diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -24,3 +24,6 @@
 
 * export the variants for hyloM and metaM.
 
+## 0.1.3.3 -- 2020-05-13
+
+* added comment
diff --git a/monadic-recursion-schemes.cabal b/monadic-recursion-schemes.cabal
--- a/monadic-recursion-schemes.cabal
+++ b/monadic-recursion-schemes.cabal
@@ -4,7 +4,7 @@
 -- http://haskell.org/cabal/users-guide/
 
 name:                monadic-recursion-schemes
-version:             0.1.3.2
+version:             0.1.3.3
 synopsis:            Recursion Schemes for Monadic version.
 description:         Yet another recursion schemes for monadic style, depends on recursion-schemes.
 homepage:            https://github.com/cutsea110/monadic-recursion-schemes.git
diff --git a/src/Data/Functor/Foldable/Monadic.hs b/src/Data/Functor/Foldable/Monadic.hs
--- a/src/Data/Functor/Foldable/Monadic.hs
+++ b/src/Data/Functor/Foldable/Monadic.hs
@@ -67,7 +67,7 @@
 
 -- | histomorphism on catamorphism variant
 histoM' :: (Monad m, Traversable (Base t), Recursive t)
-        => (Base t (Cofree (Base t) a) -> m a)
+        => (Base t (Cofree (Base t) a) -> m a) -- ^ algebra
         -> t -> m a
 histoM' phi = return . extract <=< cataM f
   where f = liftM2 (:<) <$> phi <*> return
@@ -120,8 +120,8 @@
 
 -- | metamorphism on recursive variant
 metaM :: (Monad m, Traversable (Base t), Recursive s, Corecursive t, Base s ~ Base t)
-      => (Base t t -> m t)
-      -> (s -> m (Base s s))
+      => (Base t t -> m t)   -- ^ algebra
+      -> (s -> m (Base s s)) -- ^ coalgebra
       -> s -> m t
 metaM phi psi = h
   where h = (return . embed) <=< mapM h . project