如何使用Haskell/Aeson中的类型函数解析多态值？

import Data.Aeson
import Data.Aeson.Types
import Data.Text (Text)
import qualified Data.HashMap.Strict as HM

parsePolyValue :: Value -> Parser (Maybe a)
parsePolyValue (Object obj) = do
  -- 解析对象中的"type"字段
  case HM.lookup "type" obj of
    Just (String typeName) -> case typeName of
      "Int" -> obj .: "value" >>= parseJSON >>= return . Just
      "String" -> obj .: "value" >>= parseJSON >>= return . Just
      _ -> return Nothing
    _ -> return Nothing
parsePolyValue _ = return Nothing

parsePoly :: Value -> Parser (Maybe (Either Int String))
parsePoly value = do
  -- 解析多态值
  polyValue <- parsePolyValue value
  case polyValue of
    Just (Just intValue) -> return $ Just (Left intValue)
    Just (Just stringValue) -> return $ Just (Right stringValue)
    _ -> return Nothing