Re: [ghc-steering-committee] Proposal #302: Multiway lambda: time to vote

I'm struggling with this one, so I'll procrastinate by asking two clarifying questions: 1. Why does (2) not subsume MultiWayIf? The example in the MultiWayIf note seems to work just fine with syntax (2). 2. (2) and (4) seem to lose \case{}, which is useful in practice. Am I correct in saying that we'll lose \case{}? I said I'm struggling, so I will explain why: I really want this new syntax, and I would advocate a coding style that uses it often. But I'm quite concerned about language bloat, and I don't like adding a new construct without also planning to remove redundant ones. I'm not sure how to balance these two desires. Perhaps a night's sleep will yield insight. Richard

On Jul 16, 2021, at 11:21 AM, Simon Peyton Jones via ghc-steering-committee wrote:

Friends

Sorry to be slow on #302: multi-way lambda. I was diverted by the POPL deadline.

I have not heard from Tom or Vitaly, I think, but I’ll take silence for agreement with the ballot list.

So it is time to vote!

Please go here https://docs.google.com/document/d/1NDXk5kKcLtkqhkSNESAC9jVrBn3yqS_Qe1vacAIK..., and vote.

Feel free to add pros/cons to the list on that page.

Please vote by next Thursday, 22 July, at the latest. It’s a balance of judgement, for sure, not technically complicated.

Simon

From: Simon Peyton Jones mailto:simonpj@microsoft.com> Sent: 28 June 2021 10:45 To: ghc-steering-committee@haskell.org mailto:ghc-steering-committee@haskell.org Cc: Simon Peyton Jones mailto:simonpj@microsoft.com> Subject: RE: [ghc-steering-committee] Proposal #302: `\of` (New Shepherd: Simon PJ)

Dear Steering Committee

Two weeks ago I asked

Are there any other alternatives you strongly want on the ballot?

I got these responses

Joachim, Simon, Alejandro, Arnaud: nothing to add Vitaly, Eric, Tom, Richard, Vlad: no response I’d love to hear from the five of you, please. I want to get a decision on this, and I can’t do that if I don’t hear from you.

Thanks

Simon

From: ghc-steering-committee mailto:ghc-steering-committee-bounces@haskell.org> On Behalf Of Simon Peyton Jones via ghc-steering-committee Sent: 15 June 2021 13:52 To: Joachim Breitner mailto:mail@joachim-breitner.de>; ghc-steering-committee@haskell.org mailto:ghc-steering-committee@haskell.org Subject: Re: [ghc-steering-committee] Proposal #302: `\of` (New Shepherd: Simon PJ)

| I’d like to reassing shepherding of this one. | | It seems to be clear that we want “something like this”, there are many ways | to skin the cat, so it comes down to opinion and what we need is a decision | (or a call to votes). As with anything that’s possibly quite opinionated, | it’s good to have an authorative voice, so in this case, Simon PJ. | | Simon, can you either come up with a “all things considered, I think this | variant is the (narrowly) the best” recommendation or, alternative, a | “please vote on the following options” verdict?

OK, to remind everyone Here is the proposal: https://github.com/JakobBruenker/ghc-proposals/blob/patch-1/proposals/0000-l... https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FJakobBruenker%2Fghc-proposals%2Fblob%2Fpatch-1%2Fproposals%2F0000-lambda-layout.md&data=04%7C01%7Csimonpj%40microsoft.com%7C4ae55d76732448a0c3a508d93a1955ed%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637604702751348160%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=Hvxpaew8S333OX7IvW9cfD003mLSZxpjRdjOotGVZMA%3D&reserved=0 Here is the discussion: https://github.com/ghc-proposals/ghc-proposals/pull/302 https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F302&data=04%7C01%7Csimonpj%40microsoft.com%7C4ae55d76732448a0c3a508d93a1955ed%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637604702751358154%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=1aYsUzlUaixOxNvW2%2FEkLNbImFD7yLBT273QFA76ZEI%3D&reserved=0

The basic idea is to extend to lambda all the facilities that you get with function definitions, especially multiple patterns and guards. This seems clearly a good idea, whose only obstacle is syntactic. There are no conceptual or specification challenges. The only point at issue is that of concrete syntax.

The proposal offers four possible syntactic options. After reviewing, I propose to discard (2) and (3) leaving these alternatives

Option (1) \cases { p1 p2 -> rhs1; q1 q2 -> rhs2 } Lives alongside \case, but allows multiple patterns Other keywords are possible, but I think it must be a variant on \case Option (4) Same, but use \case as the keyword Incompatible with existing \case => extended transition period, unhappy users \case { (Just x) -> rhs1; Nothing -> rhs2 } will require parens forever, which in the common case of a one argument lambda see clunky. Option (X). Reject the proposal.

Personally I favour (1). I’m relaxed about having multiple ways of saying the thing (think of let vs where), and I see no harm provided the two constructs look and behave the same. I’ve decided I like \cases precisely because it’s the plural of \case, which is exactly what is going on.

I think we’ll end up having to vote on this, which is fine when it’s a judgement call about syntax. But first:

Are there any other alternatives you strongly want on the ballot? I say “strongly” because I don’t want to open up a big new debate… we at the stage of trying to narrow options.

Thanks

Simon

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