Expression such as boundVariable + 10 fails on Gremlin Server #18

import Control.Exception.Safe (bracket, try, SomeException)
import Data.Foldable (toList)
import Data.Greskell.Greskell (Greskell) -- from greskell package
import Data.Greskell.Binder -- from greskell package
  (Binder, newBind, runBinder)
import Network.Greskell.WebSocket -- from greskell-websocket package
  (connect, close, submit, slurpResults)
import System.IO (hPutStrLn, stderr)

submitExample :: IO [Int]
submitExample =
  bracket (connect "localhost" 8182) close $ \client -> do
    let (g, binding) = runBinder $ plusTen 50
    result_handle <- submit client g (Just binding)
    fmap toList $ slurpResults result_handle

plusTen :: Int -> Binder (Greskell Int)
plusTen x = do
  var_x <- newBind x
  return $ var_x + 10

main = hspec $ specify "submit" $ do
  egot <- try submitExample :: IO (Either SomeException [Int])
  case egot of
    Left e -> do
      hPutStrLn stderr ("submit error: " ++ show e)
      hPutStrLn stderr ("  We ignore the error. Probably there's no server running?")
    Right got -> do
      hPutStrLn stderr ("submit success: " ++ show got)
      got `shouldBe` [60]