Custom JSON errors for Servant-server

Question

When using servant, I\'d like to return all errors as JSON. Currently, if a request fails to parse, I see an error message like this, returned as plain text

Answer 1

Taking inspiration from @codedmart I also use a middleware, but it does not construct the json, it only changes the content type of the response when there is an error, and keep the original error message.

startApp :: IO ()
startApp = run 8081 . (modifyResponse errorHeadersToJson) $ serve api server

errorHeadersToJson :: Response -> Response
errorHeadersToJson r
  | responseStatus r == status200 = r
  | otherwise = mapResponseHeaders text2json r

text2json :: ResponseHeaders -> ResponseHeaders
text2json h = Map.assocs (Map.fromList [("Content-Type", "application/json")] `Map.union` Map.fromList h)

The json is built beforehand with a function overriding the Servant throwError function.

data ServerError = ServerError
  { statusCode        :: Int
  , error :: String
  , message  :: String
  } deriving (Eq, Show)

$(deriveJSON defaultOptions ''ServerError)

throwJsonError :: ServantErr -> String -> Servant.Handler b
throwJsonError err "" = throwError $ err { errBody = encode $ ServerError (errHTTPCode err) ("Server error"::String) (show $ errBody err) }
throwJsonError err message = throwError $ err { errBody = encode $ ServerError (errHTTPCode err) ("Server error"::String) message }

then I can throw any error with a custom message, it will be served as a json with the correct content-type :

throwJsonError err500 "Oh no !"