{-# LANGUAGE OverloadedStrings #-}

module SimpleThings where

import Control.Applicative ((<$>), (<*>))
import Control.Monad (mzero)
import Data.Aeson
import qualified Data.Aeson as A

-- | sum type containing all possible payloads
data Payloads = FooPayload Int String
              | InviteRequestPayload String
              | IdentityValidationRequestPayload Int Int String

-- | dispatch on the value of `"msg_type"`
instance FromJSON Payloads where
    parseJSON (Object v) = v .: "msg_type" >>= handlePayloadType
          where
            -- handle foo_payload key
            handlePayloadType (A.String "foo_payload") =
                v .: "data" >>= \sub -> FooPayload <$> sub .: "id" <*> sub .: "msg"

            -- handle invite_request key
            handlePayloadType (A.String "invite_request") =
                v .: "data" >>= \sub -> InviteRequestPayload <$> sub .: "msg"

            -- handle identity_validation_data key
            handlePayloadType (A.String "identity_validation_data") =
                v .: "data" >>= \sub -> IdentityValidationRequestPayload <$> sub .: "from"
                                                                         <*> sub .: "to"
                                                                         <*> sub .: "validation_msg"

            -- default
            handlePayloadType _ = mzero
    parseJSON _                 = mzero