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

Wed Dec 9 17:02:56 UTC 2020

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 at haskell.org> On Behalf Of Simon Marlow
Sent: 23 November 2020 14:38
To: Joachim Breitner <mail at joachim-breitner.de>
Cc: ghc-steering-committee at 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://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F313&data=04%7C01%7Csimonpj%40microsoft.com%7C84a3108a4ea94f00a6ba08d88fbd6bd3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C637417393293529012%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=Le0GAZdh2JEcQssZE0BT4dI0vyfI%2FomjgqLnpTAUi3o%3D&reserved=0>
https://github.com/lexi-lambda/ghc-proposals/blob/delimited-continuation-primops/proposals/0000-delimited-continuation-primops.md<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Flexi-lambda%2Fghc-proposals%2Fblob%2Fdelimited-continuation-primops%2Fproposals%2F0000-delimited-continuation-primops.md&data=04%7C01%7Csimonpj%40microsoft.com%7C84a3108a4ea94f00a6ba08d88fbd6bd3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C637417393293539008%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=ulO4Rdn%2Fn5LHXlOpfhHDds9RxwF6t1pC9jZVOQgQPzA%3D&reserved=0>

Summary

The proposal makes no language changes, it only adds three primops<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Flexi-lambda%2Fghc-proposals%2Fblob%2Fdelimited-continuation-primops%2Fproposals%2F0000-delimited-continuation-primops.md%23proposed-change-specification&data=04%7C01%7Csimonpj%40microsoft.com%7C84a3108a4ea94f00a6ba08d88fbd6bd3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C637417393293539008%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=dBgHuMm3STiaB6be9CJ5sLsuJksOUlb6kp9SB9E1R04%3D&reserved=0>.

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<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Flexi.lambda%2Fghc%2F-%2Fcompare%2Fmaster...first-class-continuations%3Fview%3Dinline&data=04%7C01%7Csimonpj%40microsoft.com%7C84a3108a4ea94f00a6ba08d88fbd6bd3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C637417393293549000%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=OHMl2XVxh6DJdDLb68I9hqC6IOTJLRzLVujTfKRfnP8%3D&reserved=0>

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://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Flexi-lambda%2Fghc-proposals%2Fblob%2Fdelimited-continuation-primops%2Fproposals%2F0000-delimited-continuation-primops.md%23operational-semantics&data=04%7C01%7Csimonpj%40microsoft.com%7C84a3108a4ea94f00a6ba08d88fbd6bd3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C637417393293549000%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=UZ4m3i95xS%2FwQXZZ5tyC6b3M%2B%2BsNW%2BzQEAaxhHviAcc%3D&reserved=0>
  *   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://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F313%23issuecomment-732181948&data=04%7C01%7Csimonpj%40microsoft.com%7C84a3108a4ea94f00a6ba08d88fbd6bd3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C637417393293558999%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=T7Bg1Cc5JWnLu5Jei4m2U%2FdGxZLTUoWWW5d0P3rR%2Bic%3D&reserved=0>
Thoughts?

On Sat, 12 Sep 2020 at 22:59, Joachim Breitner <mail at joachim-breitner.de<mailto:mail at 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://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F313&data=04%7C01%7Csimonpj%40microsoft.com%7C84a3108a4ea94f00a6ba08d88fbd6bd3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C637417393293558999%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=pMZVPIAfsh9qyoL6DL86yHyFfiPWn4naTPBy0CLI25A%3D&reserved=0>
https://github.com/lexi-lambda/ghc-proposals/blob/delimited-continuation-primops/proposals/0000-delimited-continuation-primops.md<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Flexi-lambda%2Fghc-proposals%2Fblob%2Fdelimited-continuation-primops%2Fproposals%2F0000-delimited-continuation-primops.md&data=04%7C01%7Csimonpj%40microsoft.com%7C84a3108a4ea94f00a6ba08d88fbd6bd3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C637417393293568989%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=Zw2T0xtD1ajgsbZmMR9HuWEWKW%2Fem5XXw1%2BGMJhtYDE%3D&reserved=0>

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<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%23committee-process&data=04%7C01%7Csimonpj%40microsoft.com%7C84a3108a4ea94f00a6ba08d88fbd6bd3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C637417393293568989%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=%2FGqnupqKk9ivMF5gNqqgXyO58owCNR%2BBVZ1N6JagCUM%3D&reserved=0>

Thanks,
Joachim
--
Joachim Breitner
  mail at joachim-breitner.de<mailto:mail at joachim-breitner.de>
  http://www.joachim-breitner.de/<https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.joachim-breitner.de%2F&data=04%7C01%7Csimonpj%40microsoft.com%7C84a3108a4ea94f00a6ba08d88fbd6bd3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C637417393293578982%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=j5fdBTeQJbnPWb8tdnA%2FzgDEUoIUJhkwMrg9kfrGFrI%3D&reserved=0>

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