s.7.1 p.98 RR, I/O

Output Functions ...

putChar  :: Char -> IO ()
putStr   :: String -> IO ()
putStrLn :: String -> IO ()   -- adds a newline
print    :: Show a => a -> IO ()

Recall that (IO τ) is a Monad (s.7.2 pp.99 RR), so that >>, etc. are applicable to it. The " main program", main, must be of type (IO τ) for some type τ.

main = putStr "Hello " >> putStr "World."
       ---------------    ---------------
           IO ()              IO ()
       ----------------------------------
                     IO ()

Input Functions ...

getChar     :: IO Char     -- exception on EOF
getLine     :: IO String   -- ditto
getContents :: IO String
interact    :: (String -> String) -> IO ()
readIO      :: Read a => String -> IO a
readLn      :: Read a => IO a

In input step passes a value on to the next step after >>=

... getChar  >>=  putChar  ...
    -------       -------
    IO Char      Char->IO()
    -----------------------
              IO ()

IO type, s.8.1. prelude, p.111, RR

 

data IO a = ...abstract

 

instance Functor IO where

  fmap f x = x >>= (return . f)

 

instance Monad IO where

  (>>=) = ...

  return = ...

  fail s = ioError (userError s)

RR = revised report, CUP, 2003.