color.hs

 {-# LANGUAGE OverloadedStrings #-}

module Main where

import Data.Aeson
import qualified Data.Aeson.Types as T
import Data.Text (Text)
import Control.Applicative ((<$>))
import Control.Monad (mzero)

import qualified Data.ByteString.Char8 as BS
import qualified Data.ByteString.Lazy.Char8 as BSL

data Msg = Msg Color deriving (Show)

data Color = Red | Blue | Green deriving (Show)

instance FromJSON Color where
  parseJSON (String "green") = return $ Green
  parseJSON (String "red") = return $ Red
  parseJSON (String "blue") = return $ Blue
  parseJSON _ = mzero
  

instance FromJSON Msg where
  parseJSON (Object v) = Msg <$> v .: "color"
  parseJSON _ = mzero
  
 
parseMsgFromString :: String -> Maybe Msg
parseMsgFromString s = let bs = BSL.pack s
                       in decode bs
                               
exampleJSONMessage :: String
exampleJSONMessage = "{\"color\":\"blue\"}"

main ::IO ()
main = do 
  print $ parseMsgFromString exampleJSONMessage