Add a parallel *> and <*

[LukaJCB]

Probably two very useful operations, however giving these a good "operator name" might be fairly difficult. For now, I'll suggest something like !> and <!, but bikeshedding very welcome!

[etorreborre]

Why not remove *> on Applicative, just use the >> from Monad and use *> for Parallel?

[tpolecat]

@LukaJCB I think !> and <! are fine … I'm looking at Parallel for the first time and I think it should be constrained to CommutativeApply … need to review how we got here.

@etorreborre changing the constraints and semantics of *> and <* (which are heavily used) would be very disruptive so I don't think we should do that. >> is over-constrained and deprecated, and I think it's too soon to introduce it with new semantics, if this is what you were suggesting (I wasn't sure).

[etorreborre]

@tpolecat I was thinking that many people were using >> for sequencing actions. I haven't seen much code using *> instead. The net changes will be to go from >> and *> to *> and !> and I wonder if assigning just another meaning to *> would be a better move in the long run (and keep similar names between Haskell and Scala).

[etorreborre]

@tpolecat @LukaJCB feel free to ignore my bikeshedding opinion, I will be fine as long as we have an operator for Parallel.

[tpolecat]

>> is deprecated in favor of *> … it's an anachronism from the days before Applicative in Haskell but it's still recognizable so I think we should not have it in cats. I use <* and *> all the time but I don't know how common it is I guess … I assume lots of people are using them since they're so useful.

In any case <* and *> are also well-known from Haskell and scalaz and I think we should not change the sequential semantics. a *> b is not the same as b <* a (see State for instance) although it is for CommutativeApply and would be for Parallel with <! and !>.

[etorreborre]

I'm biased because I have used >> a lot more before I started using *>. And I agree with your last sentence.