{-# LANGUAGE TemplateHaskell #-}
import Control.Applicative
import Control.Lens hiding (op)
import Control.Monad
import Control.Monad.State
import Data.Char
import System.IO
import System.Random

data Game = Game { _values :: [Int], _right :: Int, _rounds :: Int }
makeLenses ''Game

gameLoop :: StateT Game IO ()
gameLoop = do
  flushPut "Would you like to play? y/n: "
  keepPlaying <- ("y" ==) . map toLower <$> liftIO getLine
  when keepPlaying $ do
    (x:y:r:_) <- values <<%= drop 3
    numRounds <- rounds <+= 1

    let (solution, opStr) = [(x + y, "+") , (x - y, "-")] !! (r `mod` 2)
    flushPut $ unwords ["What is", show x, opStr, show y, "? "]
    correct <- (solution ==) <$> liftIO readLn
    numRight <- right <+= if correct then 1 else 0

    liftIO . putStrLn $ unwords [message solution correct,
      "\nYou have solved", show numRight, "out of", show numRounds]
    gameLoop
  where
    flushPut = liftIO . (>> hFlush stdout) . putStr
    message _ True = "Correct!"
    message soln _ = unwords ["Sorry! the correct answer is:", show soln]

main :: IO ()
main = do
  randomValues <- randomRs (1,100) <$> getStdGen
  evalStateT gameLoop (Game randomValues 0 0)