LAST CALL to comment on the Appicative/Monad Proposal

Tue Dec 18 15:07:09 UTC 2018

Getting to ditch a similar connotation in do notation is exactly why 
I've started this thread, of course! Some instances are far less about 
having pure/return, at least without some more powerful constructs we 
can't have in a standardised Haskell just yet. It's maybe a personal 
irritation that ApplicativeDo outright fails in that situation.

I was slightly appalled when I realised just now what pronunciations of 
*> and <* I could most easily justify if nobody else comes up with a 
suggestion - I think <* might be "klap".

I do think there's value in wondering whether there are small changes to 
GHC we might request if nobody else does it. But that's partly because 
the Report has to be descriptive by default - otherwise there's no good 
institutional eye on neatening things up over time that has an eye on 
standards rather than the concerns that drive GHC.

Fundamentally, if I'd wanted to drag up MRP rather than round off an 
edge, I would've done so explicitly! If someone wants to tell me where 
to nag the GHC folks I guess I might find the time. But I'm definitely 
looking at removing warts in the context of things that are already 
happening myself.

On 18/12/2018 14:52, Cale Gibbard wrote:
> Is it just me, or is all the discussion in these threads much more 
> easily resolved if the Report is simply a report? Describe what is, 
> rather than what you wish it was, and there's much less room for 
> disagreement. A future Report can describe the way that these things 
> work differently in the future when the changes actually happen in the 
> implementation(s).
>
> I don't know about anyone else, but at least from my perspective the 
> value of the Report is in being valid documentation. The extent to 
> which it fails to describe the actual family of languages we're 
> presently writing code in is the extent to which it is failing to be a 
> useful resource for our daily lives.
>
> I dunno, I feel kind of strange when people talk about removing 
> 'return' for example, since it's very unlikely to go anywhere any time 
> soon given how much code it is referenced in. It would be practically 
> unreasonable even to try to deprecate it. Given that the Report is 
> going to discuss this part of the language, it makes sense that it 
> should be documented.
>
> Secondarily, it's a bit hard to describe why this is, but I personally 
> find it a bit obnoxious whenever someone uses 'pure' rather than 
> 'return' if the functor is known to be an instance of Monad and the 
> generality isn't needed. It's a kind of signal that the code we're 
> writing is (and perhaps needs to be) Applicative-polymorphic, or that 
> we lack a Monad instance. So when I see it, I'm forced to consider for 
> a moment why that might be, and whether I've understood the context 
> incorrectly (and if it's not the case, that's sort of irritating).
>
> So when I see a suggestion to remove 'return' altogether it's sort of 
> like "let's make everything mildly annoying to read forever" (by 
> forcing this thought about exactly how general the code is, and making 
> it slightly harder to guess the types at a glance).
>
> It's like while pure and return are equal whenever they would both 
> typecheck, they've come to have very different connotations about the 
> surrounding code.
>
> On Tue, Dec 18, 2018, 05:42 Philippa Cowderoy, <flippa at flippac.org 
> <mailto:flippa at flippac.org>> wrote:
>
>     I'm having a moment of fail trying to work out how to leave a comment.
>
>     Is there a reason (other than GHC not doing it yet) not to have do
>     notation use *> instead of >> in line with using the least
>     restrictive
>     function? I have some otherwise-nice logic programming code that
>     would
>     actively benefit from it and it seems like a missing step from here.
>
>     On 15/12/2018 23:46, 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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