Add To/FromJSONKey instances for Const (#798)

* To/FromJSONKey for Const

Author: dfithian

Data/Aeson/Types/FromJSON.hs

@@ -1459,6 +1459,9 @@ instance FromJSON a => FromJSON (Const a b) where
     {-# INLINE parseJSON #-}
     parseJSON = fmap Const . parseJSON
 
+instance (FromJSON a, FromJSONKey a) => FromJSONKey (Const a b) where
+    fromJSONKey = fmap Const fromJSONKey
+
 
 instance FromJSON1 Maybe where
     liftParseJSON _ _ Null = pure Nothing

Data/Aeson/Types/ToJSON.hs

@@ -1234,6 +1234,9 @@ instance ToJSON a => ToJSON (Const a b) where
     toEncoding (Const x) = toEncoding x
     {-# INLINE toEncoding #-}
 
+instance (ToJSON a, ToJSONKey a) => ToJSONKey (Const a b) where
+    toJSONKey = contramap getConst toJSONKey
+
 
 instance ToJSON1 Maybe where
     liftToJSON t _ (Just a) = t a

changelog.md

@@ -1,5 +1,9 @@
 For the latest version of this document, please see [https://github.com/bos/aeson/blob/master/changelog.md](https://github.com/bos/aeson/blob/master/changelog.md).
 
+### Upcoming
+
+* Add instances for `ToJSONKey` and `FromJSONKey` to `Const`.
+
 ### 1.5.3.0
 
 * Add instances for types in `strict` and `data-fix` packages, thanks to Oleg Grenrus.