See http://hackage.haskell.org/trac/ghc/ticket/3292
Turns m a -> m (). Lets one call functions for their side-effects without
having to get rid of their return values with '>> return ()'. Very useful
in many contexts (parsing, IO etc.); particularly good for 'forkIO' and 'forM_',
as they demand return types of 'IO ()' though most interesting IO functions
return non-().