Lossless Maybe encoding/decoding

Fixes #22, in a backwards-compatible way.

Author: hdgarrood

src/Data/Argonaut/Decode/Class.purs

@@ -68,9 +68,18 @@ gDecodeJson' signature json = case signature of
   mFail msg = maybe (Left msg) Right
 
 instance decodeJsonMaybe :: DecodeJson a => DecodeJson (Maybe a) where
-  decodeJson j
-    | isNull j = pure Nothing
-    | otherwise = Just <$> decodeJson j
+  decodeJson j =
+      case decode j of
+        Right x -> Right x
+        Left x -> backwardsCompat
+    where
+    decode =
+      decodeJObject >=> lookupJust >=> decodeJson
+    lookupJust =
+        maybe (Left "Missing property 'just'") Right <<< SM.lookup "just"
+    backwardsCompat
+      | isNull j = pure Nothing
+      | otherwise = Just <$> decodeJson j
 
 instance decodeJsonTuple :: (DecodeJson a, DecodeJson b) => DecodeJson (Tuple a b) where
   decodeJson j = decodeJson j >>= f

src/Data/Argonaut/Encode/Class.purs

@@ -2,7 +2,7 @@ module Data.Argonaut.Encode.Class where
 
 import Prelude
 
-import Data.Argonaut.Core (Json(), jsonNull, fromBoolean, fromNumber, fromString, fromArray, fromObject)
+import Data.Argonaut.Core (Json(), jsonNull, fromBoolean, fromNumber, fromString, fromArray, fromObject, jsonEmptyObject, jsonSingletonObject)
 import Data.Either (Either(), either)
 import Data.Foldable (foldr)
 import Data.Generic (class Generic, GenericSpine(..), toSpine)
@@ -42,8 +42,8 @@ gEncodeJson' = case _ of
       SM.insert field.recLabel (gEncodeJson' $ field.recValue unit)
 
 instance encodeJsonMaybe :: EncodeJson a => EncodeJson (Maybe a) where
-  encodeJson Nothing  = jsonNull
-  encodeJson (Just a) = encodeJson a
+  encodeJson Nothing  = jsonEmptyObject
+  encodeJson (Just a) = jsonSingletonObject "just" (encodeJson a)
 
 instance encodeJsonTuple :: (EncodeJson a, EncodeJson b) => EncodeJson (Tuple a b) where
   encodeJson (Tuple a b) = encodeJson [encodeJson a, encodeJson b]