I’m generally strongly supportive of this proposal.

                                                                                                               

Again, we might see some movement around the edges, but both Daan Leijen and Amr Sabry have given it a thumbs up, so I think it’s fundamentally sound.

 

There may be a few points to clarify, but fundamentally let’s accept.


Simon

 

 

From: ghc-steering-committee <ghc-steering-committee-bounces@haskell.org> On Behalf Of Simon Marlow
Sent: 23 November 2020 14:38
To: Joachim Breitner <mail@joachim-breitner.de>
Cc: ghc-steering-committee@haskell.org
Subject: Re: [ghc-steering-committee] Please review #313: Delimited continuation primops, Shepherd: Simon Marlow

 

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-primops/proposals/0000-delimited-continuation-primops.md

 

Summary

 

The proposal makes no language changes, it only adds three primops.

 

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-continuations?view=inline

 

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.
  • 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

Thoughts?

 

 

 

On Sat, 12 Sep 2020 at 22:59, Joachim Breitner <mail@joachim-breitner.de> 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-primops/proposals/0000-delimited-continuation-primops.md

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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