diff --git a/sql-words/sql-words.cabal b/sql-words/sql-words.cabal
index 6881adc7..2d191bfb 100644
--- a/sql-words/sql-words.cabal
+++ b/sql-words/sql-words.cabal
@@ -20,6 +20,9 @@ library
                         Language.SQL.Keyword
                         Language.SQL.Keyword.ConcatString
 
+  other-modules:
+                        Language.SQL.Keyword.Internal.Type
+
   build-depends:          base <5
   hs-source-dirs:       src
   ghc-options:          -Wall
diff --git a/sql-words/src/Language/SQL/Keyword/Internal/Type.hs b/sql-words/src/Language/SQL/Keyword/Internal/Type.hs
new file mode 100644
index 00000000..63ee2436
--- /dev/null
+++ b/sql-words/src/Language/SQL/Keyword/Internal/Type.hs
@@ -0,0 +1,139 @@
+-- |
+-- Module      : Language.SQL.Keyword.Internal.Type
+-- Copyright   : 2013 Kei Hibino
+-- License     : BSD3
+--
+-- Maintainer  : ex8k.hibino@gmail.com
+-- Stability   : experimental
+-- Portability : unknown
+--
+-- This module defines package internal types.
+module Language.SQL.Keyword.Internal.Type (
+  -- * SQL keyword type interface.
+  Keyword (..), word, wordShow,
+
+  -- * Low-level diff string interface.
+  fromDString, toDString,
+  DString, dString, showDString, isEmptyDString
+  ) where
+
+import Data.String (IsString(..))
+import Data.List (find)
+import Data.Monoid (Monoid (..), (<>))
+
+
+-- | Diff String type for low-cost concatination.
+newtype DString = DString (String -> String)
+
+-- | Make 'DString' from 'String'
+dString :: String -> DString
+dString =  DString . (++)
+
+-- | Show 'DString' into 'String'
+showDString :: DString -> String
+showDString (DString f) = f []
+
+-- | 'DString' is empty or not.
+isEmptyDString :: DString -> Bool
+isEmptyDString = null . showDString
+
+instance Show DString where
+  show = showDString
+
+instance Read DString where
+  readsPrec _ s = [(dString s, [])]
+
+instance Monoid DString where
+  mempty  = DString id
+  DString f `mappend` DString g = DString $ f . g
+
+dspace :: DString
+dspace =  dString " "
+
+-- | Type represent SQL keywords.
+data Keyword = SELECT | ALL | DISTINCT | ON
+             | GROUP | COUNT | SUM | AVG | MAX | MIN | EVERY | ANY | SOME
+             | ORDER | BY | ASC | DESC
+             | CUBE | ROLLUP | GROUPING | SETS | HAVING
+             | FOR
+
+             | LIMIT
+             | FETCH | FIRST | NEXT | ROW | ROWS | ONLY
+
+             | UNION | EXCEPT | INTERSECT
+
+             | DELETE | USING | RETURNING
+
+             | FROM | AS | WITH
+             | JOIN | INNER | LEFT | RIGHT | FULL | NATURAL | OUTER
+
+             | UPDATE | SET | DEFAULT
+
+             | WHERE
+
+             | INSERT | INTO | VALUES
+
+             | MERGE
+
+             | OVER | PARTITION
+             | DENSE_RANK | RANK | ROW_NUMBER
+             | PERCENT_RANK | CUME_DIST
+             | LAG | LEAD | FIRST_VALUE | LAST_VALUE
+
+             | CASE | END | WHEN | ELSE | THEN
+
+             | LIKE | SIMILAR
+             | AND | OR | NOT
+             | EXISTS
+
+             | IS | NULL | IN
+
+             | DATE | TIME | TIMESTAMP | INTERVAL
+
+             | Sequence DString
+             deriving (Read, Show)
+
+             {-
+                  | (:?)
+                  | (:+) | (:-) | (:*) | (:/)
+                  | OPEN | CLOSE
+             -}
+
+
+fromDString :: DString -> Keyword
+fromDString =  Sequence
+
+toDString :: Keyword -> DString
+toDString = d  where
+  d (Sequence ds) = ds
+  d  w            = dString $ show w
+
+-- | Make 'Keyword' from String
+word :: String -> Keyword
+word =  fromDString . dString
+
+-- | 'Keyword' type with OverloadedString extension,
+--   can be involved same list with string literals.
+--
+-- > selectFoo = [SELECT, "a, b, c", FROM, "foo"]
+--
+instance IsString Keyword where
+  fromString s' = found (find ((== "") . snd) (reads s')) s'  where
+   found  Nothing      s = word s
+   found (Just (w, _)) _ = w
+
+-- | 'Keyword' default concatination separate by space.
+instance Monoid Keyword where
+  mempty  = fromDString mempty
+  a `mappend` b = fromDString $ toDString a `append'` toDString b  where
+    append' p q
+      | isEmptyDString p = q
+      | isEmptyDString q = p
+      | otherwise        = p <> dspace <> q
+
+
+-- | Show 'Keyword'
+wordShow :: Keyword -> String
+wordShow =  d  where
+  d (Sequence s)   = showDString s
+  d w              = show w
diff --git a/sql-words/src/Language/SQL/Keyword/Type.hs b/sql-words/src/Language/SQL/Keyword/Type.hs
index a1c17c32..6e07d17c 100644
--- a/sql-words/src/Language/SQL/Keyword/Type.hs
+++ b/sql-words/src/Language/SQL/Keyword/Type.hs
@@ -19,123 +19,9 @@ module Language.SQL.Keyword.Type (
   stringMap
   ) where
 
