thing.hs

-- For example
type JSON = Text

-- An instance of RenderResponse knows how to turn itself into JSON
class RenderResponse a where
  render ::  a -> JSON

data Error = Error Text
data RegularResponse = RegularResponse Text -- For example
data ErrorResponse a = ErrorResponse a [Error] -- Parameterize over Response type

-- We know how to render a regular response
instance RenderResponse RegularResponse where
  render (RegularResponse text) = "{ \"message\":" <> text <> " }"

-- We know how to render an error
instance RenderResponse Error
  render (Error text) = "{ \"error\":" <> text <> " }"

-- If we know how to render the response, we know how to render it with errors
instance (RenderResponse a) => RenderResponse (ErrorResponse a) 
  render (ErrorResponse response errors) = "{ \"data\": " <> render response <> ", \"errors\": " <> errors_ <>  " }"
    where errors_ = mconcat (map render errors)

--  render (ErrorResponse "response" ["IT BROKE"])
-- { "data": {"message": "response"}, "errors": [{ "error": "IT BROKE" }] }