Haskellers think I/O is an important effect and want to manage when it's allowed. It's allowed in the IO context, which happens to be the default: main = d

Haskellers think I/O is an important effect and want to manage when it's allowed. It's allowed in the IO context, which happens to be the default: main = do putStrLn "hello world" more more = putStrLn "hello again, still in IO" If you want to, you can break the rule and do I/O from non-IO functions, by using trace: import Debug.Trace add :: Int -> Int -> Int add a b = trace ("adding " ++ show a ++ " and " ++ show b) (a + b) or unsafePerformIO: import System.IO.Unsafe add :: Int -> Int -> Int add a b = unsafePerformIO $ do appendFile "debug.log" ("adding " ++ show a ++ " and " ++ show b ++ "\n") return (a + b) Not exactly white-hot hoops of fire ? :)