MRP Summary & revised MRP 2ed (was: Monad of no `return` Proposal (MRP): Moving `return` out of `Monad`)

[Akio Takano]

Hi Herbert,

On Thu, Nov 5, 2015 at 10:46 AM, Herbert Valerio Riedel <hvr at gnu.org> wrote:
> Hello everybody,
>
> Based on the feedback gathered from the discussion, the proposal has
> been revised to address the raised concerns. The highlights are:
>
>  - A new stretched out transition scheme complying with the recently
>    enacted 3-release policy (and beyond) has been devised.
>
>  - Unifying `>>`/`*>` has been incorporated into the proposal in the
>    interest of bundling changes of obviously related changes.
>
>  - Moreover, the feasibility of automatic refactoring tooling was
>    investigated and resulted in the working `Hs2010To201x`
>    proof-of-concept.
>
> Please re-read the revised proposal at
>
>   https://ghc.haskell.org/trac/ghc/wiki/Proposal/MonadOfNoReturn
>
> for more details (or use the Wiki's diffing feature to see what changed
> relative to the original revision) if you want to comment, so we can
> focus on discussing the actual revised version.

I'm sorry to join the discussion so late, but I'd like to mention one
thing that doesn't seem to have been brought up. If I understand
correctly, this proposal can silently slow down existing programs,
sometimes even asymptotically. This applies when a monad is defined
with no explicit definition for (>>) nor for (*>).

The problem is this: currently, when a Monad instance lacks an
explicit definition for (>>), a default implementation based on (>>=)
is used. After this proposal, (>>) would be an alias for (*>), for
which there is a default implementation based on (<*>). However, at
least for some monads, the former is a much better default.

[1] is one example where it makes an asymptotic difference in runtime.
This type of difference arises when (>>=) has to linearly traverse its
LHS, but not its RHS.

[1] https://ghc.haskell.org/trac/ghc/ticket/10711#comment:1

I also constructed another example [2]. This is a standard
implementation of the strict State monad, except that (>>=) is
implemented as a NOINLINE function. You can see that using (*>) in
place of (>>) changes the memory usage of the program from constant to
linear. The difference here arises from the fact that the default
implementation for (>>) can "tail-call" its RHS as long as (>>=) also
has this tail-call property, but the default implementation of (*>)
cannot.

[2] https://gist.github.com/takano-akio/7066c511b60d6ab090c5

In my opinion introducing this kind of performance regression is quite
bad. Although I agree that it's frustrating that we are stuck with
mapM, mapM_ etc., I believe this should be fixed in a way that doesn't
make (>>) slower by default.

Regards,
Takano Akio

>
> Also, as per proposal guidelines, and more importantly, for the benefit
> of those that lost track of this rather complex discussion, I've tried
> to summarize the core of discussion over at
>
>   https://ghc.haskell.org/wiki/Proposal/MonadOfNoReturn/Discussion
>
> More importantly, based on feedback gathered as well as concerns raised
> throughout the discussion, I've revised and extended the proposal into a
> "2nd edition MRP". I feel confident the new revised proposal addresses
> the major concerns as well as adhering to the recently requested 3-yr
> compatibility policy.
>
> PS: One common oversight I noticed when reviewing the discussion is
>     that the last-minute proposal addition -- of unifying `>>`/`*>` in
>     the same vein as `pure`/`return` -- wasn't noticed by many who
>     joined the debate late.
>
>     However, Unifying `>>`/`*>` and `pure`/`return` are in my opinion
>     strongly related concerns as they share the same rationale about
>     correctness issues and it doesn't make sense to do one without the
>     other. However, the transition time-scales could be set slightly
>     different to account for the more breaking nature of effectively
>     changing `>>`'s current default method operational semantics.
>
> Thanks,
>   Herbert
>
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