import Control.Monad.Reader (Reader, runReader, ask, local)

main :: IO ()
main = do
  putStrLn ("Naive: " ++ show (fib_naive 10))
  putStrLn ("Fast: " ++ show (fib_acc (0,1) 10))
  putStrLn ("Reader: " ++ show (runReader (fib_reader 10) (0,1)))

fib_naive :: Int -> Int
fib_naive 0 = 0
fib_naive 1 = 1
fib_naive n = fib_naive (n - 1) + fib_naive (n - 2)

fib_acc :: (Int, Int) -> Int -> Int
fib_acc (i, j) 0 = i
fib_acc (i, j) 1 = j
fib_acc (i, j) n = fib_acc (j, i + j) (n - 1)

fib_reader :: Int -> Reader (Int, Int) Int
fib_reader n = do
  (i, j) <- ask
  case n of
    0 -> return i
    1 -> return j
    _ -> local (\(i, j) -> (j, i + j)) (fib_reader (n - 1))