diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+0.4.8
+-------------------------------------------------
+* Changed the `FromJSON` instance for `Record` to call `parseJSON Null` for missing fields
+* Added `FromJSON` and `ToJSON` instances for `Nullable (Field h) :* xs`
+
 0.4.7.2
 -------------------------------------------------
 * Added cassava's `ToNamedRecord`, `ToRecord`, `FromNamedRecord` and `FromRecord` instances
diff --git a/examples/eff.hs b/examples/eff.hs
new file mode 100644
--- /dev/null
+++ b/examples/eff.hs
@@ -0,0 +1,23 @@
+{-# LANGUAGE TypeApplications, OverloadedLabels, PolyKinds, DataKinds, ScopedTypeVariables, Rank2Types #-}
+import Data.Extensible
+
+import Prelude hiding (readFile)
+import Control.Exception
+import Data.ByteString (ByteString)
+
+data FileSystem r where
+  ReadFile
+    :: FilePath
+    -> FileSystem (Either SomeException ByteString)
+
+readFile
+  :: forall xs
+  . (Associate "fs" FileSystem xs
+  , Associate "fs_error" (EitherEff SomeException) xs)
+  => FilePath
+  -> Eff xs ByteString
+readFile fp = liftEff #fs (ReadFile fp)
+           >>= either (throwEff #fs_error) pure
+
+foo :: forall xs. Associate "fs" FileSystem xs => Eff xs (Either SomeException ByteString)
+foo = castEff (runEitherEff @ "fs_error" (readFile "foo") :: Eff '["fs" >: FileSystem] (Either SomeException ByteString))
diff --git a/examples/effect.hs b/examples/effect.hs
--- a/examples/effect.hs
+++ b/examples/effect.hs
@@ -1,4 +1,5 @@
-{-# LANGUAGE TemplateHaskell, DataKinds, FlexibleContexts #-}
+{-# LANGUAGE TemplateHaskell, DataKinds, TypeOperators, PolyKinds, FlexibleContexts #-}
+{-# OPTIONS_GHC -fno-warn-simplifiable-class-constraints#-}
 import Data.Extensible
 import Control.Monad.IO.Class
 import Control.Monad.Trans.Writer.Strict
diff --git a/examples/getopt.hs b/examples/getopt.hs
--- a/examples/getopt.hs
+++ b/examples/getopt.hs
@@ -4,7 +4,7 @@
 import Data.Extensible.GetOpt
 
 main :: IO ()
-main = withGetOpt opts $ \r _args -> do
+main = withGetOpt "" opts $ \r _args -> do
   putStrLn $ "verbose: " ++ show (r ^. #verbose > 0)
   putStrLn $ "extra: " ++ show (r ^? #extra. folded)
   where
diff --git a/examples/state.hs b/examples/state.hs
--- a/examples/state.hs
+++ b/examples/state.hs
@@ -1,15 +1,15 @@
 {-# LANGUAGE TypeOperators, DataKinds, TemplateHaskell, FlexibleContexts #-}
 import Data.Extensible
 import Control.Lens
-import Control.Monad.State
+import Control.Monad.State.Strict
 
 mkField "foo bar baz"
 
-statefulStuff :: State (Record '["foo" :> Int, "bar" :> Int, "baz" :> Float]) ()
+statefulStuff :: State (Record '["foo" >: Int, "bar" >: Int, "baz" >: Float]) ()
 statefulStuff = do
     v <- use foo
     bar += v
     baz .= 42
 
