scala已经配备了自身的Future类。我们先举个例子来了解scala Future的具体操作:
1 import scala.concurrent._ 2 import ExecutionContext.Implicits.global
3 object scalafuture { 4 def dbl(i: Int): Future[Int] = Future { Thread.sleep(1000) ; i + i } 5 //> dbl: (i: Int)scala.concurrent.Future[Int]
6 val fdbl = dbl(3) //> fdbl : scala.concurrent.Future[Int] = List()
7 fdbl.onSuccess { 8 case a => println(s"${a/2} + ${a/2} = $a") 9 } 10 println("calculating ...") //> calculating ...
11 Thread.sleep(2000) //> 3 + 3 = 6
12 }
这是一个标准的异步运算;在成功完成运算事件上绑定callback来获取在其它线程中的运算结果。我们也可以进行异常处理:
1 val fdz = Future { 3 / 0 } //> fdz : scala.concurrent.Future[Int] = List()
2 fdz.onFailure { 3 case e => println(s"error message {${e.getMessage}}") 4 } 5 Thread.sleep(100) //> error message {/ by zero}
又或者同时绑定运算成功和失败事件的callback函数:
1 import scala.util.{Success, Failure} 2 fdz onComplete { 3 case Success(a) => println(s"${a/2} + ${a/2} = $a") 4 case Failure(e) => println(s"error message {${e.getMessage}}") 5 } 6 Thread.sleep(100) //> error message {/ by zero}
scala Future 实现了flatMap,我们可以把几个Future组合起来用:
1 def dbl(i: Int): Future[Int] = Future { Thread.sleep(1000); i + i } 2 //> dbl: (i: Int)scala.concurrent.Future[Int]
3 def sqr(i: Int): Future[Int] = Future { i * i } //> sqr: (i: Int)scala.concurrent.Future[Int]
4 def sum(a: Int, b: Int): Future[Int] = Future { a + b } 5 //> sum: (a: Int, b: Int)scala.concurrent.Future[Int]
6 val fsum = for { 7 a <- dbl(3) 8 b <- sqr(a) 9 c <- sum(a,b) 10 } yield c //> fsum : scala.concurrent.Future[Int] = List()
11
12 fsum onSuccess { case c => println(s"the combined result is: $c") } 13 Thread.sleep(2000) //> the combined result is: 42
scala Future利用flatMap实现了流程运算:先运算dbl再sqr再sum,这个顺序是固定的即使它们可能在不同的线程里运算,因为sqr依赖dbl的结果,而sum又依赖dbl和sqr的结果。
好了,既然scala Future的功能已经比较完善了,那么scalaz的Future又有什么不同的特点呢?首先,细心一点可以发现scala Future是即时运算的,从下面的例子里可以看出:
1 import scala.concurrent.duration._ 2 val fs = Future {println("run now..."); System.currentTimeMillis() } 3 //> run now... 4 //| fs : scala.concurrent.Future[Long] = List()
5 Await.result(fs, 1.second) //> res0: Long = 1465907784714
6 Thread.sleep(1000) 7 Await.result(fs, 1.second) //> res1: Long = 1465907784714
可以看到fs是在Future构建时即时运算的,而且只会运算一次。如果scala Future中包括了能产生副作用的代码,在构建时就会立即产生副作用。所以我们是无法使用scala Future来编写纯函数的,那么在scalaz里就必须为并发编程提供一个与scala Future具同等功能但又不会立即产生副作用的类型了,这就是scalaz版本的Future。我们看看scalaz是如何定义Future的:scalaz.concurrent/Future.scala
sealed abstract class Future[+A] { ... object Future { case class Now[+A](a: A) extends Future[A] case class Async[+A](onFinish: (A => Trampoline[Unit]) => Unit) extends Future[A] case class Suspend[+A](thunk: () => Future[A]) extends Future[A] case class BindSuspend[A,B](thunk: () => Future[A], f: A => Future[B]) extends Future[B] case class BindAsync[A,B](onFinish: (A => Trampoline[Unit]) => Unit, f: A => Future[B]) extends Future[B] ...
Future[A]就是个Free Monad。它的结构化表达方式分别有Now,Async,Suspend,BindSuspend,BindAsync。我们可以用这些结构实现flatMap函数,所以Future就是Free Monad:
def flatMap[B](f: A => Future[B]): Future[B] = this match { case Now(a) => Suspend(() => f(a)) case Suspend(thunk) => BindSuspend(thunk, f) case Async(listen) => BindAsync(listen, f) cas