\documentclass{article}

\usepackage{fancyvrb}

\usepackage{comment}

\DefineVerbatimEnvironment
  {code}{Verbatim}
  {} % Add fancy options here if you like.

\excludecomment{hidden}

\begin{hidden}
module LitTeX

%default total
\end{hidden}

\begin{document}

\begin{code}
data V a = Empty | Extend a (V a)
\end{code}

\begin{code}
isCons : V a -> Bool
isCons Empty = False
isCons (Extend _ _) = True
\end{code}

\begin{hidden}
namespace Hidden
  data U a = Empty | Extend a (U a)
\end{hidden}

\end{document}