 main = print $ execState statefulStuff
-  $ foo @= 10 <: bar @= 0 <: baz @= 0 <: Nil
+  $ foo @= 10 <: bar @= 0 <: baz @= 0 <: nil
diff --git a/examples/tangle.hs b/examples/tangle.hs
--- a/examples/tangle.hs
+++ b/examples/tangle.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE TemplateHaskell, DataKinds #-}
+{-# LANGUAGE TypeOperators, PolyKinds, FlexibleContexts, FlexibleInstances, TemplateHaskell, DataKinds #-}
 import Control.Monad.Trans.Class
 import Data.Extensible
 import Data.Functor.Identity
@@ -30,4 +30,4 @@
 makeRec = runTangles
   (htabulateFor (Proxy :: Proxy MakeRec)
     $ \m -> Comp $ Field . pure <$> make m)
-  (wrench Nil)
+  (wrench nil)
diff --git a/extensible.cabal b/extensible.cabal
--- a/extensible.cabal
+++ b/extensible.cabal
@@ -1,5 +1,5 @@
 name:                extensible
-version:             0.4.7.2
+version:             0.4.8
 synopsis:            Extensible, efficient, optics-friendly data types and effects
 homepage:            https://github.com/fumieval/extensible
 bug-reports:         http://github.com/fumieval/extensible/issues
diff --git a/src/Data/Extensible.hs b/src/Data/Extensible.hs
--- a/src/Data/Extensible.hs
+++ b/src/Data/Extensible.hs
@@ -1,7 +1,7 @@
 ----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
diff --git a/src/Data/Extensible/Bits.hs b/src/Data/Extensible/Bits.hs
--- a/src/Data/Extensible/Bits.hs
+++ b/src/Data/Extensible/Bits.hs
@@ -1,6 +1,15 @@
 {-# LANGUAGE UndecidableInstances, ScopedTypeVariables, MultiParamTypeClasses, TypeFamilies #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving, DeriveGeneric #-}
-
+-----------------------------------------------------------------------
+-- |
+-- Module      :  Data.Extensible.Bits
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
+-- License     :  BSD3
+--
+-- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
+--
+-- Bit-packed records
+-----------------------------------------------------------------------
 module Data.Extensible.Bits (BitProd(..)
   , FromBits(..)
   , TotalBits
@@ -40,6 +49,7 @@
   showsPrec d x = showParen (d > 10)
     $ showString "toBitProd " . showsPrec 11 (fromBitProd x)
 
+-- | Total 'BitWidth'
 type family TotalBits h xs where
   TotalBits h '[] = 0
   TotalBits h (x ': xs) = BitWidth (h x) + TotalBits h xs
@@ -187,7 +197,10 @@
 proxyBitWidth :: Proxy h -> proxy x -> Proxy (BitWidth (h x))
 proxyBitWidth _ _ = Proxy
 
+-- | Bit-packed record
 type BitRecordOf r h = BitProd r (Field h)
+
+-- | Bit-packed record
 type BitRecord r = BitRecordOf r Identity
 
 instance (Corepresentable p, Comonad (Corep p), Functor f) => Extensible f p (BitProd r) where
diff --git a/src/Data/Extensible/Class.hs b/src/Data/Extensible/Class.hs
--- a/src/Data/Extensible/Class.hs
+++ b/src/Data/Extensible/Class.hs
@@ -5,7 +5,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Class
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -22,6 +22,7 @@
   -- * Membership
   , Membership
   , mkMembership
+  , getMemberId
   , compareMembership
   , leadership
   -- * Member
@@ -138,6 +139,7 @@
 
   hgenerateListFor p f = HCons <$> f here <*> hgenerateListFor p (f . navNext)
 
+-- | HACK: Without this, the constraints are not propagated well.
 type family ForallF (c :: k -> Constraint) (xs :: [k]) :: Constraint where
   ForallF c '[] = ()
   ForallF c (x ': xs) = (c x, Forall c xs)
diff --git a/src/Data/Extensible/Dictionary.hs b/src/Data/Extensible/Dictionary.hs
--- a/src/Data/Extensible/Dictionary.hs
+++ b/src/Data/Extensible/Dictionary.hs
@@ -3,12 +3,11 @@
 #if __GLASGOW_HASKELL__ >= 800
 {-# LANGUAGE UndecidableSuperClasses #-}
 #endif
-
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 -----------------------------------------------------------------------
---
+-- |
 -- Module      :  Data.Extensible.Dictionary
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -27,6 +26,7 @@
 import Data.Extensible.Sum
 import Data.Extensible.Internal
 import Data.Extensible.Internal.Rig
+import Data.Extensible.Nullable
 import Data.Constraint
 import Data.Extensible.Struct
 import Data.Extensible.Wrapper
@@ -170,17 +170,29 @@
     (\k m v -> HM.insert (BC.pack (symbolVal (proxyAssocKey k))) (Csv.toField v) m)
     HM.empty
 
