newtype Health    = Health Int deriving Num
newtype Attack    = Attack Int deriving Num
newtype Defense   = Defense Int deriving Num

data Fighter = MkFighter
  { health  :: Health
  , attack  :: Attack
  , defense :: Defense
  , actions :: [Action]
  }

type Knight  = Fighter
type Monster = Fighter

act x = x { actions = tail $ actions x }

duel :: Fighter -> Fighter -> Fighter
duel x y = case head $ actions x of
  AcAttack       xa -> duel (y { health = health y - (Health xa) }) (act y)
  AcPotion       xa -> duel y (x { health = health x + (Health xa) })
  AcDefenseSpell xa -> duel y (x { defense = defense x + (Defense xa) })
  AcRunAway         -> y