We’ve started playing around with the rest set of libraries at work. Some of the nice things rest does:

• Routes with built in versioning
• Automatic documentation and example generation
• Generation of Ruby, Haskell and JavaScript client libraries (and probably Python and Scala additions from us, soon)

I was originally skeptical because most of the examples rely on GHC Generics and Data.Typeable which I find to be anti-modular. Luckily you can remove most of the generic programming parts and create your instances in the usual ways.

Here’s an example from our code:

data AccountError = InvalidAccountNumber String
                  | EmptyPremiseId
                  | EmptyServicePointId
                    deriving (Show, Typeable)

instance S.JSONSchema AccountError where
  schema _ =
    S.Choice [
      S.Object [
        S.Field "tag" True (S.Constant (String "invalid_account_number")),
        S.Field "account_number" True (S.Value S.unboundedLength)
      ],
      S.Object [
        S.Field "tag" True (S.Constant (String "empty_service_point_id"))
      ],
      S.Object [
        S.Field "tag" True (S.Constant (String "empty_premise_id"))
      ]
    ]


parseAccountError :: Object -> Parser AccountError
parseAccountError o = do
  tag <- o .: "tag"
  case (tag :: String) of
    "invalid_account_number" -> InvalidAccountNumber <$> o .: "account_number"
    "empty_premise_id" -> pure EmptyPremiseId
    "empty_service_point_id" -> pure EmptyServicePointId
    s -> fail $ "Unkown tag " ++ show s

instance FromJSON AccountError where
  parseJSON = withObject "AccountError" parseAccountError

instance ToJSON AccountError where
  toJSON (InvalidAccountNumber s) = object [("tag", "invalid_account_number"), ("account_number", fromString s)]
  toJSON EmptyPremiseId = object [("tag", "empty_premise_id")]
  toJSON EmptyServicePointId = object [("tag", "empty_service_point_id")]

accountErrorCreate :: Handler (ReaderT KafkaConfig IO)
accountErrorCreate = mkInputHandler (jsonI . someI) $ lift . sendAccountError

I want to try and unify the schema, parsing and pretty-printing code. Not sure if I’ll be able to do it, though.