+-- | @'parseJSON' 'J.Null'@ is called for missing fields.
 instance Forall (KeyValue KnownSymbol (Instance1 J.FromJSON h)) xs => J.FromJSON (Field h :* xs) where
   parseJSON = J.withObject "Object" $ \v -> hgenerateFor
     (Proxy :: Proxy (KeyValue KnownSymbol (Instance1 J.FromJSON h)))
-    $ \m -> let k = symbolVal (proxyAssocKey m) in case HM.lookup (T.pack k) v of
-      Just a -> Field <$> J.parseJSON a
-      Nothing -> fail $ "Missing key: " ++ k
+    $ \m -> let k = symbolVal (proxyAssocKey m)
+      in fmap Field $ J.parseJSON $ maybe J.Null id $ HM.lookup (T.pack k) v
 
 instance Forall (KeyValue KnownSymbol (Instance1 J.ToJSON h)) xs => J.ToJSON (Field h :* xs) where
   toJSON = J.Object . hfoldlWithIndexFor
     (Proxy :: Proxy (KeyValue KnownSymbol (Instance1 J.ToJSON h)))
     (\k m v -> HM.insert (T.pack (symbolVal (proxyAssocKey k))) (J.toJSON v) m)
+    HM.empty
+
+instance Forall (KeyValue KnownSymbol (Instance1 J.FromJSON h)) xs => J.FromJSON (Nullable (Field h) :* xs) where
+  parseJSON = J.withObject "Object" $ \v -> hgenerateFor
+    (Proxy :: Proxy (KeyValue KnownSymbol (Instance1 J.FromJSON h)))
+    $ \m -> let k = symbolVal (proxyAssocKey m)
+      in fmap Nullable $ traverse J.parseJSON $ HM.lookup (T.pack k) v
+
+instance Forall (KeyValue KnownSymbol (Instance1 J.ToJSON h)) xs => J.ToJSON (Nullable (Field h) :* xs) where
+  toJSON = J.Object . hfoldlWithIndexFor
+    (Proxy :: Proxy (KeyValue KnownSymbol (Instance1 J.ToJSON h)))
+    (\k m (Nullable v) -> maybe id (HM.insert (T.pack $ symbolVal $ proxyAssocKey k) . J.toJSON) v m)
     HM.empty
 
 instance WrapForall Show h xs => Show (h :| xs) where
diff --git a/src/Data/Extensible/Effect.hs b/src/Data/Extensible/Effect.hs
--- a/src/Data/Extensible/Effect.hs
+++ b/src/Data/Extensible/Effect.hs
@@ -4,7 +4,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Effect
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
diff --git a/src/Data/Extensible/Effect/Default.hs b/src/Data/Extensible/Effect/Default.hs
--- a/src/Data/Extensible/Effect/Default.hs
+++ b/src/Data/Extensible/Effect/Default.hs
@@ -5,7 +5,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Effect.Default
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -119,6 +119,7 @@
 execWriterDef = execWriterEff
 {-# INLINE execWriterDef #-}
 
+-- | Same as @'EitherDef' ()@
 type MaybeDef = "Either" >: EitherEff ()
 
 -- | Similar to 'runMaybeT', but on 'Eff'
@@ -126,6 +127,7 @@
 runMaybeDef = runMaybeEff
 {-# INLINE runMaybeDef #-}
 
