Document Parallel type class, parSequence, parTraverse, and parMapN

[davegurnell]

Parallel is a type class that maps monads to equivalent applicatives. For example, there is an instance of Parallel to map between instances of Either and Validated.

There seem to be three key bits of syntax related to Parallel: parMapN, parSequence, and parTraverse. These combine instances of the monad by converting them to the equivalent applicative and back again:

import cats.instances.list._ // for Monoid
import cats.instances.parallel._ // for Parallel
import cats.syntax.parallel._

val either1: Either[List[String], Int] = Left(List("error1"))
val either2: Either[List[String], Int] = Left(List("error2"))
val either3: Either[List[String], Int] = Left(List("error3"))

(either1, either2, either3).parMapN(_ + _ + _)
// Left(List("error1", "error2", "error3"))