AesonGenerics.hs

data Country = US
  deriving (Eq, Generic, Read, Show, Typeable)

instance ToJSON Country where
  toEncoding = genericToEncoding $ defaultOptions {
                   constructorTagModifier = fmap toLower
                 }

instance FromJSON Country where
  parseJSON = genericParseJSON $ defaultOptions {
                   constructorTagModifier = fmap toUpper
                 }