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

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 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 + The impact assessment is stale, and the original motivation outdated + There are some corner cases that mean that behavior might potentially change, which is worrying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@gmail.com> wrote:

...

Can I draw your attention to https://github.com/haskell/core-libraries-committee/issues/325#issuecomment-... ?

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 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@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-...

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@haskell.org https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee

_______________________________________________ ghc-steering-committee mailing list ghc-steering-committee@haskell.org https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee

-- -- Matthías Páll Gissurarson http://mpg.is/ _______________________________________________ ghc-steering-committee mailing list ghc-steering-committee@haskell.org https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee

-- Arnaud Spiwack Director, Research at https://moduscreate.com and https://tweag.io.