/// Lens ///

"""
Laws:
  set l (get l a) a) = a
  get l (set l b a) = b
  set l c (set l b a) = set l c a
"""
data Lens a b = Lens (a -> b) (b -> a -> a)

get :: Lens a b -> a -> b
get (Lens f _) = f

set :: Lens a b -> b -> a -> a
set (Lens _ g) = g

/*instance Category Lens where
    id = Lens id const
    f . g = Lens (get f . get g)
                 (\x y -> set g (set f x (get g y)) y)
*/