diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # salak
 
-[![Hackage](https://img.shields.io/badge/hackage-v0.1.4-orange.svg)](https://hackage.haskell.org/package/salak)
+[![Hackage](https://img.shields.io/badge/hackage-v0.2.1-orange.svg)](https://hackage.haskell.org/package/salak)
 [![Build Status](https://travis-ci.org/leptonyu/salak.svg?branch=master)](https://travis-ci.org/leptonyu/salak)
 
 
diff --git a/salak.cabal b/salak.cabal
--- a/salak.cabal
+++ b/salak.cabal
@@ -1,78 +1,75 @@
 cabal-version: 1.12
-
--- This file has been generated from package.yaml by hpack version 0.31.1.
---
--- see: https://github.com/sol/hpack
---
--- hash: 65d137fa16b84c27133c76282e7859306d30f334f45be6cb7332a144552ef7ce
-
-name:           salak
-version:        0.2.0
-synopsis:       Configuration Loader
-description:    Configuration Loader for Production in Haskell
-category:       Library
-homepage:       https://github.com/leptonyu/salak#readme
-author:         Daniel YU
-maintainer:     Daniel YU <leptonyu@gmail.com>
-copyright:      (c) 2018 Daniel YU
-license:        BSD3
-license-file:   LICENSE
-build-type:     Simple
+name: salak
+version: 0.2.1
+license: BSD3
+license-file: LICENSE
+copyright: (c) 2018 Daniel YU
+maintainer: Daniel YU <leptonyu@gmail.com>
+author: Daniel YU
+homepage: https://github.com/leptonyu/salak#readme
+synopsis: Configuration Loader
+description:
+    Configuration Loader for Production in Haskell
+category: Library
+build-type: Simple
 extra-source-files:
     README.md
     test/salak.yml
 
 library
-  exposed-modules:
-      Data.Salak
-  other-modules:
-      Data.Salak.Types
-      Data.Salak.Environment
-      Data.Salak.CommandLine
-      Data.Salak.Aeson
-      Data.Salak.Yaml
-  hs-source-dirs:
-      src
-  ghc-options: -Wall -Wcompat -Wincomplete-record-updates -Wincomplete-uni-patterns -Wredundant-constraints -fno-warn-orphans -fno-warn-missing-signatures
-  build-depends:
-      aeson
-    , base >=4.7 && <5
-    , directory
-    , filepath
-    , scientific
-    , text
-    , unordered-containers
-    , vector
-    , yaml
-  default-language: Haskell2010
+    exposed-modules:
+        Data.Salak
+    hs-source-dirs: src
+    other-modules:
+        Data.Salak.Types
+        Data.Salak.Environment
+        Data.Salak.CommandLine
+        Data.Salak.Aeson
+        Data.Salak.Yaml
+        Data.Salak.Operation
+    default-language: Haskell2010
+    ghc-options: -Wall -Wcompat -Wincomplete-record-updates
+                 -Wincomplete-uni-patterns -Wredundant-constraints -fno-warn-orphans
+                 -fno-warn-missing-signatures
+    build-depends:
+        aeson >=1.4.2.0 && <1.5,
+        base >=4.11 && <5,
+        directory >=1.3.3.0 && <1.4,
+        filepath >=1.4.2.1 && <1.5,
+        scientific >=0.3.6.2 && <0.4,
+        text >=1.2.3.1 && <1.3,
+        unordered-containers >=0.2.9.0 && <0.3,
+        vector >=0.12.0.2 && <0.13,
+        yaml >=0.11.0.0 && <0.12
 
