Add (.:!) and partially revert d0414be

Data/Aeson.hs

@@ -78,6 +78,7 @@ module Data.Aeson
     , foldable
     , (.:)
     , (.:?)
+    , (.:!)
     , (.!=)
     , object
     -- * Parsing

Data/Aeson/TH.hs

@@ -100,7 +100,6 @@ import Data.Aeson.Types ( Value(..), Parser
                         , defaultTaggedObject
                         )
 import Data.Aeson.Types.Internal (Encoding(..))
-import Control.Monad       ( return, mapM, liftM2, fail, join )
 import Data.Bool           ( Bool(False, True), otherwise, (&&), not )
 import Data.Either         ( Either(Left, Right) )
 import Data.Eq             ( (==) )
@@ -954,7 +953,7 @@ instance (FromJSON a) => LookupField a where
           Just v  -> parseJSON v
 
 instance (FromJSON a) => LookupField (Maybe a) where
-    lookupField _ _ obj key = join <$> obj .:? key
+    lookupField _ _ = (.:?)
 
 unknownFieldFail :: String -> String -> String -> Parser fail
 unknownFieldFail tName rec key =

Data/Aeson/Types.hs

@@ -55,6 +55,7 @@ module Data.Aeson.Types
     , foldable
     , (.:)
     , (.:?)
+    , (.:!)
     , (.!=)
     , object
 

Data/Aeson/Types/Generic.hs

@@ -20,7 +20,7 @@
 module Data.Aeson.Types.Generic ( ) where
 
 import Control.Applicative ((<*>), (<$>), (<|>), pure)
-import Control.Monad ((<=<), join)
+import Control.Monad ((<=<))
 import Control.Monad.ST (ST)
 import Data.Aeson.Encode.Builder (emptyArray_)
 import Data.Aeson.Encode.Functions (builder)
@@ -656,8 +656,8 @@ instance (Selector s, GFromJSON a) => FromRecord (S1 s a) where
     {-# INLINE parseRecord #-}
 
 instance (Selector s, FromJSON a) => FromRecord (S1 s (K1 i (Maybe a))) where
-    parseRecord _ (Just lab) obj = (M1 . K1) . join <$> obj .:? lab
-    parseRecord opts Nothing obj = (M1 . K1) . join <$> obj .:? pack label
+    parseRecord _ (Just lab) obj = (M1 . K1) <$> obj .:? lab
+    parseRecord opts Nothing obj = (M1 . K1) <$> obj .:? pack label
         where
           label = fieldLabelModifier opts $
                     selName (undefined :: t s (K1 i (Maybe a)) p)

Data/Aeson/Types/Instances.hs

@@ -49,6 +49,7 @@ module Data.Aeson.Types.Instances
     , ifromJSON
     , (.:)
     , (.:?)
+    , (.:!)
     , (.!=)
     , tuple
     , (>*<)
@@ -1544,11 +1545,21 @@ obj .: key = case H.lookup key obj of
 obj .:? key = case H.lookup key obj of
                Nothing -> pure Nothing
                Just v  -> modifyFailure addKeyName
-                        $ Just <$> parseJSON v <?> Key key
+                        $ parseJSON v <?> Key key
   where
     addKeyName = (("failed to parse field " <> unpack key <> ": ") <>)
 {-# INLINE (.:?) #-}
 
+-- | Like `(.:?)` but fail if the key is present but is 'Null'.
+(.:!) :: (FromJSON a) => Object -> Text -> Parser (Maybe a)
+obj .:! key = case H.lookup key obj of
+               Nothing -> pure Nothing
+               Just v  -> modifyFailure addKeyName
+                        $ Just <$> parseJSON v <?> Key key
+  where
+    addKeyName = (("failed to parse field " <> unpack key <> ": ") <>)
+{-# INLINE (.:??) #-}
+
 -- | Helper for use in combination with '.:?' to provide default
 -- values for optional JSON object fields.
 --