如何在PureScript中为SimpleJSON创建自定义解析器Monad？

class Parseable a where
  parse :: SimpleJSON -> Either String a

newtype JSONParser a = JSONParser (SimpleJSON -> Either String a)

instance monadJSONParser :: Monad JSONParser where
  pure a = JSONParser (\_ -> Right a)
  (>>=) (JSONParser p) f = JSONParser (\json -> case p json of
                                                  Left err -> Left err
                                                  Right a -> let (JSONParser q) = f a in q json)

instance parseableString :: Parseable String where
  parse (JSONString s) = Right s
  parse _ = Left "Expected a string"

instance parseableNumber :: Parseable Number where
  parse (JSONNumber n) = Right n
  parse _ = Left "Expected a number"

instance parseableBool :: Parseable Boolean where
  parse (JSONBool b) = Right b
  parse _ = Left "Expected a boolean"

instance parseableObject :: Parseable (Map String Parseable) where
  parse (JSONObject obj) = traverse parse obj
  parse _ = Left "Expected an object"

instance parseableArray :: Parseable (Array Parseable) where
  parse (JSONArray arr) = traverse parse arr
  parse _ = Left "Expected an array"