LAST CALL to comment on the Appicative/Monad Proposal

[Cale Gibbard]

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> 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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