4. Kontrollstrukturen mit Monads

• Imperative Kontrollstrukturen lassen sich mit Monads leicht nachbilden.

  foreach :: (Monad m) => [a] -> (a -> m ()) -> m ()
  foreach []     _ = return ()
  foreach (x:xs) f = do f x
                        foreach xs f
  
  > foreach [1..10] (\x -> 
  >   putStrLn (show x)
  > )                       
  
  while :: (Monad m) => m Bool -> m () -> m ()
  while cond f = do res <- cond
                    if res then (f >> (while cond f)) 
                           else (return ())
    
  > while (return True) (
  >   putStrLn "Hello world"
  > )