listings doesn't print my keywords bold in lstlisting

Question

I've got the following problem: I want to print a listing with the listings package and lstlisting:

\documentclass{scrreprt}

\usepackage{listings}


\begin{document}

\begin{lstlisting}[language=Haskell,basicstyle=\ttfamily,keywordstyle=\bfseries]
class Show a where
  show :: a -> String
\end{lstlisting}

\end{document} 

But in the result I get, the keywords are not printed in bold font! This problem does not occur when I remove the basic style key-value-pair, but without this style, the listings look ugly.

Answer 1

Add

\usepackage{courier}

in the preamble.