On Thu, Nov 26, 2015 at 3:28 AM, Akio Takano
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. 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. -Edward