refactor out an orNull helper

autodocodec/src/Autodocodec/Class.hs

@@ -194,18 +194,7 @@ optionalFieldOrNull ::
   -- | Documentation
   Text ->
   ObjectCodec (Maybe output) (Maybe output)
-optionalFieldOrNull key doc =
-  dimapCodec f g $ OptionalKeyCodec key (maybeCodec codec) (Just doc)
-  where
-    f :: Maybe (Maybe output) -> Maybe output
-    f = \case
-      Nothing -> Nothing
-      Just Nothing -> Nothing
-      Just (Just a) -> Just a
-    g :: Maybe output -> Maybe (Maybe output)
-    g = \case
-      Nothing -> Nothing
-      Just a -> Just (Just a)
+optionalFieldOrNull key doc = orNullHelper $ OptionalKeyCodec key (maybeCodec codec) (Just doc)
 
 -- | Like 'optionalFieldOrNull', but without documentation
 optionalFieldOrNull' ::
@@ -214,8 +203,11 @@ optionalFieldOrNull' ::
   -- | Key
   Text ->
   ObjectCodec (Maybe output) (Maybe output)
-optionalFieldOrNull' key =
-  dimapCodec f g $ OptionalKeyCodec key (maybeCodec codec) Nothing
+optionalFieldOrNull' key = orNullHelper $ OptionalKeyCodec key (maybeCodec codec) Nothing
+
+-- Helper function for 'optionalFieldOrNull'.
+orNullHelper :: ObjectCodec (Maybe (Maybe value)) (Maybe (Maybe value)) -> ObjectCodec (Maybe value) (Maybe value)
+orNullHelper = dimapCodec f g
   where
     f :: Maybe (Maybe output) -> Maybe output
     f = \case

autodocodec/src/Autodocodec/Codec.hs

@@ -320,7 +320,10 @@ eitherCodec ::
   ValueCodec (Either input1 input2) (Either output1 output2)
 eitherCodec = EitherCodec
 
--- | Value or null.
+-- | Also allow @null@ during decoding of a 'Maybe' value.
+--
+-- During decoding, also accept a @null@ value as 'Nothing'.
+-- During encoding, encode as usual.
 maybeCodec :: ValueCodec input output -> ValueCodec (Maybe input) (Maybe output)
 maybeCodec = dimapCodec f g . EitherCodec nullCodec
   where