+-- | mtl-compatible either effect
 type EitherDef e = "Either" >: EitherEff e
 
 -- | Similar to 'runExceptT', but on 'Eff'
diff --git a/src/Data/Extensible/Field.hs b/src/Data/Extensible/Field.hs
--- a/src/Data/Extensible/Field.hs
+++ b/src/Data/Extensible/Field.hs
@@ -8,7 +8,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Field
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -98,12 +98,12 @@
 
 instance (pk k, pv v) => KeyValue pk pv (k ':> v)
 
--- | Combined constraint for 'Assoc'
+-- | Constraint applied to 'AssocKey'
 class (pk (AssocKey kv)) => KeyIs pk kv where
 
 instance (pk k) => KeyIs pk (k ':> v)
 
--- | Combined constraint for 'Assoc'
+-- | Constraint applied to 'AssocValue'
 class (pv (AssocValue kv)) => ValueIs pv kv where
 
 instance (pv v) => ValueIs pv (k ':> v)
diff --git a/src/Data/Extensible/GetOpt.hs b/src/Data/Extensible/GetOpt.hs
--- a/src/Data/Extensible/GetOpt.hs
+++ b/src/Data/Extensible/GetOpt.hs
@@ -2,7 +2,7 @@
 --------------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.GetOpt
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -56,6 +56,7 @@
 extendArg f (ReqArg a ph) = ReqArg (\s -> f (Just s) (a s)) ph
 extendArg f (OptArg a ph) = OptArg (f <*> a) ph
 
+-- | Simple option descriptor
 type OptDescr' = OptionDescr Identity
 
 instance Wrapper (OptionDescr h) where
@@ -76,6 +77,7 @@
     -> OptDescr' Bool
 optFlag = optionNoArg (pure . (>0))
 
+-- | Wrapper-generic version of `optNoArg`
 optionNoArg :: (Int -> h a) -> [Char] -> [String] -> String -> OptionDescr h a
 optionNoArg f ss ls expl = OptionDescr f 0 $ Option ss ls (NoArg (+1)) expl
 
@@ -95,9 +97,11 @@
     -> OptDescr' (Maybe String)
 optLastArg ss ls ph expl = OptionDescr pure Nothing $ Option ss ls (ReqArg (const . Just) ph) expl
 
+-- | Wrapper-generic version of `optReqArg`
 optionReqArg :: ([String] -> h a) -> [Char] -> [String] -> String -> String -> OptionDescr h a
 optionReqArg f ss ls ph expl = OptionDescr f [] $ Option ss ls (ReqArg (:) ph) expl
 
+-- | Construct an option with an optional argument
 optionOptArg :: ([Maybe String] -> h a) -> [Char] -> [String] -> String -> String -> OptionDescr h a
 optionOptArg f ss ls ph expl = OptionDescr f [] $ Option ss ls (OptArg (:) ph) expl
 
diff --git a/src/Data/Extensible/HList.hs b/src/Data/Extensible/HList.hs
--- a/src/Data/Extensible/HList.hs
+++ b/src/Data/Extensible/HList.hs
@@ -3,7 +3,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.HList
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
diff --git a/src/Data/Extensible/Inclusion.hs b/src/Data/Extensible/Inclusion.hs
--- a/src/Data/Extensible/Inclusion.hs
+++ b/src/Data/Extensible/Inclusion.hs
@@ -8,7 +8,7 @@
 ------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Inclusion
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -28,6 +28,7 @@
   -- * Key-value
   , IncludeAssoc
   , Associated
+  , Associated'
   , inclusionAssoc
   , shrinkAssoc
   , spreadAssoc
diff --git a/src/Data/Extensible/Internal.hs b/src/Data/Extensible/Internal.hs
--- a/src/Data/Extensible/Internal.hs
+++ b/src/Data/Extensible/Internal.hs
@@ -8,7 +8,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Inclusion
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -80,7 +80,9 @@
     $ conT ''Membership `appT` pure t `appT` varT (names !! n)
 
