Polymorphic Aeson serialization using Generic instances

{-# LANGUAGE DeriveGeneric #-}

module Integration.TestAeson where

import Data.Aeson
import GHC.Generics

data Pets = Dogs [Dog] | Cats [Cat] | Rodents [Rodent] | Reptiles [Reptile]
  deriving (Show, Generic)

data Dog = Dog {
  dogField :: String
} deriving (Show, Generic)

data Cat = Cat {
  catField :: String
} deriving (Show, Generic)

data Rodent = Rodent {
  rodentField :: String
} deriving (Show, Generic)

data Reptile = Reptile {
  reptileField :: String
} deriving (Show, Generic)

data Person = Person {
  name :: String,
  species :: String,
  pets :: Pets
} deriving (Show,Generic)

instance ToJSON Cat
instance FromJSON Cat
instance ToJSON Dog
instance FromJSON Dog
instance ToJSON Rodent
instance FromJSON Rodent
instance ToJSON Reptile
instance FromJSON Reptile
instance ToJSON Pets
instance FromJSON Pets
instance ToJSON Person
instance FromJSON Person

doit :: IO ()
doit = do
  let agerbil = Rodent "Ager"
  let amouse = Rodent "Amose"
  let me = Person "user5402" "Human" (Rodents [agerbil, amouse])
  print $ encode me

{- run this

λ> doit
"{\"species\":\"Human\",\"name\":\"user5402\",\"pets\":{\"tag\":\"Rodents\",\"contents\":[{\"rodentField\":\"Ager\"},{\"rodentField\":\"Amose\"}]}}"

-}