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

On Thu, Nov 26, 2015 at 5:08 PM, Edward Kmett wrote:

On Thu, Nov 26, 2015 at 3:28 AM, Akio Takano wrote:

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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.

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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.

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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