Monoids.elm

type Product = Product Integer

instance Monoid Product where
  empty : Product
  empty = Product 1
  
  add : Product -> Product -> Product
  add (Product x) (Product y) = Product (x * y)

instance (Monoid a, Monoid b) => Monoid (a, b) where
  empty : (a, b)
  empty = (empty, empty)
  
  add : (a, b) -> (a, b) -> (a, b)
  add (x1, x2) (y1, y2) = (add x1 y1, add x2 y2)