diff --git a/pointless-haskell.cabal b/pointless-haskell.cabal
--- a/pointless-haskell.cabal
+++ b/pointless-haskell.cabal
@@ -1,5 +1,5 @@
 Name:            pointless-haskell
-Version:         0.0.7
+Version:         0.0.8
 License:         BSD3
 License-file:    LICENSE
 Author:          Alcino Cunha <alcino@di.uminho.pt>, Hugo Pacheco <hpacheco@di.uminho.pt>
@@ -23,7 +23,7 @@
 
 Library
   Hs-Source-Dirs: src
-  Build-Depends:        base >= 3 && < 5, GHood, haskell98, process, syb >= 0.1.0.2
+  Build-Depends:        base >= 3 && < 5, GHood, process, syb >= 0.1.0.2
   exposed-modules:
         Generics.Pointless.Combinators
         Generics.Pointless.Functors,
diff --git a/src/Generics/Pointless/Functors.hs b/src/Generics/Pointless/Functors.hs
--- a/src/Generics/Pointless/Functors.hs
+++ b/src/Generics/Pointless/Functors.hs
@@ -316,16 +316,20 @@
 
 -- ** Natural Numbers
 
-data Nat = Zero | Succ Nat deriving (Eq,Show,Read,G.Typeable,G.Data)
+-- Implemented as "boxed" integers for efficiency
+data Nat = Nat Int deriving (Eq,Show,Read,G.Typeable,G.Data)
 
 type instance PF Nat = Const One :+: Id
 
 instance Mu Nat where
-    inn (Left _) = Zero
-    inn (Right n) = Succ n
-    out Zero = Left _L
-    out (Succ n) = Right n
+    inn (Left _) = Nat 0
+    inn (Right (Nat n)) = Nat (succ n)
+    out (Nat 0) = Left _L
+    out (Nat n) = if n < 1 then error "negative natural not expected" else Right (Nat $ pred n)
 
+nzero = Nat 0
+nsucc (Nat n) = Nat (succ n)
+
 -- ** Int (positive only)
 
 type instance PF Int = Const One :+: Id
@@ -343,12 +347,10 @@
 suck = inn . inr
 
 natInt :: Nat -> Int
-natInt Zero = 0
-natInt (Succ n) = succ (natInt n)
+natInt (Nat n) = n
 
 intNat :: Int -> Nat
-intNat 0 = Zero
-intNat n = Succ (intNat $ pred n)
+intNat n = if n < 0 then error "negative natural not expected" else Nat n
 
 -- ** Bool