Right type for I/O functions with argument in Haskell

Question

I want to implement a function that asks different questions in base of sex. However I fail in giving it the right Type.

askDifferentQuestion :: String -> IO String
askDifferentQuestion sex = do
  putStrLn "ciao"

main = do
  sex <- getLine
  askDifferentQuestion sex

If I execute I get

test.hs:3:3:
    Couldn't match expected type `String' with actual type `()'
    Expected type: IO String
      Actual type: IO ()
    In the return type of a call of `putStrLn'
    In a stmt of a 'do' block: putStrLn "ciao"
Failed, modules loaded: none.

Why am I doing it wrong?

Answer 1

putStrLn的类型是putStrLn String -> IO ()而不是putStrLn String -> IO String 。

Answer 2

The type IO String means an input/output action that yields a String when run. As is, askDifferentQuestion results in () , which usually indicates an insignificant value. This is because the only action to be run is putStrLn whose type is IO () , ie , you run it for its side-effect only.

Assuming your type is correct, change the definition of askDifferentQuestion to both prompt the user and return the response. For example

askDifferentQuestion :: String -> IO String
askDifferentQuestion sex = putStrLn (q sex) >> getLine
  where q "M" = "What is the airspeed velocity of an unladen swallow?"
        q "F" = "How do you like me now?"
        q "N/A" = "Fuzzy Wuzzy wasn’t fuzzy, was he?"
        q "N" = "Why do fools fall in love?"
        q "Y" = "Dude, where’s my car?"
        q _ = "Why do you park on a driveway and drive on a parkway?"