 -- | The position of @x@ in the type level set @xs@.
-newtype Membership (xs :: [k]) (x :: k) = Membership { getMemberId :: Int } deriving (Typeable, NFData)
+newtype Membership (xs :: [k]) (x :: k) = Membership
+  { getMemberId :: Int -- ^ get the position as an 'Int'.
+  } deriving (Typeable, NFData)
 
 newtype Remembrance xs x r = Remembrance (Member xs x => r)
 
@@ -100,6 +102,7 @@
 instance Hashable (Membership xs x) where
   hashWithSalt s = hashWithSalt s . getMemberId
 
+-- | Make up a 'Membership' from an integer.
 reifyMembership :: Int -> (forall x. Membership xs x -> r) -> r
 reifyMembership n k = k (Membership n)
 
@@ -121,11 +124,13 @@
 -- | A readable type search result
 data Elaborated k v = Expecting v | Missing k | Duplicate k
 
+-- | Make the result more readable
 type family Elaborate (key :: k) (xs :: [v]) :: Elaborated k v where
   Elaborate k '[] = 'Missing k
   Elaborate k '[x] = 'Expecting x
   Elaborate k xs = 'Duplicate k
 
+-- | Find a type associated to the specified key.
 type family FindAssoc (n :: Nat) (key :: k) (xs :: [Assoc k v]) where
   FindAssoc n k ((k ':> v) ': xs) = (n ':> v) ': FindAssoc (1 + n) k xs
   FindAssoc n k ((k' ':> v) ': xs) = FindAssoc (1 + n) k xs
@@ -173,6 +178,7 @@
 -- | Unicode flipped alias for 'Member'
 type x ∈ xs = Member xs x
 
+-- | First element
 type family Head (xs :: [k]) :: k where
   Head (x ': xs) = x
 
@@ -182,6 +188,7 @@
   FindType x (y ': ys) = MapSucc (FindType x ys)
   FindType x '[] = '[]
 
+-- | Last element
 type family Last (x :: [k]) :: k where
   Last '[x] = x
   Last (x ': xs) = Last xs
diff --git a/src/Data/Extensible/Internal/Rig.hs b/src/Data/Extensible/Internal/Rig.hs
--- a/src/Data/Extensible/Internal/Rig.hs
+++ b/src/Data/Extensible/Internal/Rig.hs
@@ -2,7 +2,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Internal.Rig
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -44,17 +44,20 @@
 over = coerce
 {-# INLINE over #-}
 
+-- | Reifies the structure of 'Iso's
 data Exchange a b s t = Exchange (s -> a) (b -> t)
 
 instance Profunctor (Exchange a b) where
   dimap f g (Exchange sa bt) = Exchange (sa . f) (g . bt)
   {-# INLINE dimap #-}
 
+-- | Recover tho functions from an Iso/
 withIso :: Optic (Exchange a b) Identity s t a b -> ((s -> a) -> (b -> t) -> r) -> r
 withIso l r = case l (Exchange id Identity) of
   Exchange f g -> r f (coerce g)
 {-# INLINE withIso #-}
 
