Bluefin prevents handles leaking

4

It’s really nice to see this trick to be used in an effect system, I think it’s way underused.
Maybe something that you might not immediately realize, but this also works outside of bluefin. E.g. if you have IO, you can write a newtype ScopedHandle s = UnsafeScopedHandle {getHandle :: Handle} and a function withScopedHandle :: FilePath -> (forall s. ScopedHandle s -> IO r) -> IO r and you will not be able to escape the handle out of the continuation.
Take this in contrast to the usual ways of preventing something like this, like:

  • just using a continuation: make pure the continuation, here you have your handle
  • make the continuation return IO () instead of IO r: make an IORef, write the handle to the IORef, here you have your handle
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