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

27 Nov 2015

      I understand that you'd prefer it as a replacement for this plan than as a
supplement, but I don't think that that would work all that well. We
already have half a dozen combinators in Control.Monad generalized, more
are generalizing in 7.10. e.g. We don't want to introduce a 'replicateA_'
for instance as the pattern has been to generalize the existing 'M'
combinators where possible.

What you propose would leave us paralyzed in an difficult to explain middle
ground indefinitely.

As for making a thenM or thenDefault or thenDefaultByBind to make it easier
to define (*>), I'm 100% on board with that.

-Edward

On Fri, Nov 27, 2015 at 4:50 AM, Akio Takano  wrote:
...
On Thu, Nov 26, 2015 at 5:08 PM, Edward Kmett  wrote:
...
On Thu, Nov 26, 2015 at 3:28 AM, Akio Takano  wrote:
...
This is true, but I think it's much better to avoid breaking people's
code in the first place. Also, since the breakage can be silent, one
will not always be able to make a fix promptly.
We're not talking about making this change until we can get some
warnings in
place.
I would appreciate such a warning. I was concerned because I didn't
find a plan for one on the proposal page. In particular, neither the
proposed -fwarn-mrp-compat flag nor the implemented
-fwarn-noncanonical-monad-instances flag seemed to protect a
programmer from the issue of silent performance regression, when there
is no explicit definition for (*>) nor for (>>).
...
That said, in the presence of some existing combinators that have already
been generalized from Monad to Applicative you may want to ensure that
these
...
definitions have been fixed already.
...
...
In situations where (<*>) is asymptotically more efficient than (>>=)
then
the default definition in terms of (<*>) wins.
You are right. I hadn't thought about this.
...
Right now, if you run through hackage there are lots of places where
(>>)
has been manually improved but the (*>) has not -- or vice versa. We
have
two places where people should apply an optimization and many have
only
...
realized that they should optimize one or the other.
The key here is to encourage folks to actually define (*>) when it
matters.
I understand this, but perhaps there is a way to achieve this without
slowing down existing code. How about introducing a new warning
(enabled with -Wall) that is triggered when a type satisfies the
following 3 conditions?
1. The type has a Monad instance and an Applicative instance declared
in the same module, with the same set of constraints.
2. (*>) is not defined as (*>) = (>>). i.e. either it has a
non-trivial definition or its definition is left out.
3. (>>) is not defined as (>>) = (*>). i.e. either it has a
non-trivial definition or its definition is left out.
This way, people can be warned when (*>) and (>>) can share an
implementation but they don't.
This is pretty much what Herbert has been working on, except with the
definition biased in favor of (>>) = (*>) being expected, and the other
becoming a warning as that definition blows up when and if we later move
(>>) out of the class.
Probably I wasn't clear, but I actually wanted to suggest a warning as
a replacement for (the (>>)-related half of) MRP, not as a migration
path to it. If the goal is to encourage people to have a good
implementation for (*>), it may be achievable with just a warning,
with no change to the class hierarchy.
That said, I can imagine having a warning like this as a migration
path. In that case we could provide a function like
thenDefaultByBind :: (Monad m) => m a -> m b -> m b
thenDefaultByBind a b = a >>= \_ -> b
and give people an option to define
(*>) = thenDefaultByBind
in their Applicative instances.
- Akio
...
-Edward