+-- | @'review' :: AReview s a -> a -> s@
 review :: Optic' Tagged Identity s a -> a -> s
 review = coerce
 {-# INLINE review #-}
diff --git a/src/Data/Extensible/Label.hs b/src/Data/Extensible/Label.hs
--- a/src/Data/Extensible/Label.hs
+++ b/src/Data/Extensible/Label.hs
@@ -3,7 +3,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Label
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
diff --git a/src/Data/Extensible/Match.hs b/src/Data/Extensible/Match.hs
--- a/src/Data/Extensible/Match.hs
+++ b/src/Data/Extensible/Match.hs
@@ -2,7 +2,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.League
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
diff --git a/src/Data/Extensible/Nullable.hs b/src/Data/Extensible/Nullable.hs
--- a/src/Data/Extensible/Nullable.hs
+++ b/src/Data/Extensible/Nullable.hs
@@ -2,7 +2,7 @@
 ------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Nullable
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
diff --git a/src/Data/Extensible/Plain.hs b/src/Data/Extensible/Plain.hs
--- a/src/Data/Extensible/Plain.hs
+++ b/src/Data/Extensible/Plain.hs
@@ -3,7 +3,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Plain
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
diff --git a/src/Data/Extensible/Product.hs b/src/Data/Extensible/Product.hs
--- a/src/Data/Extensible/Product.hs
+++ b/src/Data/Extensible/Product.hs
@@ -5,7 +5,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Product
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -136,6 +136,7 @@
 hfoldMapWithIndex f = hfoldrWithIndex (\i -> mappend . f i) mempty
 {-# INLINE hfoldMapWithIndex #-}
 
+-- | Perform a strict left fold over the elements.
 hfoldlWithIndex :: (forall x. Membership xs x -> r -> h x -> r) -> r -> h :* xs -> r
 hfoldlWithIndex f r xs = hfoldrWithIndex (\i x c a -> c $! f i a x) id xs r
 {-# INLINE hfoldlWithIndex #-}
@@ -146,6 +147,7 @@
 hfoldrWithIndexFor p f r xs = henumerateFor p xs (\i -> f i (hlookup i xs)) r
 {-# INLINE hfoldrWithIndexFor #-}
 
+-- | Constrained 'hfoldlWithIndex'
 hfoldlWithIndexFor :: (Forall c xs) => proxy c
   -> (forall x. c x => Membership xs x -> r -> h x -> r) -> r -> h :* xs -> r
 hfoldlWithIndexFor p f r xs = hfoldrWithIndexFor p (\i x c a -> c $! f i a x) id xs r
@@ -157,6 +159,7 @@
 hfoldMapWithIndexFor p f = hfoldrWithIndexFor p (\i -> mappend . f i) mempty
 {-# INLINE hfoldMapWithIndexFor #-}
 
+-- | Constrained 'hfoldMap'
 hfoldMapFor :: (Forall c xs, Monoid a) => proxy c
   -> (forall x. c x => h x -> a) -> h :* xs -> a
 hfoldMapFor p f = hfoldMapWithIndexFor p (const f)
diff --git a/src/Data/Extensible/Record.hs b/src/Data/Extensible/Record.hs
--- a/src/Data/Extensible/Record.hs
+++ b/src/Data/Extensible/Record.hs
@@ -2,7 +2,7 @@
 ------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Record
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
diff --git a/src/Data/Extensible/Struct.hs b/src/Data/Extensible/Struct.hs
--- a/src/Data/Extensible/Struct.hs
+++ b/src/Data/Extensible/Struct.hs
@@ -7,7 +7,7 @@
 ------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Struct
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -78,6 +78,7 @@
 get (Struct m) (getMemberId -> I# i) = primitive $ unsafeCoerce# readSmallArray# m i
 {-# INLINE get #-}
 
+-- | Atomically modify an element in a 'Struct'.
 atomicModify :: PrimMonad m
   => Struct (PrimState m) h xs -> Membership xs x -> (h x -> (h x, a)) -> m a
 atomicModify (Struct m) (getMemberId -> I# i) f = primitive
@@ -91,6 +92,7 @@
           _ -> retry y s'
 {-# INLINE atomicModify #-}
 
+-- | Strict version of 'atomicModify'.
 atomicModify' :: PrimMonad m
   => Struct (PrimState m) h xs -> Membership xs x -> (h x -> (h x, a)) -> m a
 atomicModify' s i f = atomicModify s i
@@ -98,6 +100,7 @@
   >>= (return $!)
 {-# INLINE atomicModify' #-}
 
