Futures

Futures are mainly tokens returned by an actor that promises a result sometime in the future. The typical place one encounters a future is from an actor. The !! method of an actor returns a future. However in addition the Futures object provides several methods for doing parallel processing without having to write actors (or threads for that matter.)

Note: My use of scala.util.Random requires Scala 2.8 but I think the rest is 2.7.7.

1. scala> import scala.actors.Futures._                
2. import scala.actors.Futures._
4. /* 
5. The most basic future example.  
6. Just runs the function in a seperate thread and returns a Future object
7. for the result
8. */
9. scala> future {Thread.sleep(10000);println("hi");10}
10. res2: scala.actors.Future[Int] = < function0>
12. /* 
13. Use one of the future methods for obtaining the actual result
14. this one blocks tile ready, but other can check if result is ready and so on
15. */
16. scala> res2()
17. hi
18. res3: Int = 10
20. /*
21. This is more interesting. 
22. The method creates several futures and the using the awaitAll method 
23. waits for all the futures to complete before continuing
24. */
25. scala> def bubbles = {                                                                                                             
26.      | val bubbles = for( i <- 1 to 20) yield {
27.      |   future {
28.      |     Thread.sleep(scala.util.Random.nextInt(20)*500)
29.      |     println("pop "+i)
30.      |     "pop "+i
31.      |   }
32.      | }
33.      | awaitAll(30000, bubbles:_*) foreach println _                                                                               
34.      | }
35. bubbles: Unit
37. scala> bubbles
38. pop 9
39. pop 12
40. pop 11
41. pop 5
42. pop 3
43. pop 14
44. pop 20
45. pop 10
46. pop 1
47. pop 4
48. pop 16
49. pop 6
50. pop 7
51. pop 2
52. pop 8
53. pop 18
54. pop 15
55. pop 19
56. pop 17
57. pop 13
58. Some(pop 1)
59. Some(pop 2)
60. Some(pop 3)
61. Some(pop 4)
62. Some(pop 5)
63. Some(pop 6)
64. Some(pop 7)
65. Some(pop 8)
66. Some(pop 9)
67. Some(pop 10)
68. Some(pop 11)
69. Some(pop 12)
70. Some(pop 13)
71. Some(pop 14)
72. Some(pop 15)
73. Some(pop 16)
74. Some(pop 17)
75. Some(pop 18)
76. Some(pop 19)
77. Some(pop 20)
79. scala> import java.net._                                                                                        
80. import java.net._
82. scala> import scala.io._
83. import scala.io._
85. /*
86. A kind of funny example.  We set off two futures to download the google page
87. and print the first that finishes
88. */
89. scala> def load = {
90.      | val com = future {
91.      |   val comStream = new URL("http://www.google.com").openStream()                                                                       
92.      |   Source.fromInputStream(comStream).getLines().mkString("\n")
93.      | }
94.      | val ch = future {
95.      |   val chStream = new URL("http://www.google.ch").openStream()
96.      |   Source.fromInputStream(chStream).getLines().mkString("\n")
97.      | }  
98.      | awaitEither(ch,com)                                                                                               
99.      | }
100. load: Any
102. scala> load
103. res0: Any = 
104. & !doctype html...