Как избежать квадратичного увеличения экземпляров класса типов?

Подумайте:

{-# OPTIONS -fglasgow-exts #-}

data Second = Second
data Minute = Minute
data Hour = Hour

-- Look Ma', a phantom type!
data Time a = Time Int

instance Show (Time Second) where
  show (Time t) = show t ++ "sec" 

instance Show (Time Minute) where
  show (Time t) = show t ++ "min" 

instance Show (Time Hour) where
  show (Time t) = show t ++ "hrs" 

sec :: Int -> Time Second
sec t = Time t

minute :: Int -> Time Minute
minute t = Time t 

hour :: Int -> Time Hour
hour t = Time t 

class TimeAdder a b c | a b -> c where
  add :: Time a -> Time b -> Time c

instance TimeAdder Second Second Second where
  add (Time s1) (Time s2) = sec (s1 + s2)

instance TimeAdder Second Minute Second where
  add (Time s) (Time m) = sec (s + 60*m)

instance TimeAdder Second Hour Second where
  add (Time s) (Time h) = sec (s + 3600*h)

instance TimeAdder Minute Second Second where
  add (Time m) (Time s) = sec (60*m + s)

instance TimeAdder Minute Minute Minute where
  add (Time m1) (Time m2) = minute (m1 + m2)

instance TimeAdder Minute Hour Minute where
  add (Time m) (Time h) = minute (m + 60*h)

instance TimeAdder Hour Second Second where
  add (Time h) (Time s) = sec (3600*h + s)

instance TimeAdder Hour Minute Minute where
  add (Time h) (Time m) = minute (60*h + m)

instance TimeAdder Hour Hour Hour where
  add (Time h1) (Time h2) = hour (h1 + h2)

add (minute 5) (hour 2)
--125min

Хотя я очень взволнован, что такая безумная штука работает, мне интересно, как квадратичный взрыв TimeAdder экземпляров можно было бы избежать.