Set default values for omitted fields with Haskell Aeson

I'm using Aeson to accept user configuration in JSON format, where some fields may be omitted and the default values would be used. According to doc I should write something like this:

import           Data.Aesonimport           GHC.Genericsdata Person = Person  { name :: String  , age  :: Int  } deriving (Generic, Show)instance FromJSON Person where  omittedField = Just $ Person "Unnamed" (-1)  parseJSON = genericParseJSON defaultOptions { allowOmittedFields = True }main :: IO ()main = do print (eitherDecode @Person "{}")          print (eitherDecode @Person "{\"name\":\"Bob\"}")          print (eitherDecode @Person "{\"name\":\"Bob\",\"age\":42}")

Does not really work:

Left "Error in $: parsing Main.Person(Person) failed, key \"name\" not found"Left "Error in $: parsing Main.Person(Person) failed, key \"age\" not found"Right (Person {name = "Bob", age = 42})