\@ifnextchar inside pmatrix?

Question

I created a function which handles a superscript followed after the function. I do that by using \@ifnextchar^. Everything works well, except in any pmatrix environment. Why?

As a quick-and-dirty example I created the (useless) function \fv to debug it a little:

\documentclass{report}
\usepackage{amsmath}

\makeatletter
\newcommand{\fv}[1]{
    \@ifnextchar^{A}{B}
}
\makeatother

\begin{document}

    \begin{align*}
        \begin{pmatrix}
            \fv{x}, \fv{x}^y
        \end{pmatrix} = \fv{x} = \fv{x}^y
    \end{align*}

\end{document}

I had expected a result in the form (B,A^y)=B=A^y. But what I get is (B,B^y)=B=A^y.

Is there a way to detect the superscript in pmatrix environments as well?

Answer 1

The problem is in your macro: \@ifnextchar is picking up the space character.

Try

\newcommand{\fv}[1]{%
    \@ifnextchar^{A}{B}%
}

The % I inserted masks the space, so \@ifnextchar has a chance to "see" the next character.

(explanation by Heiko Oberdiek:) LaTeX's original \@ifnextchar gobbles spaces, thus the extra space would not hurt. But amsmath redefines \@ifnextchar inside the matrix environment. The redefined version (\new@ifnextchar, defined in amsgen.sty) does not ignore the space.

Hence, your second example, using the original \@ifnextchar, even works with the space in place.