Add Endo style instances for Join

Author: garyb

src/Data/Profunctor/Join.purs

@@ -5,6 +5,7 @@ import Prelude
 import Data.Functor.Invariant (class Invariant)
 import Data.Newtype (class Newtype)
 import Data.Profunctor (class Profunctor, dimap)
+import Data.Monoid (class Monoid)
 
 -- | Turns a `Profunctor` into a `Invariant` functor by equating the two type
 -- | arguments.
@@ -17,5 +18,11 @@ derive newtype instance ordJoin :: Ord (p a a) => Ord (Join p a)
 instance showJoin :: Show (p a a) => Show (Join p a) where
   show (Join x) = "(Join " <> show x <> ")"
 
+instance semigroupJoin :: Semigroupoid p => Semigroup (Join p a) where
+  append (Join a) (Join b) = Join (a <<< b)
+
+instance monoidJoin :: Category p => Monoid (Join p a) where
+  mempty = Join id
+
 instance invariantJoin :: Profunctor p => Invariant (Join p) where
   imap f g (Join a) = Join (dimap g f a)