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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">I’m generally strongly supportive of this proposal.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">There may be a few points to clarify, but fundamentally let’s accept.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><br>
Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-steering-committee <ghc-steering-committee-bounces@haskell.org>
<b>On Behalf Of </b>Simon Marlow<br>
<b>Sent:</b> 23 November 2020 14:38<br>
<b>To:</b> Joachim Breitner <mail@joachim-breitner.de><br>
<b>Cc:</b> ghc-steering-committee@haskell.org<br>
<b>Subject:</b> Re: [ghc-steering-committee] Please review #313: Delimited continuation primops, Shepherd: Simon Marlow<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Committee,<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
We have been asked to review <br>
<br>
<o:p></o:p></p>
<h2>#313: Delimited continuation primops<o:p></o:p></h2>
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<p class="MsoNormal"><a href="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" target="_blank">https://github.com/ghc-proposals/ghc-proposals/pull/313</a><br>
<a href="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" target="_blank">https://github.com/lexi-lambda/ghc-proposals/blob/delimited-continuation-primops/proposals/0000-delimited-continuation-primops.md</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>Summary</b><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The proposal makes no language changes, it only adds <a href="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">
three primops</a>.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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:
<a href="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">
https://gitlab.haskell.org/lexi.lambda/ghc/-/compare/master...first-class-continuations?view=inline</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>Decision</b><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I'm going to tentatively recommend acceptance. <o:p></o:p></p>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
This is a sensible choice for the primtives, being the most general of the alternatives, as
<a href="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">
explained in the proposal.</a><o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
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.<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
I suspect there may be some difficulties around unsafePerformIO, so I <a href="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">
raised that on the github thread</a><o:p></o:p></li></ul>
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<p class="MsoNormal">Thoughts?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Sat, 12 Sep 2020 at 22:59, Joachim Breitner <<a href="mailto:mail@joachim-breitner.de">mail@joachim-breitner.de</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Dear Committee,<br>
<br>
this is your secretary speaking:<br>
<br>
Delimited continuation primops<br>
has been proposed by Alexis King<br>
<a href="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" target="_blank">https://github.com/ghc-proposals/ghc-proposals/pull/313</a><br>
<a href="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" target="_blank">https://github.com/lexi-lambda/ghc-proposals/blob/delimited-continuation-primops/proposals/0000-delimited-continuation-primops.md</a><br>
<br>
I’ll propose Simon Marlow as the shepherd.<br>
<br>
Please guide us to a conclusion as outlined in <br>
<a href="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" target="_blank">https://github.com/ghc-proposals/ghc-proposals#committee-process</a><br>
<br>
Thanks,<br>
Joachim<br>
-- <br>
Joachim Breitner<br>
<a href="mailto:mail@joachim-breitner.de" target="_blank">mail@joachim-breitner.de</a><br>
<a href="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" target="_blank">
http://www.joachim-breitner.de/</a><br>
<br>
<br>
_______________________________________________<br>
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