. With amsmath it has different spacing which makes it feel less cramped and more like a normal colon. – Marcel Krüger May 31 '21 at 06:32

Macro for a math map f:A\to B

1 Answers1