Add maybeKey combinator (#37)

* Add maybeKey combinator

* remove prefixes

lib/Data/Aeson/Combinators/Decode.hs

@@ -41,6 +41,7 @@ module Data.Aeson.Combinators.Decode (
   , jsonNull
 -- *** Objects
   , key
+  , maybeKey
   , at
 -- *** Arrays
   , index
@@ -426,6 +427,20 @@ key t (Decoder d) = Decoder $ \case
   val      -> typeMismatch "Object" val
 {-# INLINE key #-}
 
+-- | Same as 'key' but works with omitted attributes in payloads and produces parsed values in the context of 'Maybe'.
+--   Note that this combinator behaves differently to a combination of 'maybe' and 'key', which produce error if
+--   the attribute is missing from the json object.
+-- >>> decode (maybeKey "data" int) "{}"
+-- Just Nothing
+--
+--- >>> decode (maybeKey "data" int) "{\"data\": 42}"
+-- Just (Just 42)
+maybeKey :: Key -> Decoder a -> Decoder (Maybe a)
+maybeKey t (Decoder d) = Decoder $ \case
+  Object v -> (v .:? t) >>= maybe (pure Nothing) (fmap Just . d)
+  val      -> typeMismatch "Object" val
+{-# INLINE maybeKey #-}
+
 
 -- | Extract JSON value from JSON object keys
 --