diff --git a/DB-record/src/Database/Record/Singleton.hs b/DB-record/src/Database/Record/Singleton.hs
index 6e90b2c4..ad8d21b7 100644
--- a/DB-record/src/Database/Record/Singleton.hs
+++ b/DB-record/src/Database/Record/Singleton.hs
@@ -4,7 +4,20 @@
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE UndecidableInstances #-}
 
+-- |
+-- Module      : Database.Record.Singleton
+-- Copyright   : 2013 Kei Hibino
+-- License     : BSD3
+--
+-- Maintainer  : ex8k.hibino@gmail.com
+-- Stability   : experimental
+-- Portability : unknown
+--
+-- This module defines persistable instances for
+-- general single field record.
 module Database.Record.Singleton (
+  -- * Singleton type constructor
+  -- $singletonTypeConstructor
   Singleton, runSingleton, singleton,
 
   singletonWidth,
@@ -23,28 +36,39 @@ import Database.Record.FromSql (FromSql(recordFromSql), recordFromSql')
 import Database.Record.ToSql (ToSql(recordToSql), recordToSql')
 
 
--- | Singleton value record.
+{- $singletonTypeConstructor
+Singleton type constructor is useful to avoid overlapped instances.
+-}
+
+-- | General single field record.
 newtype Singleton a = Singleton { runSingleton :: a }
 
+-- | Wrap into 'Singleton'.
 singleton :: a -> Singleton a
 singleton = Singleton
 
+-- | Axiom of 'PersistableRecordWidth' proof object. Single field recrod width is 1.
 singletonWidth :: PersistableRecordWidth (Singleton a)
 singletonWidth =  persistableRecordWidth 1
 
+-- | Derived 'PersistableRecordWidth' proof object for 'Singleton'.
 instance PersistableWidth (Singleton a) where
   persistableWidth = singletonWidth
 
+-- | Derivation rule of 'PersistableRecord' for 'Singleton'.
 persistableSingletonFromValue :: PersistableRecordWidth (Singleton a) -> PersistableSqlValue q a -> PersistableRecord q (Singleton a)
 persistableSingletonFromValue pw pv =
   persistableRecord pw (singleton . toValue pv . head) ((:[]) . fromValue pv . runSingleton)
 
+-- | Derived 'PersistableRecord' of 'Singleton'
 derivedPersistableSingleton :: (PersistableWidth (Singleton a), PersistableValue q a) => PersistableRecord q (Singleton a)
 derivedPersistableSingleton =  persistableSingletonFromValue persistableWidth persistableValue
 
 
+-- | Inference rule for 'RecordFromSql'.
 instance Persistable q (Singleton a) => FromSql q (Singleton a)  where
   recordFromSql = recordFromSql'
 
+-- | Inference rule for 'RecordToSql'.
 instance Persistable q (Singleton a) => ToSql q (Singleton a) where
   recordToSql = recordToSql'