Haskell :: Aeson :: parse ADT based on field value

Question

I'm using an external API which returns JSON responses. One of the responses is an array of objects and these objects are identified by the field value inside them. I'm having some trouble understanding how the parsing of such JSON response could be done with Aeson.

Here is a simplified version of my problem:

newtype Content = Content { content :: [Media] } deriving (Generic)

instance FromJSON Content

data Media =
  Video { objectClass :: Text
        , title :: Text } |
  AudioBook { objectClass :: Text
            , title :: Text }

In API documentation it is said that the object can be identified by the field objectClass which has value "video" for our Video object and "audiobook" for our AudioBook and so on. Example JSON:

[{objectClass: "video", title: "Some title"}
,{objectClass: "audiobook", title: "Other title"}]

The question is how can this type of JSON be approached using Aeson?

instance FromJSON Media where
  parseJSON (Object x) = ???

Answer 1

You basically need a function Text -> Text -> Media:

toMedia :: Text -> Text -> Media
toMedia "video"     = Video "video"
toMedia "audiobook" = AudioBook "audiobook"

The FromJSON instance is now really simple (using <$> and <*> from Control.Applicative):

instance FromJSON Media where
    parseJSON (Object x) = toMedia <$> x .: "objectClass" <*> x .: "title"

However, at this point you're redundant: the objectClass field in Video or Audio doesn't give you more information than the actual type, so you might remove it:

data Media = Video     { title :: Text }
           | AudioBook { title :: Text }

toMedia :: Text -> Text -> Media
toMedia "video"     = Video
toMedia "audiobook" = AudioBook

Also note that toMedia is partial. You probably want to catch invalid "objectClass" values:

instance FromJSON Media where
    parseJSON (Object x) = 
        do oc <- x .: "objectClass"
           case oc of
               String "video"     -> Video     <$> x .: "title"
               String "audiobook" -> AudioBook <$> x .: "title"
               _                  -> empty

{- an alternative using a proper toMedia
toMedia :: Alternative f => Text -> f (Text -> Media)
toMedia "video"     = pure Video
toMedia "audiobook" = pure AudioBook
toMedia _           = empty

instance FromJSON Media where
    parseJSON (Object x) = (x .: "objectClass" >>= toMedia) <*> x .: "title"
-}

And last, but not least, remember that valid JSON uses strings for the name.