gistfile1.txt

data MonO

-- This is the type of arrows for the category of monoids.  Note that
-- for a monoid, regarded as a category, there is a single object
-- (called MonO here).  Arrows are formed by members of the monoid's
-- carrier, m.
data MonC a b where
  M :: Monoid c => c -> MonC MonO MonO

instance Monoid m => Category MonC where
  dom (M (_ :: m)) = M (mempty :: m)
  cod (M (_ :: m)) = M (mempty :: m)
  (M a) ∘ (M b) = M ((mappend a b) :: m)