The article is kind of overcomplicating things in favor of really trying preserve `forever`.

Here's an easy one with `whileM` (the one parameter version that is effectively a do-while loop, where the while condition is the expression's return value, there is a two parameter version that takes an independent boolean parameter).

  main :: IO ()
  main = do
    wsUrl <- fetchConnectionUrl
    conn <- connectWebSocket wsUrl

    whileM $ do
      message <- readMessage conn

      case message of
        MessageA val -> putStrLn "Message A" $> True
        MessageB val -> putStrLn "Message B" $> True
        Disconnect -> pure False

EDIT: ah I read a little too quickly. The author wants to have arbitrary breaks in the while loop. In that case you can use https://hackage.haskell.org/package/loop-while-1.0.0/docs/Co... which is morally the same thing without the `mzero` jargon (which really is just a generalized `break` here). The other alternative is to just stick with whileM and just normal structured programming if-else blocks that eventually end up returning true or false.