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

Not going into any details, if you're in a do block, you can (informally/inaccurately) consider <- as getting the value out of the IO.

For example, the following function takes a line from getLine, and passes it to a pure function that just takes a String

main = do
  line <- getLine
  putStrLn (wrap line)

wrap :: String -> String
wrap line = "'" ++ line ++ "'"

If you compile this as wrap, and on the command line run

echo "Hello" | wrap

you should see

'Hello'