Scalaz（44）－ concurrency ：scalaz Future，尚不完整的多线程类型

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：

 import scala.concurrent._
 import ExecutionContext.Implicits.global
 object scalafuture {
   def dbl(i: Int): Future[Int] = Future { Thread.sleep() ; i + i }
                                       //> dbl: (i: Int)scala.concurrent.Future[Int]
   val fdbl = dbl()                   //> fdbl  : scala.concurrent.Future[Int] = List()
   fdbl.onSuccess {
     case a => println(s"${a/2} + ${a/2} = $a")
   }
   println("calculating ...")          //> calculating ...
   Thread.sleep()                  //> 3 + 3 = 6
 }

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：

   val fdz = Future {  /  }      //> fdz  : scala.concurrent.Future[Int] = List()
   fdz.onFailure {
     case e => println(s"error message {${e.getMessage}}")
   }
   Thread.sleep()               //> error message {/ by zero}

又或者同时绑定运算成功和失败事件的callback函数：

   import scala.util.{Success, Failure}
   fdz onComplete {
     case Success(a) => println(s"${a/2} + ${a/2} = $a")
     case Failure(e) => println(s"error message {${e.getMessage}}")
   }
   Thread.sleep()               //> error message {/ by zero}

scala Future 实现了flatMap，我们可以把几个Future组合起来用：

   def dbl(i: Int): Future[Int] = Future { Thread.sleep(); i + i }
                                                   //> dbl: (i: Int)scala.concurrent.Future[Int]
   def sqr(i: Int): Future[Int] = Future { i * i } //> sqr: (i: Int)scala.concurrent.Future[Int]
   def sum(a: Int, b: Int): Future[Int] = Future { a + b }
                                           //> sum: (a: Int, b: Int)scala.concurrent.Future[Int]
   val fsum = for {
     a <- dbl()
     b <- sqr(a)
     c <- sum(a,b)
   } yield c                               //> fsum  : scala.concurrent.Future[Int] = List()
 
   fsum onSuccess { case c => println(s"the combined result is: $c") }
   Thread.sleep()                     //> 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：

   import scala.concurrent.duration._
   val fs = Future {println("run now..."); System.currentTimeMillis() }
                                          //> run now...
                                          //| fs  : scala.concurrent.Future[Long] = List()
   Await.result(fs, .second)             //> res0: Long = 1465907784714
   Thread.sleep()
   Await.result(fs, .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)
    case BindSuspend(thunk, g) =>
      Suspend(() => BindSuspend(thunk, g andThen (_ flatMap f)))
    case BindAsync(listen, g) =>
      Suspend(() => BindAsync(listen, g andThen (_ flatMap f)))
  }

free structure类型可以支持算式／算法关注分离，也就是说我们可以用scalaz Future来描述程序功能而不涉及正真运算。scalaz Future的构建方式如下：

 import scalaz._
 import Scalaz._
 import scalaz.concurrent._
 import scala.concurrent.duration._
 object scalazFuture {
 val fnow = Future.now {println("run..."); System.currentTimeMillis()}
                                           //> run...
                                           //| fnow  : scalaz.concurrent.Future[Long] = Now(1465909860301)
 val fdelay = Future.delay {println("run..."); System.currentTimeMillis()}
                                           //> fdelay  : scalaz.concurrent.Future[Long] = Suspend(<function0>)
 val fapply = Future {println("run..."); System.currentTimeMillis()}
                                           //> fapply  : scalaz.concurrent.Future[Long] = Async(<function1>)

可以看到fnow是个即时运算的构建器，而这个now就是一个lift函数, 它负责把一个普通无副作用运算升格成Future。fdelay,fapply分别把运算存入trampoline进行结构化了。我们必须另外运算trampoline来运行结构内的运算：

 fdelay.run                                        //> run...
                                                   //| res0: Long = 1465910524847
 Thread.sleep()
 fdelay.run                                        //> run...
                                                   //| res1: Long = 1465910525881
 fapply.run                                        //> run...
                                                   //| res2: Long = 1465910525883
 Thread.sleep()
 fapply.run                                        //> run...
                                                   //| res3: Long = 1465910526884

scalaz Future只有在运算时才会产生副作用，而且可以多次运算。

我们可以用即时（blocking）、异步、定时方式来运算Future：

 fapply.unsafePerformSync                          //> run...
                                                   //| res4: Long = 1465958049118
 fapply.unsafePerformAsync {
   case a => println(a)
 }
 Thread.sleep()
 fapply.unsafePerformSyncFor( second)             //> run...
                                                   //| 1465958051126
                                                   //| run...
                                                   //| res5: Long = 1465958052172

