\documentclass{article}
\begin{document}
\begin{writehaskell}

import Text.LaTeX

import Control.Concurrent
import Control.Concurrent.MVar

test :: IO String
test = do
  v <- newEmptyMVar
  forkOS $ putMVar v "concurrent"
  takeMVar v

\end{writehaskell}
I like to write \iohatex{fmap fromString test} programs.
\end{document}