\documentclass{article}
\begin{document}

Recursive definition of the factorial function in Haskell.

\begin{writehaskell}[visible]
fact :: Int -> Int
fact 0 = 1
fact n = n * fact (n-1)
\end{writehaskell}

The factorial function grows really fast. While 5 factorial is
\evalhaskell{fact 5}, 10 factorial is \evalhaskell{fact 10}.
\end{document}