TypeFamilies.hs

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}

type Position = (Int, Int)
newtype PlayerData = PlayerData { _pos :: Position }
data GameStateData = GameStateData PlayerData [Position]



-- player typeclass and instance
class Player p where
  playerPosition :: p -> Position
  playerMoveTo :: Position -> p -> p

instance Player PlayerData where
  playerPosition = _pos
  playerMoveTo pos player = player { _pos = pos }
  
  
  
-- gamestate typeclass and implementation
class Player (PlayerType g) => GameState g where
  type PlayerType g :: *
  getPlayer :: g -> PlayerType g
  getMonsterPositions :: g -> [Position]

instance GameState GameStateData where
  type PlayerType GameStateData = PlayerData
  getPlayer           (GameStateData p _) = p
  getMonsterPositions (GameStateData _ mPoses) = mPoses