Question

我正在尝试使用一些具有默认值的参数来定义一个case类，但默认值需要一个隐式参数。我尝试过这样的事情：

case class ChannelLatches(started: TestLatch, stopped: TestLatch)(implicit system: ActorSystem) {
  def this()(implicit system: ActorSystem) = this(new TestLatch(), new TestLatch())(system)
}

和此：

case class ChannelLatches(started: TestLatch, stopped: TestLatch)(implicit system: ActorSystem) {
   def this()(implicit system: ActorSystem) = this(new TestLatch(), new TestLatch())(system)
}

但在这两种情况下，编译器都无法识别我的新构造函数。有什么指针吗？

Answer 1

问题不在case类或其构造函数中。当您收到像

这样的编译错误时

scala> val channelLatches = new ChannelLatches
<console>:11: error: could not find implicit value for parameter system: ActorSystem
       val channelLatches = new ChannelLatches
                            ^

这意味着你没有一个类型为ActorSystem的隐式变量作为范围内的单个标识符。

你的代码示例（它们是完全相同的代码，对吗？）和@Eastsun的例子都是完全合法的代码：

scala> class ActorSystem
defined class ActorSystem

scala> class TestLatch
defined class TestLatch

scala> case class ChannelLatches(started: TestLatch = new TestLatch, stopped: TestLatch = new TestLatch)(implicit system: ActorSystem)
defined class ChannelLatches

scala> implicit val actor = new ActorSystem
actor: ActorSystem = ActorSystem@586f403e

scala> val channelLatches = new ChannelLatches
channelLatches: ChannelLatches = ChannelLatches(TestLatch@521a74af,TestLatch@46e2b745)

请注意隐式val actor，它使编译器可以隐式提供缺少的参数。

有关隐式参数的介绍，请参阅A Tour of Scala: Implicit Parameters。

---编辑2012-03-05：添加了另一个示例，其中ChannelLatches是Something的内部类

如果你希望ChannelLatches是某个内部类，你真的不需要将ActorSystem实例传递给内部类实例，因为内部类可以访问外部对象的值。 A Tour of Scala: Inner Classes

scala> class ActorSystem
defined class ActorSystem

scala> class TestLatch
defined class TestLatch

scala> class Something(implicit val as: ActorSystem) {
     |   case class ChannelLatches(started: TestLatch, stopped: TestLatch) {
     |     def this() = this(new TestLatch(), new TestLatch())
     |  
     |     def actorSystem = as
     |   }
     | }
defined class Something

scala> implicit val as = new ActorSystem
as: ActorSystem = ActorSystem@5481be8a

scala> val s = new Something
s: Something = Something@7139acf

scala> val cl = new s.ChannelLatches
cl: s.ChannelLatches = ChannelLatches(TestLatch@38764254,TestLatch@5bfcb5c1)

scala> cl.actorSystem == as
res0: Boolean = true

用隐式参数重载case类构造函数？

1 个答案: