\dicumentclass{article}
\usepackage{listings}
\begin{document}
\begin{lstlisting}[language=R]
fib <- function(n) {
  if (n < 2)
    n
  else
    fib(n - 1) + fib(n - 2)
}
fib(10) # => 55
\end{lstlisting}
\end{document}