Re: LAST CALL to comment on the Applicative/Monad Proposal

[Herbert Valerio Riedel]

Hello Mario et al.,

On Tue, Dec 18, 2018 at 4:17 AM Mario Blažević  wrote:
> While you're reviewing AMP, please take a bit of time to also comment on
> the related new MonadPlus excise proposal at
> https://github.com/haskell/rfcs/pull/23
>
> The proposal is very short so it should be an easy decision. Thank you.

As I've explained already on the RFC, I refuse (i.e. to make this very
clear: I'm exercising my veto) to accept the AMP change without also
at least a commitment to MFP and MRP. And note that the committee was
created with the explicit request from SPJ that it ought to be a
consensus-based decision process rather than a democratic voting
process -- and there isn't consensus in this matter as far as I'm
concerned.

The discussion started on the right foot with Richard rightly saying

> The redundant functions (like return) are all well and good in an 
> implementation such as GHC, but I think they should be struck from the 
> Report. Let's design it like we should have from the beginning, cleanly. GHC 
> could then include its non-conformance with this new standard as an 
> infelicity. Perhaps GHC can even deprecate return, but I don't have a problem 
> with just living with the infelicity.

That's the ideal we should strive for and what I expected the
discussion to follow-through but unfortunately it got lost along the
way.

Herbert
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Re: LAST CALL to comment on the Applicative/Monad Proposal

[Mario Blažević]

While you're reviewing AMP, please take a bit of time to also comment on 
the related new MonadPlus excise proposal at 
https://github.com/haskell/rfcs/pull/23


The proposal is very short so it should be an easy decision. Thank you.


On 2018-12-15 6:46 p.m., Mario Blažević wrote:
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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