Enhancement: for-yield with Future Monad is a common concurrency bug

[philliptaylorpro]

It would be good if Scapegoat could see this:

import scala.concurrent._
import scala.concurrent.ExecutionContext.Implicits.global

for {
    a <- Future { "a" }
    b <- Future { "b" }
} yield { println(a + b) }

and warn Missed concurrency opportunity. Future b waits for Future a but may not need to

Conditions:

• For yield
• Top level Monad is scala.concurrent.Future[_]
• Future { "b" } doesn't refer to variable A

Details:
A for-yield statement is a series of flatMaps with a map at the end, therefore the code above waits for a to return successfully before starting b. By rearranging the for-loop you can theoretically speed up the application. For example this would be faster assuming the Futures were slower:

Future.sequence(Seq(Future { "a" }, Future { "b" })).map {
    case a :: b :: Nil => println(a + b)
    case _ =>
}

It's just an idea.

[sksamuel]

Great idea.