[ghc-steering-committee] Please review #687: Monad of No Return

Wed Apr 16 10:00:22 UTC 2025

Since Friday, the pull request has been marked as a draft. I've informed
the author that we're waiting on CLC feedback, and that we'll leave
the proposal as "under revision" until then.

On Mon, 14 Apr 2025 at 03:42, Arnaud Spiwack <arnaud.spiwack at tweag.io>
wrote:

> I also agree that _if_ we do the changes that the proposal wants, then the
> revised timeline/steps sound better than the original.
>
> On Sat, 12 Apr 2025 at 05:39, Matthías Páll Gissurarson via
> ghc-steering-committee <ghc-steering-committee at haskell.org> wrote:
>
>> I agree, we should wait for the CLC before voting on this proposal.
>>
>> To summarize:
>>
>> 10 years ago, the CLC voted on the "Monad of No Return" proposal, which
>> removes `return` and `(>>)` from `Monad`, to be replaced by `pure` and
>> `(*>)` from `Applicative`, with `return` and `(>>)` being replaced with
>> top-level bindings. The implementation of this was to proceed in multiple
>> phases:
>> + Phase 1 (implemented in 8.2, default in 9.2) that warns when `return`
>> and `(>>)` are overwritten with non-canonical implementations (something
>> other than `pure` and `(*>)`)
>> + Phase 2: `return` and `(>>)` become top-level bindings and GHC ignores
>> canonical implementation of these
>> + Phase 3: Warn about all `return` and `(>>)` overwrites
>> + Phase 4: Disallow overwrites of `return` and `(>>)`, removing them from
>> Monad.
>>
>> This proposal proposes merging these into 2 phases:
>> + Phase 2: Make non-canonical implementations of `(>>)` and `return` a
>> compilation error, and add a warning for canonical ones
>> + Phase 3: Move `return` and `(>>)` to the top-level and remove them from
>> Monad.
>>
>> I'm in favor of *this specific change*, i.e. reducing the phases and
>> easing migration.
>>
>> However, there are multiple concerns raised with the original MoNR
>> proposal, as compiled by Julian:
>> + `(*>)` is not as performant as `(>>)` in some cases, which affects a
>> large portion of the ecosystem, as this is an "unseen default".
>> + The impact assessment is stale, and the original motivation outdated
>> + There are some corner cases that mean that behavior might potentially
>> change, which is worrying
>>
>> So there's a question of whether we should keep going with the "Monad of
>> No Return" proposal at all (of which this proposal is a modification), and
>> if so, in what form we should do so.
>>
>> As Simon points out, the CLC is discussing this, and I agree that the
>> time is not right for a vote at the moment. We should think about the
>> original proposal, and I encourage you to weigh in on the [CLC discussion](
>> https://github.com/haskell/core-libraries-committee/issues/325) (it's
>> "internal", but multiple observers have already chimed in)
>>
>> Unless anyone disagrees in the next 2 days (i.e. by Monday), I will
>> inform the authors of this proposal of this, and suggest that we move the
>> proposal back to "under revision" until we've gotten feedback from the CLC.
>>
>>
>> On Fri, 11 Apr 2025 at 10:01, Simon Peyton Jones <
>> simon.peytonjones at gmail.com> wrote:
>>
>>> Can I draw your attention to
>>> https://github.com/haskell/core-libraries-committee/issues/325#issuecomment-2796062617
>>> ?
>>>
>>> The CLC plan to revisit the proposal, and while continued discussion
>>> here is fine, I don't think we could act until the CLC concludes.  My
>>> thought: let's move this back to "under revision" rather than shepherd it
>>> to a conclusion.
>>>
>>> We (maybe Adam or Matthias?) should write to the authors to explain
>>> this.  In a sympathetic and supportive way -- I think we all want to see
>>> Haskell progress towards beauty and not just keep bad warts; but it's just
>>> a tricky one.
>>>
>>> Simon
>>>
>>> On Fri, 11 Apr 2025 at 08:50, Matthías Páll Gissurarson via
>>> ghc-steering-committee <ghc-steering-committee at haskell.org> wrote:
>>>
>>>> Thank you all! This is a slightly nuanced one, so I’ll write a summary
>>>> today for us to vote on.
>>>>
>>>> /Matti Palli
>>>>
>>>>
>>>> On Tue, Apr 8, 2025 at 09:39 Adam Gundry via ghc-steering-committee <
>>>> ghc-steering-committee at haskell.org> wrote:
>>>>
>>>>> Dear Committee,
>>>>>
>>>>> Benjamin proposes to revive and move forward the Monad of No Return
>>>>> proposal:
>>>>>
>>>>> https://github.com/ghc-proposals/ghc-proposals/pull/687
>>>>>
>>>>> https://github.com/L0neGamer/ghc-proposals/blob/amending-mrp/proposals/0000-amending-mrp.rst
>>>>>
>>>>> I'd like to nominate Matthías as the shepherd.
>>>>>
>>>>> Please guide us to a conclusion as outlined in
>>>>> https://github.com/ghc-proposals/ghc-proposals#committee-process
>>>>>
>>>>> Cheers,
>>>>>
>>>>> Adam
>>>>>
>>>>> --
>>>>> Adam Gundry, Haskell Consultant
>>>>> Well-Typed LLP, https://www.well-typed.com/
>>>>>
>>>>> Registered in England & Wales, OC335890
>>>>> 27 Old Gloucester Street, London WC1N 3AX, England
>>>>> <https://www.google.com/maps/search/27+Old+Gloucester+Street,+London+WC1N+3AX,+England?entry=gmail&source=g>
>>>>> _______________________________________________
>>>>> ghc-steering-committee mailing list
>>>>> ghc-steering-committee at haskell.org
>>>>>
>>>>> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>>>>>
>>>> _______________________________________________
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>>>>
>>>
>>
>> --
>> --  Matthías Páll Gissurarson <http://mpg.is/>
>> _______________________________________________
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>>
>
>
> --
> Arnaud Spiwack
> Director, Research at https://moduscreate.com and https://tweag.io.
>

-- 
--  Matthías Páll Gissurarson <http://mpg.is/>
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