module Main where

import qualified Data.ByteString.Char8 as BS
import Data.Maybe (fromMaybe)
import Data.Char (isSpace)
import Control.Monad.State.Lazy
import Control.Applicative

-----BEGIN PARSER-----

type Parser a = StateT BS.ByteString Maybe a

skipSpaces (x, s) = (x, BS.dropWhile isSpace s)

readInt :: Parser Int
readInt = StateT $ fmap skipSpaces . BS.readInt

-----END PARSER------

-----BEGIN COMBINATORS-----

readPair :: Parser (Int, Int)
readPair = do
  x <- readInt
  y <- readInt
  return (x, y)

readIntList :: Int -> Parser [Int]
readIntList n = sequence  $ replicate n readInt

readSizedList :: Parser [Int]
readSizedList = readInt >>= readIntList

-----END COMBINATORS-----

-- Testing...

data Struct = Struct Int (Int, Int) [Int]
            deriving (Show)

orFail = fromMaybe (BS.pack "EPIC FAIL!!!\n")

parsePrint :: Show a => Parser a -> (BS.ByteString -> BS.ByteString)
parsePrint parseMe = orFail . evalStateT (do x <- parseMe
                                             return $ BS.pack $ show x ++ "\n")

testPair = parsePrint (Struct <$> readInt <*> readPair <*> readSizedList)

main = BS.interact testPair