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

[Matthías Páll Gissurarson]

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+ The impact assessment is stale, and the original motivation outdated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/
>>>
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>

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