-import Data.String (IsString(fromString))
-import Data.List (find)
-import Data.Monoid (Monoid (..), (<>))
+import Language.SQL.Keyword.Internal.Type (Keyword (..), word, wordShow)
 
 
-newtype DString = DString (String -> String)
-
-dString :: String -> DString
-dString =  DString . (++)
-
-showDString :: DString -> String
-showDString (DString f) = f []
-
-isEmptyDString :: DString -> Bool
-isEmptyDString = null . showDString
-
-instance Show DString where
-  show = showDString
-
-instance Read DString where
-  readsPrec _ s = [(dString s, [])]
-
-instance Monoid DString where
-  mempty  = DString id
-  DString f `mappend` DString g = DString $ f . g
-
-dspace :: DString
-dspace =  dString " "
-
--- | Type represent SQL keywords.
-data Keyword = SELECT | ALL | DISTINCT | ON
-             | GROUP | COUNT | SUM | AVG | MAX | MIN | EVERY | ANY | SOME
-             | ORDER | BY | ASC | DESC
-             | CUBE | ROLLUP | GROUPING | SETS | HAVING
-             | FOR
-
-             | LIMIT
-             | FETCH | FIRST | NEXT | ROW | ROWS | ONLY
-
-             | UNION | EXCEPT | INTERSECT
-
-             | DELETE | USING | RETURNING
-
-             | FROM | AS | WITH
-             | JOIN | INNER | LEFT | RIGHT | FULL | NATURAL | OUTER
-
-             | UPDATE | SET | DEFAULT
-
-             | WHERE
-
-             | INSERT | INTO | VALUES
-
-             | MERGE
-
-             | OVER | PARTITION
-             | DENSE_RANK | RANK | ROW_NUMBER
-             | PERCENT_RANK | CUME_DIST
-             | LAG | LEAD | FIRST_VALUE | LAST_VALUE
-
-             | CASE | END | WHEN | ELSE | THEN
-
-             | LIKE | SIMILAR
-             | AND | OR | NOT
-             | EXISTS
-
-             | IS | NULL | IN
-
-             | DATE | TIME | TIMESTAMP | INTERVAL
-
-             | Sequence DString
-             deriving (Read, Show)
-
-             {-
-                  | (:?)
-                  | (:+) | (:-) | (:*) | (:/)
-                  | OPEN | CLOSE
-             -}
-
-
-fromDString :: DString -> Keyword
-fromDString =  Sequence
-
-toDString :: Keyword -> DString
-toDString = d  where
-  d (Sequence ds) = ds
-  d  w            = dString $ show w
-
--- | Make 'Keyword' from String
-word :: String -> Keyword
-word =  fromDString . dString
-
--- | 'Keyword' type with OverloadedString extension,
---   can be involved same list with string literals.
---
--- > selectFoo = [SELECT, "a, b, c", FROM, "foo"]
---
-instance IsString Keyword where
-  fromString s' = found (find ((== "") . snd) (reads s')) s'  where
-   found  Nothing      s = word s
-   found (Just (w, _)) _ = w
-
--- | 'Keyword' default concatination separate by space.
-instance Monoid Keyword where
-  mempty  = fromDString mempty
-  a `mappend` b = fromDString $ toDString a `append'` toDString b  where
-    append' p q
-      | isEmptyDString p = q
-      | isEmptyDString q = p
-      | otherwise        = p <> dspace <> q
-
-
--- | Show 'Keyword'
-wordShow :: Keyword -> String
-wordShow =  d  where
-  d (Sequence s)   = showDString s
-  d w              = show w
-
 -- | Concatinate keywords into 'String' like unwords
 unwordsSQL :: [Keyword] -> String
 unwordsSQL =  unwords . map wordShow