data TurnPhase = Phase1 | Phase2 deriving Show

data GamePhase = GamePhase { _playerTurn :: Int
                           , _turnPhase  :: TurnPhase
                           } deriving Show
makeLenses ''GamePhase

processEndPhase :: Game -> Game
processEndPhase game = game
  where
    update gamePhase =
      case gamePhase ^. turnPhase of Phase1 -> gamePhase & turnPhase .~ Phase2
                                  -- Phase2 -> ...Here I want to both reset _turnPhase to Phase1 and increment _playerTurn

-- Obviously, I could just return a whole new structure, like GamePhase { _playerTurn = oldTurn + 1, _turnPhase = Phase1 }.
-- But I was hoping lenses would provide me with a concise, inline way of doing the same thing, especially because
-- my data structures are probably going to get a lot more complicated than this.

-- Also, I'd like to avoid using State monads unless that's absolutely what's called for here. I don't totally understand them.