Aeson: parsing dynamic keys as type field

Question

Let's say there is a JSON like:

{
  "bob_id" : {
    "name": "bob",
    "age" : 20
  },
  "jack_id" : {
    "name": "jack",
    "age" : 25
  }
}

Is it possible to parse it to [Person] with Person defined like below?

data Person = Person {
   id   :: Text
  ,name :: Text
  ,age  :: Int
}

Answer 1

You cannot define an instance for [Person] literally, because aeson already includes an instance for [a], however you can create a newtype, and provide an instance for that.

Aeson also includes the instance FromJSON a => FromJSON (Map Text a), which means if aeson knows how to parse something, it knows how to parse a dict of that something.

You can define a temporary datatype resembling a value in the dict, then use the Map instance to define FromJSON PersonList, where newtype PersonList = PersonList [Person]:

data PersonInfo = PersonInfo { infoName :: Text, infoAge :: Int }

instance FromJSON PersonInfo where
    parseJSON (Object v) = PersonInfo <$> v .: "name" <*> v .: "age"
    parseJSON _ = mzero

data Person = Person { id :: Text, name :: Text, age :: Int }
newtype PersonList = PersonList [Person]

instance FromJSON PersonList where
    parseJSON v = fmap (PersonList . map (\(id, PersonInfo name age) -> Person id name age) . M.toList) $ parseJSON v