+-- | Apply a function to an element atomically.
 atomicModify_ :: PrimMonad m
   => Struct (PrimState m) h xs -> Membership xs x -> (h x -> h x) -> m (h x)
 atomicModify_ (Struct m) (getMemberId -> I# i) f = primitive
@@ -110,6 +113,7 @@
         _ -> retry y s'
 {-# INLINE atomicModify_ #-}
 
+-- | Strict version of 'atomicModify_'.
 atomicModify'_ :: PrimMonad m
   => Struct (PrimState m) h xs -> Membership xs x -> (h x -> h x) -> m (h x)
 atomicModify'_ s i f = atomicModify_ s i f >>= (return $!)
@@ -217,6 +221,7 @@
 hlength (HProduct ar) = I# (sizeofSmallArray# ar)
 {-# INLINE hlength #-}
 
+-- | Concatenate type level lists
 type family (++) (xs :: [k]) (ys :: [k]) :: [k] where
   '[] ++ ys = ys
   (x ': xs) ++ ys = x ': xs ++ ys
@@ -298,6 +303,7 @@
 hfrozen m = runST $ m >>= unsafeFreeze
 {-# INLINE[0] hfrozen #-}
 
+-- | Turn a product into a 'Struct' temporarily.
 hmodify :: (forall s. Struct s h xs -> ST s ()) -> h :* xs -> h :* xs
 hmodify f m = runST $ do
   s <- thaw m
diff --git a/src/Data/Extensible/Sum.hs b/src/Data/Extensible/Sum.hs
--- a/src/Data/Extensible/Sum.hs
+++ b/src/Data/Extensible/Sum.hs
@@ -6,7 +6,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Sum
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
diff --git a/src/Data/Extensible/TH.hs b/src/Data/Extensible/TH.hs
--- a/src/Data/Extensible/TH.hs
+++ b/src/Data/Extensible/TH.hs
@@ -2,7 +2,7 @@
 ------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.TH
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
@@ -44,6 +44,7 @@
   let name = mkName $ if isLower x then x : xs else '_' : x : xs
   in mkFieldAs name s
 
+-- | @'mkFieldAs' (mkName "foo") "bar"@ defines a field for "bar" as @foo@.
 mkFieldAs :: Name -> String -> DecsQ
 mkFieldAs name s = do
   let st = litT (strTyLit s)
@@ -86,6 +87,7 @@
 decEffectSuite :: DecsQ -> DecsQ
 decEffectSuite = customDecEffects True True
 
+-- | Generate effect suite with custom settings.
 customDecEffects :: Bool -- ^ generate a synonym of the set of actions
     -> Bool -- ^ generate synonyms for individual actions
     -> DecsQ -> DecsQ
diff --git a/src/Data/Extensible/Tangle.hs b/src/Data/Extensible/Tangle.hs
--- a/src/Data/Extensible/Tangle.hs
+++ b/src/Data/Extensible/Tangle.hs
@@ -2,14 +2,21 @@
 ------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Struct
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>
 --
 -- Extensible tangles
 ------------------------------------------------------------------------
-module Data.Extensible.Tangle where
+module Data.Extensible.Tangle
+  ( TangleT(..)
+  , lasso
+  , hitchAt
+  , runTangleT
+  , evalTangleT
+  , runTangles
+  ) where
 
 import Control.Applicative
 import Control.Monad.Trans.RWS.Strict
diff --git a/src/Data/Extensible/Wrapper.hs b/src/Data/Extensible/Wrapper.hs
--- a/src/Data/Extensible/Wrapper.hs
+++ b/src/Data/Extensible/Wrapper.hs
@@ -3,7 +3,7 @@
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Extensible.Wrapper
--- Copyright   :  (c) Fumiaki Kinoshita 2017
+-- Copyright   :  (c) Fumiaki Kinoshita 2018
 -- License     :  BSD3
 --
 -- Maintainer  :  Fumiaki Kinoshita <fumiexcel@gmail.com>