\begin{slide}{Anwendung der Maybe-Monade}

\begin{verbatim}
case ( evaluate e l ) of
    Nothing -> Nothing
    Just a  -> case ( evaluate e r ) of
        Nothing -> Nothing
        Just b  -> Just ( a + b )
\end{verbatim}
mittels der Monad-Instanz von Maybe:  
\begin{verbatim}
evaluate e l >>= \ a ->
    evaluate e r >>= \ b ->
        return ( a + b)
\end{verbatim}
  
\end{slide}
