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Comment by hardwaregeek | Hacker News Reader

Does Roc have escape hatches for mutability? That's something I value in Rust, good escape hatches from the borrow checker, so if something gets too difficult, you can just `Arc<Mutex<RefCell<T>>>` it.

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I don't think so (could be wrong), but it does perform optimistic mutation on your functional code.

There is a talk somewhere where Richard "demos" this.

yeah that seems cool! I'm a little skeptical of pure functional programming with no escape hatch. I rarely need the escapes in Rust but when I do, I really do.

It isn't "no escape hatch". It is platform dependent escape hatches. A platform can give you full access to mutation and libffi if it wants.

It should be possible to have a platform that has a mutable reference with an API like this:

    main = 
        x = pf.NewRef!
        pf.Write! x 5
        y = pf.Read! x

That's right, although the Rust equivalent of that would be more like `Future<Arc<Mutex<RefCell<T>>>>` than `Arc<Mutex<RefCell<T>>>` - so the ergonomics would be a bit different!

Haskell:

    import Data.IORef
    
    main :: IO ()
    main = do
      x <- newIORef (0 :: Int)
      writeIORef x 5
      y <- readIORef x
      print y

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