结构化状态Async代表了scalaz Future的多线程处理特性：

/**
   * Create a `Future` from an asynchronous computation, which takes the form
   * of a function with which we can register a callback. This can be used
   * to translate from a callback-based API to a straightforward monadic
   * version. See `Task.async` for a version that allows for asynchronous
   * exceptions.
   */
  def async[A](listen: (A => Unit) => Unit): Future[A] =
    Async((cb: A => Trampoline[Unit]) => listen { a => cb(a).run })
 
  /** Create a `Future` that will evaluate `a` using the given `ExecutorService`. */
  def apply[A](a: => A)(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Future[A] = Async { cb =>
    pool.submit { new Callable[Unit] { def call = cb(a).run }}
  }
 
  /** Create a `Future` that will evaluate `a` after at least the given delay. */
  def schedule[A](a: => A, delay: Duration)(implicit pool: ScheduledExecutorService =
      Strategy.DefaultTimeoutScheduler): Future[A] =
    Async { cb =>
      pool.schedule(new Callable[Unit] {
        def call = cb(a).run
      }, delay.toMillis, TimeUnit.MILLISECONDS)
    }

我们看到apply和schedule在构建Future时对运算线程进行了配置。

如果我们需要模仿scala Future的功效可以用unsafeStart:

 val fs = fapply.unsafeStart              //> run...
                                          //| fs  : scalaz.concurrent.Future[Long] = Suspend(<function0>)
 fs.run                                   //> res6: Long = 1465958922401
 Thread.sleep()
 fs.run                                   //> res7: Long = 1465958922401

我们也可以用scala Future的callback方式用async函数把自定义的callback挂在构建的Future上：

 def fu(t: Long): Future[String] =
   Future.async[String]{k => k(s"the curreent time is: ${t.toString}!!!")}
                                                   //> fu: (t: Long)scalaz.concurrent.Future[String]
 fu(System.currentTimeMillis()).run                //> res8: String = the curreent time is: 1465958923415!!!

scala Future和scalaz Future之间可以相互转换：

 import scala.concurrent.{Future => sFuture}
 import scala.concurrent.ExecutionContext
 import scala.util.{Success,Failure}
 def futureTozFuture[A](sf: sFuture[A])(implicit ec: ExecutionContext): Future[A] =
   Future.async {cb => sf.onComplete {
     case Success(a) => cb(a)
 //    case Failure(e) => cb(e)
   }}                            //> futureTozFuture: [A](sf: scala.concurrent.Future[A])(implicit ec: scala.con
                                 //| current.ExecutionContext)scalaz.concurrent.Future[A]
 def zFutureTosFuture[A](zf: Future[A]): sFuture[A] = {
   val prom = scala.concurrent.Promise[A]
   zf.unsafePerformAsync {
      case a => prom.success(a)是
   }
   prom.future
 }

突然发现scalaz Future是没有异常处理（exception）功能的。scalaz提供了concurrent.Task类型填补了Future的这部分缺陷。我们会在下篇讨论Task。
我们用上面scala Future的例子来示范scalaz Future的函数组合能力：

   def dbl(i: Int): Future[Int] = Future { i + i } //> dbl: (i: Int)scalaz.concurrent.Future[Int]
   def sqr(i: Int): Future[Int] = Future { i * i } //> sqr: (i: Int)scalaz.concurrent.Future[Int]
   def sum(a: Int, b: Int): Future[Int] = Future { a + b }
                                   //> sum: (a: Int, b: Int)scalaz.concurrent.Future[Int]
   val fsum = for {
     a <- dbl()
     b <- sqr(a)
     c <- sum(a,b)
   } yield c                       //> fsum  : scalaz.concurrent.Future[Int] = BindAsync(<function1>,<function1>)
 
   fsum.unsafePerformAsync {
     case a => println(s"result c is:$a")
   }
   Thread.sleep()              //> result c is:42