Write a function from IO a -> a?

Question

Take the function getLine.

it has a type

getLine :: IO String

How do I extract the String from this IO value. More generally, how do I

Answer 1

If you know C then consider the question "How can I get the string from gets?" An IO String is not some string that's made hard to get to, it's a procedure that can return a string - like reading from a network or stdin. You want to run the procedure to obtain a string.

A common way to run IO actions in a sequence is do notation:

main = do
   someString <- getLine
   -- someString :: String
   print someString

In the above you run the getLine operation to obtain a String value then use the value however you wish.