Princess vs Lion in Elm

princess-vs-lion.elm

module PrincessVsLion where
import Keyboard

data GameState = Ongoing Int Int | LionWon | PrincessWon

proceedGame keyPresses state = case state of
  Ongoing princess lion ->
      checkEnd (princess + (keyPresses `div` 2)) (lion + 1)
  x -> checkRestart keyPresses x

checkEnd princess lion =
  if | lion == princess -> LionWon
     | princess >= 13 -> PrincessWon
     | otherwise -> Ongoing princess lion

checkRestart keyPresses state =
  if | keyPresses > 0 -> initState
     | otherwise -> state

input = diff 0 (sampleOn (fps 2) (count Keyboard.space))
initState = Ongoing 4 0

gameState = foldp proceedGame initState input

main = lift asText gameState

diff initValue signal =
  let roll2 x3 (x2, x1) = (x3, x2)
      withPrevious = foldp roll2 (initValue, initValue) signal
      diff2 (x2, x1) = x2 - x1
  in lift diff2 withPrevious