diff --git a/forest.cabal b/forest.cabal
--- a/forest.cabal
+++ b/forest.cabal
@@ -1,5 +1,5 @@
 name:           forest
-version:        0.2
+version:        0.2.1
 synopsis:       Tree and Forest types
 license:        MPL-2.0
 license-file:   LICENSE
@@ -24,13 +24,14 @@
     Data.Tree.Forest
 
   build-depends:
-    aeson >= 0.2 && < 1.3,
+    aeson >= 0.2 && < 1.4,
     base >= 4.2 && < 5,
+    comonad >= 5.0 && < 6,
     deepseq >= 1.1 && < 2,
+    free >= 4.12 && < 6,
     hashable >= 1.2 && < 2,
     profunctors >= 3.2 && < 6,
-    semigroupoids >= 1.0 && < 6,
-    base >= 4 && < 5
+    semigroupoids >= 1.0 && < 6
 
   if impl(ghc < 8)
     build-depends:
diff --git a/src/Data/Tree/Forest.hs b/src/Data/Tree/Forest.hs
--- a/src/Data/Tree/Forest.hs
+++ b/src/Data/Tree/Forest.hs
@@ -30,6 +30,7 @@
     , _Leaf
     , _Node
     , _Forest
+    , free
 
     -- * Higher order traversals
     -- ** Trees
@@ -133,10 +134,18 @@
 import           Prelude hiding (foldr1)
 
 
+-- comonad -------------------------------------------------------------------
+import           Control.Comonad.Trans.Env (EnvT (EnvT))
+
+
 -- deepseq -------------------------------------------------------------------
 import           Control.DeepSeq (NFData, rnf)
 
 
+-- free ----------------------------------------------------------------------
+import           Control.Monad.Free (Free (Free, Pure))
+
+
 -- hashable ------------------------------------------------------------------
 import           Data.Hashable (Hashable, hashWithSalt)
 
@@ -202,6 +211,20 @@
 
 
 ------------------------------------------------------------------------------
+-- | This is an @Functor g => Iso' (Tree g s a) (Free (EnvT s g) a)@
+free :: (Profunctor p, Functor f, Functor g)
+    => p (Free (EnvT s g) a) (f (Free (EnvT s g) a))
+    -> p (Tree g s a) (f (Tree g s a))
+free = dimap to (fmap from)
+  where
+    from (Pure a) = Leaf a
+    from (Free (EnvT s ts)) = Node s (Forest (fmap from ts))
+    to (Leaf a) = Pure a
+    to (Node s (Forest ts)) = Free (EnvT s (fmap to ts))
+{-# INLINE free #-}
+
+
+------------------------------------------------------------------------------
 instance (NFData s, NFData a, NFData (f (Tree f s a))) =>
     NFData (Tree f s a)
   where
@@ -627,7 +650,7 @@
 
 
 ------------------------------------------------------------------------------
-instance Alternative f => Monoid (Forest f s a) where
+instance (Alt f, Alternative f)=> Monoid (Forest f s a) where
     mempty = empty
     {-# INLINE mempty #-}
     mappend = (<|>)