diff --git a/relational-join/src/Database/Relational/Query/Pi.hs b/relational-join/src/Database/Relational/Query/Pi.hs
index 1fa4ce74..e25413cf 100644
--- a/relational-join/src/Database/Relational/Query/Pi.hs
+++ b/relational-join/src/Database/Relational/Query/Pi.hs
@@ -1,12 +1,26 @@
+-- |
+-- Module      : Database.Relational.Query.Pi
+-- Copyright   : 2013 Kei Hibino
+-- License     : BSD3
+--
+-- Maintainer  : ex8k.hibino@gmail.com
+-- Stability   : experimental
+-- Portability : unknown
+--
+-- This module defines typed projection path objects.
+-- Contains normal interfaces.
 module Database.Relational.Query.Pi (
-  PiUnit,
-  pairPiFstUnit, pairPiSndUnit,
-
-  fst'', snd'',
-  fst', snd',
-
+  -- * Projection path
   Pi((:*)),
   leafIndex,
+
+  fst', snd',
+
+  -- * Projection path unit
+  PiUnit, offset,
+  pairPiFstUnit, pairPiSndUnit,
+
+  fst'', snd''
   ) where
 
 import Database.Record
@@ -18,30 +32,38 @@ import Database.Relational.Query.Pi.Unsafe
   (PiUnit, offset, Pi ((:*), Leaf), definePiUnit)
 
 
+-- | Get index of flat SQL value list from typed projection path.
 leafIndex :: Pi r f -> Int
 leafIndex =  rec  where
   rec :: Pi r f -> Int
   rec (Leaf pi0) = offset pi0
   rec (pi0 :* x) = offset pi0 + rec x
 
+-- | Projection path unit like fst of tuple.
 pairPiFstUnit :: PiUnit (c a b) a
 pairPiFstUnit =  definePiUnit 0
 
+-- | Devivation rule of projection path unit like snd of tuple.
 pairPiSndUnit' :: PersistableRecordWidth a -> PiUnit (c a b) b
 pairPiSndUnit' pw = definePiUnit (runPersistableRecordWidth pw)
 
+-- | Devivated projection path unit like snd of tuple.
 pairPiSndUnit :: PersistableWidth a => PiUnit (c a b) b
 pairPiSndUnit =  pairPiSndUnit' persistableWidth
 
 
+-- | Projection path unit for fst of tuple.
 fst'' :: PiUnit (a, b) a
 fst'' =  pairPiFstUnit
 
+-- | Projection path unit for snd of tuple.
 snd'' :: PersistableWidth a =>  PiUnit (a, b) b
 snd'' =  pairPiSndUnit
 
+-- | Projection path for fst of tuple.
 fst' :: Pi (a, b) a
 fst' =  Leaf fst''
 
+-- | Projection path for snd of tuple.
 snd' :: PersistableWidth a =>  Pi (a, b) b
 snd' =  Leaf snd''
diff --git a/relational-join/src/Database/Relational/Query/Pi/Unsafe.hs b/relational-join/src/Database/Relational/Query/Pi/Unsafe.hs
index cc476772..dba9cce0 100644
--- a/relational-join/src/Database/Relational/Query/Pi/Unsafe.hs
+++ b/relational-join/src/Database/Relational/Query/Pi/Unsafe.hs
@@ -1,30 +1,50 @@
 {-# LANGUAGE ExistentialQuantification #-}
 
+-- |
+-- Module      : Database.Relational.Query.Pi.Unsafe
+-- Copyright   : 2013 Kei Hibino
+-- License     : BSD3
+--
+-- Maintainer  : ex8k.hibino@gmail.com
+-- Stability   : experimental
+-- Portability : unknown
+--
+-- This module defines typed projection path objects.
+-- Contains internal structure and unsafe interfaces.
 module Database.Relational.Query.Pi.Unsafe (
+  -- * Projection path unit
   PiUnit, offset, definePiUnit,
 
+  -- * Projection path
   Pi ((:*), Leaf),
 
   defineColumn
   ) where
 
-newtype PiUnit r ft = PiUnit Int
--- data PiUnit r ft = PiUnit
+-- | Projection path unit from record type 'r' into column type 'ct'.
+newtype PiUnit r ct = PiUnit Int
+-- data PiUnit r ct = PiUnit
 --                { offset  :: Int
---                , column  :: Column r ft
+--                , column  :: Column r ct
 --                }
 
-offset :: PiUnit r ft -> Int
+-- | Get index of flat SQL value list from typed projection path unit.
+offset :: PiUnit r ct -> Int
 offset (PiUnit off) = off
 
-data Pi r ft = forall r' . PiUnit r r' :* Pi r' ft
-             |             Leaf (PiUnit r ft)
+-- | Projection path from record type 'r' into column type 'ct'.
+data Pi r ct = forall r' . PiUnit r r' :* Pi r' ct
+             |             Leaf (PiUnit r ct)
 
 infixr 9 :*
 
 
-defineColumn :: Int -> Pi r ft
+-- | Unsafely define projection path from record type 'r' into column type 'ct'.
+defineColumn :: Int     -- ^ Index of flat SQL value list
+             -> Pi r ct -- ^ Result projection path
 defineColumn =  Leaf . PiUnit
 
-definePiUnit :: Int -> PiUnit r ft
+-- | Unsafely define projection path unit from record type 'r' into column type 'ct'.
+definePiUnit :: Int         -- ^ Index of flat SQL value list
+             -> PiUnit r ct -- ^ Result projection path
 definePiUnit =  PiUnit