Submarine Shooter

submarine.hs

import System.IO.Unsafe (unsafePerformIO)
import System.IO (hFlush, stdout)
import System.Random (randomRIO)
import Control.Monad (foldM_)
import Data.Functor ((<$>))
import System.Exit (exitSuccess)

-- prerequisite
mapSize :: Int
mapSize = 9
type Submarine = (Int,Int) 

-- utilities
genPoint :: Submarine
genPoint = (rand mapSize, rand mapSize)

rand :: Int -> Int
rand n = unsafePerformIO $ randomRIO (1, n)

input :: IO Int
input = read <$> getLine

-- game
mainloop :: IO ()
mainloop = foldM_ gameRound genPoint [1..]

gameRound :: Submarine -> Int -> IO Submarine
gameRound sm rn = do
  putStrLn $ "------ " ++ show rn ++ " ------"
  putStr "  X=? "
  hFlush stdout
  x <- input
  putStr "  Y=? "
  hFlush stdout
  y <- input
  judge sm (x, y)
     
judge :: Submarine -> (Int, Int) -> IO Submarine
judge sm@(zx, zy) (x, y) = do
  sm' <- case sqrt.fromIntegral $ (zx-x)^2 + (zy-y)^2 of 
    r | 4 < r     -> putStrLn "4>R"    >> return sm
      | 3 < r     -> putStrLn "3<R<=4" >> return sm
      | 2 < r     -> putStrLn "2<R<=3" >> return sm
      | 1 < r     -> putStrLn "1<R<=2" >> return sm
      | 0 < r     -> putStrLn "R=1"    >> return (moved sm)
      | otherwise -> end True
  if (1,1) <= sm' && sm' <= (mapSize, mapSize)
    then return sm'
    else end False

moved :: Submarine -> Submarine
moved (x,y) = (x + (rand 3 - 2),
               y + (rand 3 - 2))

end :: Bool -> IO b
end flag = do
  if flag 
    then putStrLn "OK!"
    else putStrLn "DOING BADLY!"
  putStrLn "------ END ------"
  exitSuccess

main :: IO ()
main = mainloop