[Haskell-cafe] Best way to implement "scoped exceptions"?

Tom Ellis tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk
Thu Dec 29 15:41:22 UTC 2022

On Thu, Dec 29, 2022 at 01:31:47PM +0000, Li-yao Xia wrote:
> The recently implemented Delimited Continuation Primops proposal[1] features
> tagged prompts. That seems like just what you are looking for.
> [1]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0313-delimited-continuation-primops.rst

Very interesting, thanks!  My initial attempt at implementing scoped
exceptions using the delimited continuation primops was bad:

    withScopedExceptionBad ::
      ((e -> IO (Either e r)) -> IO r) -> IO (Either e r)
    withScopedExceptionBad body = do
      promptTag <- newPromptTag
      prompt promptTag $ do
        l <- control0 promptTag $ \myThrow -> do
          r <- body (myThrow . pure)
          pure (Right r)
        pure (Left l)
It was very head-scratching trying to work out how it should be
implemented.  Then with the help of a gist by sebfisch[1] I managed

    withScopedException ::
      ((forall a. e -> IO a) -> IO r) -> IO (Either e r)
    withScopedException body = do
      promptTag <- newPromptTag
      prompt promptTag $ do
        r <- body (\e -> control0 promptTag (\_ -> pure (Left e)))
        pure (Right r)

Surprisingly to me, for scoped exceptions, the handler passed to
control0 should *ignore* its argument.  That's because its argument
allows it to escape from the call to control0, but we want to escape
from the call to prompt.  This is very powerful and mind-bending stuff
and allows me to do exactly what I wanted.

Thanks Li-yao!


[1] https://gist.github.com/sebfisch/2235780

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