omit Nothing while encoding to json

src/JRec.hs

@@ -10,7 +10,7 @@ module JRec
     append,
     union,
     insert,
-    insertOrSet
+    insertOrSet,
   )
 where
 
@@ -63,8 +63,8 @@ append = R.combine
 
 -- Merges records, removing duplicates
 --
--- Left-biased. Does not sort. 
--- 
+-- Left-biased. Does not sort.
+--
 -- O(n * m) type check complexity.
 union ::
   forall lhs rhs res.
@@ -83,18 +83,19 @@ union = R.union
 -- | Insert a field into a record that does not already contain it
 --
 -- O(n) type check complexity.
-insert :: 
-  forall label value lts res. 
+insert ::
+  forall label value lts res.
   ( KnownNat (1 + R.RecSize lts),
     KnownNat (R.RecSize lts),
     KnownSymbol label,
     R.RecCopy lts lts res,
     res ~ ((label := value) : lts),
     R.RemoveAccessTo label lts ~ lts
-  ) => 
+  ) =>
   label := value ->
-  Rec lts -> Rec res
-insert = R.combine . Rec 
+  Rec lts ->
+  Rec res
+insert = R.combine . Rec
 
 -- | Insert a field into a record. Set it if it already exists
 --

src/JRec/Internal.hs

@@ -32,6 +32,7 @@ import qualified Control.Monad.State as S
 import Data.Aeson
 import Data.Aeson.Types (Parser)
 import Data.Constraint
+import Data.Maybe (catMaybes)
 import Data.Proxy
 import qualified Data.Text as T
 import Data.Typeable
@@ -115,7 +116,7 @@ instance RecEq lts lts => Eq (Rec lts) where
 
 #ifdef WITH_AESON
 instance
-  ( RecApply lts lts ToJSON
+  ( RecApply lts lts EncodeField
   ) =>
   ToJSON (Rec lts)
   where
@@ -124,6 +125,7 @@ instance
 
 instance (RecSize lts ~ s, KnownNat s, RecJsonParse lts) => FromJSON (Rec lts) where
   parseJSON = recJsonParser defaultJSONOptions
+
 #endif
 
 instance RecNfData lts lts => NFData (Rec lts) where
@@ -552,13 +554,26 @@ reflectRecFold _ f r =
 showRec :: forall lts. (RecApply lts lts Show) => Rec lts -> [(String, String)]
 showRec = reflectRec @Show Proxy (\k v -> (k, show v))
 
-recToValue :: forall lts. (RecApply lts lts ToJSON) => JSONOptions -> Rec lts -> Value
-recToValue options r =
-  object $ reflectRec @ToJSON Proxy (\k v -> (T.pack (fieldTransform options k), toJSON v)) r
+class ToJSON a => EncodeField a where
+  encodeField :: a -> Maybe Value
+  encodeKV :: T.Text -> a -> Series
 
-recToEncoding :: forall lts. (RecApply lts lts ToJSON) => JSONOptions -> Rec lts -> Encoding
+instance ToJSON a => EncodeField a where
+  encodeField = pure . toJSON
+  encodeKV = (.=)
+
+instance {-# OVERLAPS #-} ToJSON a => EncodeField (Maybe a) where
+  encodeField = fmap toJSON
+  encodeKV _ Nothing = mempty
+  encodeKV k v = k .= v
+
+recToValue :: forall lts. (RecApply lts lts EncodeField) => JSONOptions -> Rec lts -> Value
+recToValue options r =
+  object $ catMaybes $ reflectRec @EncodeField Proxy (\k v -> (T.pack (fieldTransform options k),) <$> encodeField v) r
+
+recToEncoding :: forall lts. (RecApply lts lts EncodeField) => JSONOptions -> Rec lts -> Encoding
 recToEncoding options r =
-  pairs $ mconcat $ reflectRec @ToJSON Proxy (\k v -> (T.pack (fieldTransform options k) .= v)) r
+  pairs $ mconcat $ reflectRec @EncodeField Proxy (\k v -> (T.pack (fieldTransform options k) `encodeKV` v)) r
 
 recJsonParser ::
   forall lts s.

test/JRecSpec.hs

@@ -91,14 +91,14 @@ spec = do
   describe "optional fields" $ do
     it "encode" $ do
       encode (Rec (#a := (1 :: Int), #b := (Nothing :: Maybe Int)))
-        `shouldBe` "{\"a\":1,\"b\":null}"
+        `shouldBe` "{\"a\":1}"
     it "encode" $ do
       encode (Rec (#a := (1 :: Int), #b := (Just 2 :: Maybe Int)))
         `shouldBe` "{\"a\":1,\"b\":2}"
     it "decode" $ do
-      decode "{\"a\": 1, \"b\": null}"
-        `shouldBe` Just (Rec (#a := (1 :: Int), #b := (Nothing :: Maybe Int)))
       decode "{\"a\": 1}"
         `shouldBe` Just (Rec (#a := (1 :: Int), #b := (Nothing :: Maybe Int)))
+      decode "{\"a\": 1, \"b\": null}"
+        `shouldBe` Just (Rec (#a := (1 :: Int), #b := (Nothing :: Maybe Int)))
       decode "{\"a\": 1, \"b\": 2}"
         `shouldBe` Just (Rec (#a := (1 :: Int), #b := (Just 2 :: Maybe Int)))