Re: [ghc-steering-committee] Please review #313: Delimited continuation primops, Shepherd: Simon Marlow

I like delimited continuations a lot. They are a fantastic tool of delicious sweetness. So I'm delighted at the idea of delimited continuations coming to GHC. I may not be very objective here. I haven't looked at the proposal for a while. But, last I checked, I found it rather convincing. I have little opinion about the implementation, which touches parts of GHC of which I know very little. But if Simon and Simon, you both find the prototype reasonable, I'll be quite happy to accept the proposal. On Mon, Nov 23, 2020 at 3:38 PM Simon Marlow wrote:

Committee,

We have been asked to review #313: Delimited continuation primops https://github.com/ghc-proposals/ghc-proposals/pull/313

https://github.com/lexi-lambda/ghc-proposals/blob/delimited-continuation-pri...

*Summary*

The proposal makes no language changes, it only adds three primops https://github.com/lexi-lambda/ghc-proposals/blob/delimited-continuation-pri... .

The main motivation is to support building efficient implementations of Algebraic Effect systems, which depend on being able to efficiently capture a continuation. Currently this is done explicitly, which imposes a severe performance penalty.

These primops are the minimal support needed to be able to capture a continuation and apply it at runtime, together with some basic type safety via the PromtTag type to ensure that at least we don't replace a continuation with a computation of a different type. (there are other ways to go wrong with these primops though, they're not a safe interface by themselves: they need to be wrapped in a safe library).

The primops are implemented by copying chunks of stack into the heap. This is something that GHC's runtime already does a lot of, so it's not a new concept, although it does require a new closure type and knock-on changes across several files in the runtime (though it's mainly mechanical). There's a prototype implementation here: https://gitlab.haskell.org/lexi.lambda/ghc/-/compare/master...first-class-co...

*Decision*

I'm going to tentatively recommend acceptance.

- This is a sensible choice for the primtives, being the most general of the alternatives, as explained in the proposal. https://github.com/lexi-lambda/ghc-proposals/blob/delimited-continuation-pri... - Would the new primops impose a significant ongoing maintenance burden? Having looked at the patch, although there are some missing pieces, I don't think the new concepts impose any significant new requirements on other parts of the runtime. - I suspect there may be some difficulties around unsafePerformIO, so I raised that on the github thread https://github.com/ghc-proposals/ghc-proposals/pull/313#issuecomment-7321819...

Thoughts?

On Sat, 12 Sep 2020 at 22:59, Joachim Breitner wrote:

...

Dear Committee,

this is your secretary speaking:

Delimited continuation primops has been proposed by Alexis King https://github.com/ghc-proposals/ghc-proposals/pull/313

https://github.com/lexi-lambda/ghc-proposals/blob/delimited-continuation-pri...

I’ll propose Simon Marlow as the shepherd.

Please guide us to a conclusion as outlined in https://github.com/ghc-proposals/ghc-proposals#committee-process

Thanks, Joachim -- Joachim Breitner mail@joachim-breitner.de http://www.joachim-breitner.de/

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