SSCCE of using Aeson to decode polymorphic JSON

poly-aeson.hs

#!/usr/bin/env stack
-- stack --install-ghc --resolver lts-5.13 runghc --package aeson

{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson
import Data.Aeson.Types
import qualified Data.Text as T

main :: IO ()
main = do
  let polyInt = decode "{\"polyField\":1234}" :: Maybe (Poly Int)
  print polyInt
  let polyConcrete = decode "{\"polyField\":{\"concreteField\":\"false\"}}" :: Maybe (Poly Concrete)
  print polyConcrete

data Poly a = Poly
              { polyField :: a
              } deriving Show

instance (FromJSON a) => FromJSON (Poly a) where
  parseJSON (Object v) = Poly <$>
                         v .: "polyField"
  parseJSON invalid = typeMismatch "Poly" invalid

data Concrete = Concrete
                { concreteField :: T.Text
                } deriving Show

instance FromJSON Concrete where
  parseJSON (Object v) = Concrete <$>
                         v .: "concreteField"
  parseJSON invalid = typeMismatch "Concrete" invalid