diff --git a/monoid-extras.cabal b/monoid-extras.cabal
--- a/monoid-extras.cabal
+++ b/monoid-extras.cabal
@@ -1,5 +1,5 @@
 name:                monoid-extras
-version:             0.3.2.2
+version:             0.3.2.3
 synopsis:            Various extra monoid-related definitions and utilities
 description:         Various extra monoid-related definitions and utilities,
                      such as monoid actions, monoid coproducts, \"deletable\"
@@ -33,9 +33,9 @@
 
   build-depends:     base >= 4.3 && < 4.8,
                      groups < 0.5,
-                     groupoids < 4,
+                     groupoids < 5,
                      semigroups >= 0.8 && < 0.12,
-                     semigroupoids < 4
+                     semigroupoids < 5
 
   hs-source-dirs:    src
 
diff --git a/src/Data/Monoid/Action.hs b/src/Data/Monoid/Action.hs
--- a/src/Data/Monoid/Action.hs
+++ b/src/Data/Monoid/Action.hs
@@ -1,6 +1,5 @@
-{-# LANGUAGE MultiParamTypeClasses
-           , FlexibleInstances
-  #-}
+{-# LANGUAGE FlexibleInstances     #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
 
 -----------------------------------------------------------------------------
 -- |
@@ -17,7 +16,7 @@
        ( Action(..)
        ) where
 
-import Data.Semigroup
+import           Data.Semigroup
 
 ------------------------------------------------------------
 --  Monoid and semigroup actions
@@ -32,7 +31,7 @@
 --   * @act (m1 ``mappend`` m2) = act m1 . act m2@
 --
 --   Semigroup instances are required to satisfy the second law but with
---   '(<>)' instead of 'mappend'.  Additionally, if the type @s@ has
+--   ('<>') instead of 'mappend'.  Additionally, if the type @s@ has
 --   any algebraic structure, @act m@ should be a homomorphism.  For
 --   example, if @s@ is also a monoid we should have @act m mempty =
 --   mempty@ and @act m (s1 ``mappend`` s2) = (act m s1) ``mappend``
@@ -60,7 +59,25 @@
   act :: m -> s -> s
   act = const id
 
+-- | @()@ acts as the identity.
+instance Action () l where
+  act () = id
+
 -- | @Nothing@ acts as the identity; @Just m@ acts as @m@.
 instance Action m s => Action (Option m) s where
   act (Option Nothing)  s = s
   act (Option (Just m)) s = act m s
+
+-- | @Endo@ acts by application.
+--
+--   Note that in order for this instance to satisfy the @Action@
+--   laws, whenever the type @a@ has some sort of algebraic structure,
+--   the type @Endo a@ must be considered to represent /homomorphisms/
+--   (structure-preserving maps) on @a@, even though there is no way
+--   to enforce this in the type system.  For example, if @a@ is an
+--   instance of @Monoid@, then one should only use @Endo a@ values
+--   @f@ with the property that @f mempty = mempty@ and @f (a <> b) =
+--   f a <> f b@.
+instance Action (Endo a) a where
+  act = appEndo
+
diff --git a/src/Data/Monoid/MList.hs b/src/Data/Monoid/MList.hs
--- a/src/Data/Monoid/MList.hs
+++ b/src/Data/Monoid/MList.hs
@@ -113,9 +113,6 @@
 --   heterogeneous monoidal lists on each other.
 newtype SM m = SM m
 
-instance Action () l where
-  act _ a = a
-
 instance (Action (SM a) l2, Action l1 l2) => Action (a, l1) l2 where
   act (a,l) = act (SM a) . act l