diff --git a/Mapping.cabal b/Mapping.cabal
--- a/Mapping.cabal
+++ b/Mapping.cabal
@@ -2,7 +2,7 @@
 -- documentation, see http://haskell.org/cabal/users-guide/
 
 name:                Mapping
-version:             0.1.0.0
+version:             0.1.0.1
 synopsis:            Mapping
 -- description:         
 homepage:            https://github.com/zaoqi/Mapping.hs
diff --git a/Mapping.hs b/Mapping.hs
--- a/Mapping.hs
+++ b/Mapping.hs
@@ -25,7 +25,7 @@
 
 type List a = [a]
 
-data Mapping k v = MappingNil | MappingNode (Mapping k v) k v (Mapping k v)
+data Mapping k v = MappingNil | MappingNode (Mapping k v) k v (Mapping k v) deriving Show
 
 mappingRef :: Ord k => Mapping k v -> k -> v -> v
 mappingRef (MappingNode l mk v r) k d =