December 2018 - Haskell-prime

LAST CALL to comment on the Appicative/Monad Proposal
by Mario Blažević 02 Apr '19

02 Apr '19

The very first RFC created (https://github.com/haskell/rfcs/pull/1) the Applicative/Monad Proposal, has now reached the Last Call stage. In order to ground the discussion, I have taken some time to update the Prelude and the text of the Haskell Report with its effects before the call. The rendered report is available at https://github.com/blamario/rfcs/blob/amp/report/report/haskell.pdf for your review. TL;DR: The proposed changes to the report add the latest design of the Applicative and Alternative classes, but otherwise are intentionally minimal. Any further modifications, like the MonadFail proposal or moving return out of the Monad class, should be relegated to new RFCs. In some more detail, the changes are: 1. Applicative has been added as a subclass of Functor and superclass of Monad, its methods and laws as currently defined in the base library. The class and all its methods (pure, (<*>), (<*), (*>), and liftA2) are exported from Prelude, but no other Applicative-related functions (like liftA3) are. 2. The Functor class definition has been moved from module Control.Monad to Control.Applicative in order to avoid circular imports. Note that neither module is a part of the language specification. 3. The Monad class has been left unmodified, apart from making Applicative its superclass and adding return a == pure a as a law. 4. Alternative has been added to the Control.Applicative module, but not to Prelude. This is the same treatment already meted to MonadPlus. I'm unsure why MonadPlus even exists in the report, as it has no relevance to the language specification, and I would gladly remove both classes. Please take some time within the following three weeks (including some extra allowance for the upcoming holiday breaks) to vote for or against the proposal, or to leave a comment with suggestions for its improvement.

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Re: LAST CALL to comment on the Appicative/Monad Proposal
by Doug McIlroy 18 Dec '18

18 Dec '18

I am very glad to see Applicative take its place in the report: one less mystery in understanding Haskell in the wild. The following comments pertain to presentation. 13.1 Functor class "The Functor class is used for types that can be mapped over." "is used for" is extremely vague. Better wording would be "The functor class comprises types that can be mapped over." The same comment applies to section 6.3.5. 13.2 Applicative class A verbal hint about the mnemonic intent of liftA2, like those for liftA and liftA3 in 13.4, would be helpful. Why is the "methods" subsection empty? 13.3 Alternative class This class hasn't been fully integrated into the report. It is missing from the Standard Prelude (Section 9) and from its natural place after 6.3.6. <|> appears here without definition and is not in the index. Why is the "methods" subsection empty? Doug McIlroy

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Re: Helium II
by Anthony Clayden 03 Dec '18

03 Dec '18

On *Thu Nov 29 15:00:19 UTC 2018, Doug McIlroy wrote:* > >* Frankly after you've filtered out all that, I'd be astonished if you** have anything left. It seems to be usual practice on Hackage to switch** on a swag of LANGUAGE pragmas even if this module isn't using them.* >>* (MPTCs, FlexibleInstances/Contexts in particular.)* > A depressing remark, which comes close to saying > there is no true Haskell. ? There wasn't a true Haskell in 1998. Already Hugs (in HugsMode) and GHC (with glasgow-extns) had MPTCs, FlexibleInstances/Contexts, FunDeps, UndecidableInstances. Overlapping instances was optional extra. > It's one reason I hang mostly with Hugs. Interesting. Does that mean Hugs98? How on earth do you manage without at least some of the above? > GHC pretty well repudiates Haskell 2010. So does Hugs (in HugsMode): a large proportion of the extra features in H2010 were suggested by the Hugs teams; and Hugs 2006 was already far advanced beyond it. > May Haskell 2020 receive more respect! I think you'll need to adjust your expectations: I doubt H2020 will happen. There was a glimmer of activity last month, but this list has resumed its moribund state. So I think we can put last month down to dead cat bounce. If H2020 does happen, HugsMode will still be well ahead of whatever gets added. AntC

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