diff --git a/src/Data/Params/Functor.hs b/src/Data/Params/Functor.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/Params/Functor.hs
@@ -0,0 +1,49 @@
+{-# LANGUAGE TemplateHaskell #-}
+
+module Data.Params.Functor
+    where
+
+import Control.Category
+import Prelude hiding ((.), id, Functor(..), Applicative(..))
+
+import Data.Maybe
+import Data.Params
+
+-------------------------------------------------------------------------------
+
+class Functor lens tb where
+  fmap' :: ( b ~ GetParam lens tb, ta ~ SetParam lens a tb )  
+      => TypeLens p lens 
+      -> (a -> b) -> ta -> tb
+
+fmap :: 
+  ( Functor lens tb
+  , b ~ GetParam lens tb
+  , ta ~ SetParam lens a tb
+  ) => TypeLens Base lens
+    -> (a -> b) -> ta -> tb
+fmap lens = fmap' (lens._base)
+
+-------------------
+-- Either
+
+mkParams ''Either
+
+instance Functor p b => Functor (Param_b p) (Either a b) where
+  fmap' lens f (Left a) = Left a
+  fmap' lens f (Right b) = Right $ fmap' (zoom lens) f b
+
+instance Functor p a => Functor (Param_a p) (Either a b) where
+  fmap' lens f (Left a) = Left $ fmap' (zoom lens) f a
+  fmap' lens f (Right b) = Right b
+
+instance Functor Base t where
+  fmap' _ f a = f a
+
+-------------------
+-- Maybe
+
+mkParams ''Maybe
+instance Functor p a => Functor (Param_a p) (Maybe a) where
+  fmap' lens f Nothing = Nothing
+  fmap' lens f (Just a) = Just $ fmap' (zoom lens) f a
diff --git a/typeparams.cabal b/typeparams.cabal
--- a/typeparams.cabal
+++ b/typeparams.cabal
@@ -1,5 +1,5 @@
 Name:                typeparams
-Version:             0.0.3.0
+Version:             0.0.3.1
 Synopsis:            Lens-like interface for type level parameters; allows unboxed unboxed vectors and supercompilation
 Description:         
     This library provides a lens-like interface for working with type parameters. In the code:
@@ -46,7 +46,7 @@
         Data.Params
         Data.Params.Frac
         Data.Params.PseudoPrim
-        -- Data.Params.Instances
+        Data.Params.Functor
         Data.Params.Vector
         -- Data.Params.Vector.Storable
         -- Data.Params.Vector.StorableRaw