error using lstlisting inside minipage inside align*

Question

I am trying to use a lstlisting environment inside a minipage, inside an align* (fromo amsmath)... However, lualatex throws an error. This is the minimum working examp

\documentclass{article}

\usepackage{amsmath}
\usepackage{listingsutf8}

\begin{document}

\begin{align*}
f(a\in A) &= x\\
f(x\in X) &= \begin{minipage}[t]{4cm}
\begin{lstlisting}[mathescape=true]
(let $m$=mean($X$)
    $g(m) \circ h(x)$)
\end{lstlisting}
\end{minipage}
\end{align*}

\end{document}

The error is below:

! Argument of \lst@next has an extra }.
<inserted text> 
\par 
l.16 \end{align*}

? 
! Emergency stop.
<inserted text> 
\par 
l.16 \end{align*}

So, is there any way to do what I want? I'd like to define a function by cases, using align*, but with indented code on the right side

I'm not sure if it helps to mention that I'm typesetting the denotational semantics of a language, using some Lisp-like syntax on the right side of the equations defining semantic functions. A pseudo-example, formatted as plain text, follows.

      C[ v := e ]s = s[ E[e]/v ]
C[ some-command ]s = (let x=...
                         (if ...
                          then ...
                          else ...))

edit: As suggested by egreg, I tried the tabbing environment:

\documentclass{article}

\usepackage{amsmath}
\usepackage{listingsutf8}
\lstset{%
    showstringspaces=false,
    basicstyle=\ttfamily,
    commentstyle=\fontfamily{cmtt}\fontshape{it}\selectfont,
    frame=single
}

\begin{document}

\begin{center}
\begin{tabbing}
$f(a\in A)$ = \= $x$\\
$f(x\in X)$ = \> (let $m$=mean($X$) in\\
\> $\;\;\;\;$    \= $g(m) \circ h(x)$)
\end{tabbing}
\end{center}

\begin{center}
    \begin{tabbing}
        $f(a\in A)$ = \= $x$\\
        $f(x\in X)$ = \> \begin{minipage}[t]{4cm}
\begin{lstlisting}[mathescape=true]
(let $m$=mean($X$) in:
    $g(m) \circ h(x)$)
\end{lstlisting}
\end{minipage}
\end{tabbing}
\end{center}
\end{document}

This is the result:

There are two attempts in the document. The first seems cumbersone to typeset (and controlling indentation would be somewhat painful); the second is easier to type, but the first line of the lstlisting inside the minipage isn't aligned with the equation (I've included the frame in the lstlisting environment only to make the alignment clear).

Also, the equal signs don't seem to be aligned in both cases.

Is this easy to fix?

Answer 1

You can't use lstlisting in the body of an align* environment, because it's the same as using it in the argument to a command, which is not allowed.

I suggest tabbing:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align*}
f(a\in A) &= x\\
f(x\in X) &=
  \begin{minipage}[t]{\textwidth}\ttfamily
  \begin{tabbing}
    (let \= $m=\texttt{mean}(X)$ in\\
         \> $g(m) \circ h(x)$)
  \end{tabbing}
  \end{minipage}
\end{align*}

\end{document}

This takes advantage from the fact that a minipage that only encloses a tabbing environment is as wide as the widest line in the tabbing.