diff --git a/Data/Type/Natural.hs b/Data/Type/Natural.hs
--- a/Data/Type/Natural.hs
+++ b/Data/Type/Natural.hs
@@ -45,11 +45,13 @@
                     , sN15, sN16, sN17, sN18, sN19, sN20
                     ) where
 import           Data.Singletons
-import           Prelude          (Bool (..), Eq (..), Integral (..), Ord ((<)),
+import           Data.Type.Monomorphic
+import           Prelude          (Int, Bool (..), Eq (..), Integral (..), Ord ((<)),
                                    Show (..), error, id, otherwise, ($), (.), undefined)
 import qualified Prelude          as P
 import           Proof.Equational
 
+
 --------------------------------------------------
 -- * Natural numbers and its singleton type
 --------------------------------------------------
@@ -627,3 +629,11 @@
 sNatToInt :: P.Num n => SNat x -> n
 sNatToInt SZ     = 0
 sNatToInt (SS n) = sNatToInt n P.+ 1
+
+instance Monomorphicable (Sing :: Nat -> *) where
+  type MonomorphicRep (Sing :: Nat -> *) = Int
+  demote  (Monomorphic sn) = sNatToInt sn
+  promote n
+      | n < 0     = error "negative integer!"
+      | n == 0    = Monomorphic sZ
+      | otherwise = withPolymorhic (n P.- 1) $ \sn -> Monomorphic $ sS sn
diff --git a/type-natural.cabal b/type-natural.cabal
--- a/type-natural.cabal
+++ b/type-natural.cabal
@@ -2,7 +2,7 @@
 -- documentation, see http://haskell.org/cabal/users-guide/
 
 name:                type-natural
-version:             0.0.1.1
+version:             0.0.2.0
 synopsis:            Type-level natural and proofs of their properties.
 description:         Type-level natural numbers and proofs of their properties.
 homepage:            https://github.com/konn/type-natural
@@ -22,6 +22,7 @@
 library
   exposed-modules:     Data.Type.Natural
   -- other-modules:       
-  build-depends:       base                     ==4.6.*
-               ,       singletons               ==0.8.*
+  build-depends:       base                     == 4.6.*
+               ,       singletons               == 0.8.*
                ,       equational-reasoning     == 0.0.*
+               ,       monomorphic              >= 0.0.3