diff --git a/hw-fingertree.cabal b/hw-fingertree.cabal
--- a/hw-fingertree.cabal
+++ b/hw-fingertree.cabal
@@ -1,7 +1,7 @@
 cabal-version:  2.2
 
 name:           hw-fingertree
-version:        0.1.0.2
+version:        0.1.0.3
 synopsis:       Generic finger-tree structure, with example instances
 description:    A general sequence representation with arbitrary
                 annotations, for use as a base for implementations of
diff --git a/src/HaskellWorks/Data/FingerTree.hs b/src/HaskellWorks/Data/FingerTree.hs
--- a/src/HaskellWorks/Data/FingerTree.hs
+++ b/src/HaskellWorks/Data/FingerTree.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE CPP                    #-}
 {-# LANGUAGE DeriveAnyClass         #-}
+{-# LANGUAGE DeriveFunctor          #-}
 {-# LANGUAGE DeriveGeneric          #-}
 {-# LANGUAGE FlexibleInstances      #-}
 {-# LANGUAGE FunctionalDependencies #-}
@@ -128,7 +129,7 @@
   | Two a a
   | Three a a a
   | Four a a a a
-  deriving (Show, Generic, NFData)
+  deriving (Show, Generic, NFData, Functor)
 
 instance Foldable Digit where
   foldMap f (One a)        = f a