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

Sat Jul 17 11:21:10 UTC 2021

Hi Richard - I'm curious about the coding style that you would advocate
that uses it often. Can you give an example?

Personally I thought I would use \case when we introduced it, but in
practice I find I want it rarely enough that I actually don't use it even
in the places where it would be marginally shorter, purely because I'm
optimising for code readability and I don't think \case helps (one could go
either way on that though).

I do find MultiWayIf to be useful in practice - it looks nice, its meaning
is obvious, and it helps code readability. But as I've said before I would
never use it if it was called \cases :)

Cheers
Simon

On Sat, 17 Jul 2021 at 03:29, Richard Eisenberg <lists at richarde.dev> wrote:

> 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 <ghc-steering-committee at haskell.org> 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_Qe1vacAIKnDs/edit?usp=sharing>,
> 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 <simonpj at microsoft.com>
> *Sent:* 28 June 2021 10:45
> *To:* ghc-steering-committee at haskell.org
> *Cc:* Simon Peyton Jones <simonpj at 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 <ghc-steering-committee-bounces at haskell.org
> > *On Behalf Of *Simon Peyton Jones via ghc-steering-committee
> *Sent:* 15 June 2021 13:52
> *To:* Joachim Breitner <mail at joachim-breitner.de>;
> ghc-steering-committee at 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-lambda-layout.md
>    <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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