Question

How do we print the output of a function that returns an IO String to the stdout? I am not able to use show or print.

Answer 1

If you want to print the result of the function foo :: Int -> IO String (for example), you can do

main = do
    str <- foo 12
    putStrLn str

or, without the do-notation,

main = foo 12 >>= putStrLn.

The do-notation is syntactic sugar for the second, which uses the fundamental (>>=) combinator, which has the type

(>>=) :: Monad m => m a -> (a -> m b) -> m b

IO is an instance of the Monad class, so you can use it here.

foo :: Int -> IO String
foo 12 :: IO String

putStrLn :: String -> IO ()

(foo 12) >>= putStrLn :: IO ()