 test-suite spec
-  type: exitcode-stdio-1.0
-  main-is: Spec.hs
-  other-modules:
-      Data.Salak
-      Data.Salak.Aeson
-      Data.Salak.CommandLine
-      Data.Salak.Environment
-      Data.Salak.Types
-      Data.Salak.Yaml
-      Paths_salak
-  hs-source-dirs:
-      test
-      src
-  ghc-options: -Wall -Wcompat -Wincomplete-record-updates -Wincomplete-uni-patterns -Wredundant-constraints -fno-warn-orphans -fno-warn-missing-signatures
-  build-depends:
-      QuickCheck
-    , aeson
-    , aeson-pretty
-    , base >=4.7 && <5
-    , bytestring
-    , directory
-    , filepath
-    , hspec ==2.*
-    , scientific
-    , text
-    , unordered-containers
-    , vector
-    , yaml
-  default-language: Haskell2010
+    type: exitcode-stdio-1.0
+    main-is: Spec.hs
+    hs-source-dirs: test src
+    other-modules:
+        Data.Salak
+        Data.Salak.Aeson
+        Data.Salak.CommandLine
+        Data.Salak.Environment
+        Data.Salak.Operation
+        Data.Salak.Types
+        Data.Salak.Yaml
+        Paths_salak
+    default-language: Haskell2010
+    ghc-options: -Wall -Wcompat -Wincomplete-record-updates
+                 -Wincomplete-uni-patterns -Wredundant-constraints -fno-warn-orphans
+                 -fno-warn-missing-signatures
+    build-depends:
+        QuickCheck >=2.12.6.1 && <2.13,
+        aeson >=1.4.2.0 && <1.5,
+        aeson-pretty >=0.8.7 && <0.9,
+        base >=4.11 && <5,
+        bytestring >=0.10.8.2 && <0.11,
+        directory >=1.3.3.0 && <1.4,
+        filepath >=1.4.2.1 && <1.5,
+        hspec ==2.*,
+        scientific >=0.3.6.2 && <0.4,
+        text >=1.2.3.1 && <1.3,
+        unordered-containers >=0.2.9.0 && <0.3,
+        vector >=0.12.0.2 && <0.13,
+        yaml >=0.11.0.0 && <0.12
diff --git a/src/Data/Salak.hs b/src/Data/Salak.hs
--- a/src/Data/Salak.hs
+++ b/src/Data/Salak.hs
@@ -30,7 +30,7 @@
   , Properties(..)
   , Key
   , FromProperties(..)
-  , Return(..)
+  , Return
   -- * Properties Loader Helper
   , insert
   , makePropertiesFromEnvironment
@@ -39,12 +39,15 @@
   , makePropertiesFromJson
   , makePropertiesFromYaml
   , FileName
+  -- * Operations
+  , module Data.Salak.Operation
   ) where
 
 import           Data.Maybe
 import           Data.Salak.Aeson
 import           Data.Salak.CommandLine
 import           Data.Salak.Environment
+import           Data.Salak.Operation
 import           Data.Salak.Types
 import           Data.Salak.Yaml
 import           Data.Text              (Text, unpack)
diff --git a/src/Data/Salak/Aeson.hs b/src/Data/Salak/Aeson.hs
--- a/src/Data/Salak/Aeson.hs
+++ b/src/Data/Salak/Aeson.hs
@@ -7,7 +7,7 @@
 import           Data.Aeson
 import qualified Data.HashMap.Strict as M
 import           Data.Salak.Types
-import           Data.Vector         (fromList, toList)
+import           Data.Vector         (toList)
 
 -- | Load `Properties` from JSON `Value`
 makePropertiesFromJson :: Value -> Properties -> Properties
@@ -29,19 +29,3 @@
     g2 []    = []
     g2 (a:_) = [a]
     g3 (as,bs) (a,b) = (as++a,bs++b)
-
-instance FromProperties Value where
-  fromProperties (Properties []        []) = Empty
-  fromProperties (Properties [PBool p] []) = OK $ Bool   p
-  fromProperties (Properties [PNum  p] []) = OK $ Number p
-  fromProperties (Properties [PStr  p] []) = OK $ String p
-  fromProperties (Properties ps [])        = Array . fromList <$> traverse (fromProperties.singleton) ps
-  fromProperties (Properties _ [m])        = OK $ Object $ M.map (fromReturn Null . fromProperties) m
-  fromProperties (Properties _  ms)        = Array . fromList <$> traverse (fromProperties.singletonMap) ms
-
-instance {-# OVERLAPPABLE #-} FromJSON a => FromProperties a where
-  fromProperties a = do
-    v :: Value <- fromProperties a
-    case fromJSON v of
-      Success r -> OK r
-      Error   e -> Fail e
diff --git a/src/Data/Salak/Operation.hs b/src/Data/Salak/Operation.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/Salak/Operation.hs
@@ -0,0 +1,25 @@
+module Data.Salak.Operation where
+
+import           Data.Salak.Types
+import           Data.Text        (Text, unpack)
+
+infixl 5 .?>
+(.?>) :: FromProperties a => Properties -> Text -> Return a
+(.?>) = flip lookup'
+
+infixl 5 .|=
+(.|=) :: Return a -> a -> a
+(.|=) (Right a)       _ = a
+(.|=) (Left (Fail e)) _ = error e
+(.|=) _               d = d
+
+infixl 5 .?=
+(.?=) :: Return a -> a -> Return a
+(.?=) a b = Right (a .|= b)
+
+infixl 5 .>>
+(.>>) :: FromProperties a => Properties -> Text -> a
+(.>>) p key = case p .?> key of
+  Right v           -> v
+  Left (EmptyKey k) -> error $ "Config " <> unpack k <> " not found"
+  Left (Fail e)     -> error e
diff --git a/src/Data/Salak/Types.hs b/src/Data/Salak/Types.hs
--- a/src/Data/Salak/Types.hs
+++ b/src/Data/Salak/Types.hs
@@ -97,20 +97,28 @@
 lookup k = from . lookup' k
   where
     from :: Return a -> Maybe a
-    from (OK a)   = Just a
-    from Empty    = Nothing
-    from (Fail e) = error e
+    from (Right a)           = Just a
+    from (Left (EmptyKey _)) = Nothing
+    from (Left (Fail     e)) = error e
 
