Add support for decoding missing record fields to Nothing (#93)

Co-authored-by: Thomas Honeyman <admin@thomashoneyman.com>

Author: jvliwanag

CHANGELOG.md

@@ -8,6 +8,8 @@ Breaking changes:
 
 New features:
 
+- Added support for decoding missing record fields to `Nothing` (#93 by @jvliwanag)
+
 Bugfixes:
 
 Other improvements:

src/Data/Argonaut/Decode/Class.purs

@@ -19,7 +19,7 @@ import Data.String (CodePoint)
 import Data.Symbol (class IsSymbol, reflectSymbol)
 import Data.Tuple (Tuple)
 import Foreign.Object as FO
-import Prelude (class Ord, Unit, Void, bind, ($), (<<<))
+import Prelude (class Ord, Unit, Void, bind, ($), (<$>))
 import Prim.Row as Row
 import Prim.RowList as RL
 import Record as Record
@@ -111,7 +111,7 @@ instance gDecodeJsonNil :: GDecodeJson () RL.Nil where
   gDecodeJson _ _ = Right {}
 
 instance gDecodeJsonCons
-  :: ( DecodeJson value
+  :: ( DecodeJsonField value
      , GDecodeJson rowTail tail
      , IsSymbol field
      , Row.Cons field value rowTail row
@@ -122,12 +122,27 @@ instance gDecodeJsonCons
     let
       _field = Proxy :: Proxy field
       fieldName = reflectSymbol _field
+      fieldValue = FO.lookup fieldName object
 
-    case FO.lookup fieldName object of
-      Just jsonVal -> do
-        val <- lmap (AtKey fieldName) <<< decodeJson $ jsonVal
+    case decodeJsonField fieldValue of
+      Just fieldVal -> do
+        val <- lmap (AtKey fieldName) fieldVal
         rest <- gDecodeJson object (Proxy :: Proxy tail)
         Right $ Record.insert _field val rest
 
       Nothing ->
         Left $ AtKey fieldName MissingValue
+
+class DecodeJsonField a where
+  decodeJsonField :: Maybe Json -> Maybe (Either JsonDecodeError a)
+
+instance decodeFieldMaybe
+  :: DecodeJson a
+  => DecodeJsonField (Maybe a) where
+  decodeJsonField Nothing = Just $ Right Nothing
+  decodeJsonField (Just j) = Just $ decodeJson j
+
+else instance decodeFieldId
+  :: DecodeJson a
+  => DecodeJsonField a where
+  decodeJsonField j = decodeJson <$> j

test/Test/Main.purs

@@ -74,6 +74,7 @@ main = flip runReaderT 0 do
   suite "Encode/Decode NonEmpty Check" nonEmptyCheck
   suite "Encode/Decode Checks" encodeDecodeCheck
   suite "Encode/Decode Record Checks" encodeDecodeRecordCheck
+  suite "Decode Optional Field Check" decodeOptionalFieldCheck
   suite "Combinators Checks" combinatorsCheck
   suite "Manual Combinators Checks" manualRecordDecode
   suite "Error Message Checks" errorMsgCheck
@@ -93,6 +94,27 @@ encodeDecodeRecordCheck = do
     let redecoded = decodeJson (encodeJson rec)
     pure $ Right rec == redecoded <?> (show redecoded <> " /= Right " <> show rec)
 
+decodeOptionalFieldCheck :: Test
+decodeOptionalFieldCheck = do
+  barMissingJson <- jsonParser' """{ }"""
+  barNullJson <- jsonParser' """{ "bar": null }"""
+  barPresentJson <- jsonParser' """{ "bar": [] }"""
+
+  test "Decode missing field" do
+    case decodeJson barMissingJson of
+      Right ({ bar: Nothing } :: FooRecord) -> pure unit
+      _ -> failure ("Failed to properly decode JSON string: " <> stringify barMissingJson)
+
+  test "Decode null field" do
+    case decodeJson barNullJson of
+      Right ({ bar: Nothing } :: FooRecord) -> pure unit
+      _ -> failure ("Failed to properly decode JSON string: " <> stringify barNullJson)
+
+  test "Decode present field" do
+    case decodeJson barPresentJson of
+      Right ({ bar: Just [] } :: FooRecord) -> pure unit
+      _ -> failure ("Failed to properly decode JSON string: " <> stringify barPresentJson)
+
 genTestJson :: Gen Json
 genTestJson = resize 5 genJson
 
@@ -434,3 +456,7 @@ instance decodeJsonFooNested' :: DecodeJson FooNested' where
     bar <- x .:? "bar"
     baz <- x .:? "baz" .!= false
     pure $ FooNested' { bar, baz }
+
+type FooRecord =
+  { bar :: Maybe (Array Int)
+  }