Gumballs!

Main.hs

data Action = Coin | Turn deriving Show
data Locked = Locked | Unlocked deriving Show
data State = State { locked :: Locked, coins :: Int, gumballs :: Int } deriving Show

update :: Action -> State -> State
update Coin s@(State { locked = Locked, gumballs = g }) | g == 0 = s
update Coin s@(State { locked = Locked, coins = c })             = s { locked = Unlocked, coins = c + 1 }
update Turn s@(State { locked = Unlocked, gumballs = g })        = s { locked = Locked, gumballs = g - 1 }
update _ s                                                       = s

main = putStrLn $ show $ update Turn $ update Coin State { locked = Locked, coins = 0, gumballs = 100 }