 -- | Find `Properties` by key and convert to specific Haskell value.
 lookup' :: FromProperties a => Text -> Properties -> Return a
-lookup' = go . toKeys
+lookup' k = go (toKeys k)
   where
     go [] p                      = fromProperties p
     go (a:as) (Properties _ [m]) = case M.lookup a m of
-      Just n -> go as n
-      _      -> Empty
-    go _ _                       = Empty
+      Just n -> case go as n of
+        Left (EmptyKey ke) -> Left $ EmptyKey $ joinKey a ke
+        v                  -> v
+      _      -> Left $ EmptyKey $ T.intercalate "." (a:as)
+    go ke _                      = Left $ EmptyKey $ T.intercalate "." ke
 
+
+joinKey :: Text -> Text -> Text
+joinKey "" k = k
+joinKey k "" = k
+joinKey a b  = a <> "." <> b
+
 -- | Insert batch properties to `Properties`
 makeProperties :: [(Text, Property)] -> Properties -> Properties
 makeProperties = flip (foldl go)
@@ -118,52 +126,27 @@
     go m (k,v) = insert (toKeys k) v m
 
 -- | Return of `FromProperties`
-data Return a
-  = Empty
-  | OK a
-  | Fail String
-  deriving Show
-
-instance Functor Return where
-  fmap f (OK a)   = OK (f a)
-  fmap _ Empty    = Empty
-  fmap _ (Fail b) = Fail b
-
-instance Applicative Return where
-  pure = OK
-  (OK f) <*> (OK a) = OK (f a)
-  (Fail x) <*> (Fail y) = Fail $ x ++ ";" ++ y
-  (Fail x) <*> _ = Fail x
-  _ <*> (Fail y) = Fail y
-  _ <*> _ = Empty
-
-instance Monad Return where
-  (OK a)   >>= f = f a
-  Empty    >>= _ = Empty
-  (Fail b) >>= _ = Fail b
-
-fromReturn :: b -> Return b -> b
-fromReturn _ (OK a) = a
-fromReturn a _      = a
+type Return = Either ErrResult
+data ErrResult = EmptyKey Text | Fail String deriving Show
 
 -- | Convert `Properties` to Haskell value.
 class FromProperties a where
   fromProperties :: Properties -> Return a
 
 instance FromProperties Property where
-  fromProperties (Properties [a] _) = OK a
-  fromProperties (Properties [] _)  = Empty
-  fromProperties _                  = Fail "property has multi values"
+  fromProperties (Properties [a] _) = Right a
+  fromProperties (Properties [] _)  = Left $ EmptyKey ""
+  fromProperties _                  = Left $ Fail "property has multi values"
 
 instance {-# OVERLAPPABLE #-} FromProperties a => FromProperties [a] where
-  fromProperties (Properties ps ms) = traverse fromProperties $ fmap singleton ps ++ fmap singletonMap ms
+  fromProperties (Properties ps ms) = traverse fromProperties $ fmap singleton ps <> fmap singletonMap ms
 
 instance FromProperties Scientific where
   fromProperties = fromProperties >=> go
     where
-      go (PNum a) = OK a
+      go (PNum a) = Right a
       go (PStr a) = to readMaybe $ T.unpack a
-      go _        = Fail "bool cannot convert to number"
+      go _        = Left $ Fail "bool cannot convert to number"
 
