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.
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2 Answers
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 ()
1 Comment
schuelermine
Generally, you could say ´printio = do oi <- io \n print io´
How do we print the output of a function that returns an IO String to the stdout?
Well let's see. Here's a function that returns an IO String:
dumbFunction :: a -> IO String
dumbFunction x = getLine
dumbFunction is a dumb function (but a function nonetheless!). It ignores its input, and then returns getLine, which has the type IO String.
So you tell me, how do you print getLine :: IO String? The answer is, you don't! This is what we call an "IO action". Note that an IO action is not a function, because it does not take input. (However, IO actions might acquire input from, well, IO operations such as reading stdin, as getLine does. But it is not considered a "function" because it does not accept any traditional input)
So instead of printing out the action itself, you probably want to run the action, and then print the result. This can be done as Daniel Fischer described (with <-, which can be thought of as the "run" operator).