Parsing nested JSON into a list of Tuples with Aeson - json

Question

Say I have the following structure:
data AddressDto = AddressDto
{ addressDtoId :: UUID
, addressDtoCode :: Text
, addressDtoCity :: Maybe Text
, addressDtoStreet :: Text
, addressDtoPostCode :: Text
} deriving (Eq, Show, Generic)
instance FromJSON AddressDto where
parseJSON = genericParseJSON $ apiOptions "addressDto"
instance ToJSON AddressDto where
toJSON = genericToJSON $ apiOptions "addressDto"
toEncoding = genericToEncoding $ apiOptions "addressDto"
This works as you would expect.
Now say I want to parse a JSON structure of the format:
{ UUID: AddressDto, UUID: AddressDto, UUID: AddressDto }
A reasonable Haskell representation would seem to be:
data AddressListDto = AddressListDto [(UUID, AddressDto)]
Creating a helper function like so:
keyAndValueToList :: Either ServiceError AddressListDto -> Text -> DiscountDto -> Either ServiceError AddressListDto
keyAndValueToList (Left err) _ _ = Left err
keyAndValueToList (Right (AddressListDto ald)) k v = do
let maybeUUID = fromString $ toS k
case maybeUUID of
Nothing -> Left $ ParseError $ InvalidDiscountId k
Just validUUID -> Right $ AddressListDto $ (validUUID, v) : ald
and finally an instance of FromJSON:
instance FromJSON AddressListDto where
parseJSON = withObject "tuple" $ \o -> do
let result = foldlWithKey' keyAndValueToList (Right $ AddressListDto []) o
case result of
Right res -> pure res
Left err -> throwError err
This fails to compile with:
Couldn't match type ‘aeson-1.4.5.0:Data.Aeson.Types.Internal.Value’
with ‘AddressDto’
Expected type: unordered-containers-0.2.10.0:Data.HashMap.Base.HashMap
Text AddressDto
Two questions:
1) How do I make sure the nested values in the hashmap get parsed correctly to an AddressDto.
2) How do I avoid forcing the initial value into an either? Is there a function I could use instead of foldlWithKey' that didn't make me wrap the initial value like this?

Answer 1

Funny answer. To implement parseJSON, you can use
parseJSON :: Value -> Parser (HashMap Text AddressDto)
...but even better, why not just use a type alias instead of a fresh data type, so:
type AddressListDto = HashMap UUID AddressDto
This already has a suitable FromJSON instance, so then you don't even need to write any code yourself.