Contrived example of using the Maybe monad to make inherently unsafe operations safe by representing failures as Nothing. You can turn a sequence of functions that produce Maybes into an imperative chunk of code that only has to handle the success case.

maybemonad.hs

safeHead :: [a] -> Maybe a
safeHead []     = Nothing
safeHead (x:xs) = Just x

safeDiv :: Int -> Int -> Maybe Int
safeDiv _ 0 = Nothing
safeDiv x y = Just (div x y)

divideHeads :: [Int] -> [Int] -> Maybe Int
divideHeads xs ys = do
  -- using Maybe monad, Nothing will safely terminate early
  x <- safeHead xs
  y <- safeHead ys
  safeDiv x y

main :: IO ()
main = do
  print (divideHeads [] []) -- Nothing, can't head empty
  print (divideHeads [4, 2] [2]) -- Just 2
  print (divideHeads [1, 2] [0, 5]) -- Nothing from divide by zero