alternativeParsers.hs

module Main where

import Data.Aeson
import Data.Aeson.Types
import Control.Applicative (Alternative(..))

data State = State { stateName :: String, stateNum :: Int } deriving (Eq, Show)

parseState :: Value -> Parser State
parseState = withObject "State" $ \v -> State <$> v .: "name" <*> v .: "id"

parseProvince :: Value -> Parser State
parseProvince = withObject "State" $ \v -> State <$> v .: "provinceName" <*> pure 20

instance FromJSON State where
  parseJSON v =
    parseState v <|> parseProvince v

main :: IO ()
main = do
  let state = "{\"name\": \"WA\", \"id\": 1}"
      province = "{\"provinceName\": \"NY\"}"
  print (decode state :: Maybe State)
  print (decode province :: Maybe State)