fail.hs

{-# LANGUAGE OverloadedStrings #-}
import Data.Aeson

data Name = Name {
    firstName :: String,
    lastName :: String
} deriving (Show, Eq)

instance FromJSON Name where
  parseJSON = withObject "Name" $ \v -> Name
    <$> v .: "firstName"
    <*> v .: "lastName"

main = print (decode "{\"firstName\": \"Rachel\", \"lastName\": true}" :: Maybe Name)