爲什麼這個簡單的Scala理解不會執行期貨？

Question

我堅持弄清楚爲什麼這不起作用：

import scala.concurrent.future 
import scala.concurrent.Future 
import scala.concurrent.ExecutionContext 

import scala.concurrent.ExecutionContext.Implicits.global 

object FutureTest { 

    def main(args: Array[String]) { 
    val result1 = future("a") 
    val result2 = future("b") 
    val result3 = future("c") 

    val res = for { 
     r1 <- result1 
     r2 <- result2 
     r3 <- result3 
    } yield (r1 + r2 + r3) 

    for { r <- res} yield(println(r)) 
    } 
} 

我期待這個打印「ABC」，但沒有真正發生。

Answer 1

您正在執行一個獨立的程序，問題是，主線程終止以後才能完成，看到的東西，你可以這樣做：

import scala.concurrent.future 
import scala.concurrent.Future 
import scala.concurrent.ExecutionContext 

import scala.concurrent.ExecutionContext.Implicits.global 

object FutureTest { 

    def main(args: Array[String]) { 
    val result1 = future("a") 
    val result2 = future("b") 
    val result3 = future("c") 

    val res = for { 
     r1 <- result1 
     r2 <- result2 
     r3 <- result3 
    } yield (r1 + r2 + r3) 

    val printing = for { r <- res} yield(println(r)) 

    Await.ready(printing, Duration.Inf) 
    } 
}