diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -25,3 +25,6 @@
 
 ## 0.2.3
 * function: diff; find changes between two tries
+
+## 0.2.4
+* added 'functor' instance for the Diff datatype
diff --git a/hsrc_lib/Data/VCache/Trie.hs b/hsrc_lib/Data/VCache/Trie.hs
--- a/hsrc_lib/Data/VCache/Trie.hs
+++ b/hsrc_lib/Data/VCache/Trie.hs
@@ -484,6 +484,11 @@
 data Diff a = InL a | Diff a a | InR a
     deriving (Show, Eq)
 
+instance Functor Diff where
+    fmap f (InL a) = InL (f a)
+    fmap f (Diff a b) = Diff (f a) (f b)
+    fmap f (InR b) = InR (f b)
+
 -- thoughts: it might be worthwhile to have a variation
 -- that preserves common subtrees and elements, i.e. such
 -- that both trees can be reconstructed from the diff.
@@ -491,6 +496,9 @@
 -- | Compute differences between two tries. The provided functions 
 -- determine the difference type for values in just the left or right
 -- or both. 
+--
+-- Note: this function is optimized for cases where tries are in the
+-- same VSpace and share many subtrees.
 diff :: (Eq a) => Trie a -> Trie a -> [(ByteString, Diff a)]
 diff = diffRoot where
     diffRoot a b = diffChild mempty (trie_root a) (trie_root b)
diff --git a/vcache-trie.cabal b/vcache-trie.cabal
--- a/vcache-trie.cabal
+++ b/vcache-trie.cabal
@@ -1,5 +1,5 @@
 Name: vcache-trie
-Version: 0.2.3
+Version: 0.2.4
 Synopsis: patricia tries modeled above VCache 
 Category: Database
 Description: