Parsing JSON in Haskell

parse_json.hs

{-# LANGUAGE OverloadedStrings, DeriveGeneric #-}

import Data.Text (Text)
import Data.Aeson
import GHC.Generics
import qualified Data.ByteString.Lazy as B

data Person =
  Person { first  :: !Text
         , last   :: !Text
         , age    :: Int
           } deriving (Show, Generic)

instance FromJSON Person
instance ToJSON   Person

main :: IO ()
main = do
  d <-(eitherDecode <$> B.readFile "people.json") :: IO (Either String [Person])
  case d of
    Left err -> putStrLn err
    Right ps -> print $ map (\x -> first x) ps