import IO

-- getLine
-- getChar
-- print

incMaybe :: Maybe Int -> Maybe Int
incMaybe m = m >>= (\x -> return (x+1))

sumMaybe :: Maybe Int -> Maybe Int -> Maybe Int
sumMaybe m1 m2 = m1 >>= (\x -> m2 >>= (\y -> return (x + y)))

sumMaybe1 m1 m2 = do
  x <- m1
  y <- m2
  return x + y

{--
class  Monad m  where
    (>>=)            :: m a -> (a -> m b) -> m b
    return           :: a -> m a    
    
    (>>)             :: m a -> m b -> m b
    fail             :: String -> m a

    m >> k           =  m >>= \_ -> k
--}

doubleElements :: [Int] -> [Int]
doubleElements l = l >>= (\x -> [x,x])

decProduct1 a b = [ (x,y) | x <- a, y <- b ]

decProduct2 a b = a >>= (\x -> b >>= (\y -> return (x,y)))

decProduct3 a b = do
  x <- a
  y <- b
  return (x,y)

-- Main

mainDoubleLine :: IO ()
mainDoubleLine = getLine >>= (\x -> putStrLn x >> putStrLn x)

mainDoubleLines :: IO ()
mainDoubleLines = getLine >>= (\x -> if x == "q" then return () else putStrLn x >> putStrLn x >> mainDoubleLines)

getInt :: IO Int
getInt = getLine >>= (\s -> return (read s))

mainDoubleInts :: IO ()
mainDoubleInts = getInt >>= (\x -> if x == 0 then return () else print (2*x) >> mainDoubleInts)

mainFilter :: IO ()
mainFilter = interact (\s -> [c | c <- s, c >= '0', c <= '9'])

-- type IO a  =  RealWorld -> (a, RealWorld)

-- ReadLine

mainRead = do 
  x <- openFile "/tmp/foo.txt" ReadMode
  y <- hGetLine x
  putStr y
  hClose x
  
  
mainRead2 = do 
  n <- getLine
  x <- openFile n ReadMode
  y <- hGetLine x
  putStr y
  hClose x
  
mainWrite = do
  x <- openFile "/tmp/foo.txt" WriteMode
  hPutStr x "sss"
  hClose x