 instance FromProperties String where
   fromProperties a = T.unpack <$> fromProperties a
@@ -171,8 +154,8 @@
 instance FromProperties Text where
   fromProperties = fromProperties >=> go
     where
-      go (PStr a) = OK a
-      go a        = OK $ T.pack $ show a
+      go (PStr a) = Right a
+      go a        = Right $ T.pack $ show a
 
 instance FromProperties Float where
   fromProperties a = toRealFloat <$> fromProperties a
@@ -215,25 +198,27 @@
 
 to :: (b -> Maybe a) -> b -> Return a
 to v b = case v b of
-  Just a  -> OK a
-  Nothing -> Fail "number convert failed"
+  Just a  -> Right a
+  Nothing -> Left $ Fail "number convert failed"
 
 instance FromProperties Bool where
   fromProperties = fromProperties >=> go
     where
-      go (PBool a) = OK a
+      go (PBool a) = Right a
       go (PStr  a) = g2 $ T.toLower a
-      go _         = Fail "number cannot convert to bool"
+      go _         = Left $ Fail "number cannot convert to bool"
       g2 :: Text -> Return Bool
-      g2 "true"  = OK True
-      g2 "false" = OK False
-      g2 _       = Fail "string value cannot convert to bool"
+      g2 "1"     = Right True
+      g2 "true"  = Right True
+      g2 "0"     = Right False
+      g2 "false" = Right False
+      g2 _       = Left $ Fail "string value cannot convert to bool"
 
 instance FromProperties Char where
   fromProperties = fromProperties >=> go
     where
       go (PStr s)
-        | T.null s  = Empty
-        | otherwise = OK $ T.head s
-      go _          = Fail "cannot convert to char"
+        | T.null s  = Left  $ EmptyKey ""
+        | otherwise = Right $ T.head s
+      go _          = Left  $ Fail "cannot convert to char"
 
diff --git a/test/Spec.hs b/test/Spec.hs
--- a/test/Spec.hs
+++ b/test/Spec.hs
@@ -4,12 +4,11 @@
 
 module Main where
 
-import           Data.Aeson
 import qualified Data.HashMap.Strict    as M
-import           Data.Maybe
 import           Data.Salak
 import           Data.Salak.CommandLine
 import           Data.Salak.Environment
+import           Data.Salak.Operation   ()
 import qualified Data.Salak.Types       as P
 import           Data.Text              (Text, intercalate, pack)
 import           System.Environment
@@ -32,11 +31,11 @@
   , ext  :: Int
   } deriving (Eq, Show)
 
-instance FromJSON Config where
-  parseJSON = withObject "Config" $ \v -> Config
-        <$> v .: "name"
-        <*> v .: "dir"
-        <*> (fromMaybe 1 <$> v .:? "ext")
+instance FromProperties Config where
+  fromProperties v = Config
+        <$> v .?> "name"
+        <*> v .?> "dir"
+        <*> v .?> "ext" .?= 1
 
 specProperty = do
   context "empty" $ do
@@ -63,9 +62,9 @@
     it "Reject replacement" $ do
       insert k "1" m `shouldBe` m
     it "quickCheck" $ do
-      quickCheck $ \a' (b :: Integer) -> let a = pack a' in Just b == P.lookup a (insert (toKeys a) (PNum $ fromInteger b) empty)
-      quickCheck $ \a' (b :: Bool)    -> let a = pack a' in Just b == P.lookup a (insert (toKeys a) (PBool b) empty)
-      quickCheck $ \a' (b :: String)  -> let a = pack a' in Just b == P.lookup a (insert (toKeys a) (PStr $ pack b) empty)
+      quickCheck $ \a' (b :: Int)    -> let a = pack a' in Just b == P.lookup a (insert (toKeys a) (PNum $ fromIntegral b) empty)
+      quickCheck $ \a' (b :: Bool)   -> let a = pack a' in Just b == P.lookup a (insert (toKeys a) (PBool b) empty)
+      quickCheck $ \a' (b :: String) -> let a = pack a' in Just b == P.lookup a (insert (toKeys a) (PStr $ pack b) empty)
   context "lookup" $ do
     it "normal" $ do
       let m = insert ["a"] (PNum 1) empty