如何使用 Scala 的单例对象类型? [英] How to use Scala's singleton-object types?

问题描述

我正在编写一个类作为一系列单例对象的基类.在每个单例对象中，都会有代表某些属性的 val，我想写一个方法，对于每个单例对象，只接受它创建的对象.

I'm writing a class which serves as base class for a series of singleton objects. In each singleton objects, there will be vals representing certain properties, and I want to write a method which, for each singleton object, only accepts objects created by it.

所以我有以下几点:

class Obj[M <: Maker]

class Maker {
  implicit val me: this.type = this
  def make[M <: Maker](implicit maker: M) = new Obj[M]
  def accept(obj: Obj[this.type]) = {...}
}

到目前为止，一切都很好.然后我想声明这些单例对象之一:

So far, so good. Then I want to declare one of these singleton objects:

object M extends Maker {
  val a = make
}

但是，如果我尝试这样做:

But then, if I try this:

M.accept(M.a)

然后我得到一个编译时错误:

then I get a compile-time error:

type mismatch; found : com.test.Obj[object com.test.M] required: com.test.Obj[com.test.M.type]

我的问题:

1. object com.test.M 的类型是什么，它与 com.test.M.type 有什么不同?
2. 我怎样才能以更智能的方式做到这一点?

1. What's the type object com.test.M, and how is it different from com.test.M.type?
2. How can I do this in a smarter way?

推荐答案

与时俱进，我的好人！我在 24 小时前修复了这个问题.接下来，我希望能看到迅猛龙追逐渡渡鸟，一边在点播屏幕保护程序上查看股票报价，一边疯狂地挥舞着他们的马车鞭子.

Get with the times, my good man! I fixed this over 24 hours ago. Next I expect to see velociraptors chasing dodos, furiously cracking their buggy whips while looking up stock quotes on their pointcast screensavers.

有问题的提交是:http://lampsvn.epfl.ch/trac/Scala/changeset/23622

// 1130.scala
class Obj[M <: Maker]

class Maker {
  implicit val me: this.type = this
  def make[M <: Maker](implicit maker: M) = new Obj[M]
  def accept(obj: Obj[this.type]) = ()
}

object M extends Maker {
  val a = make
}

object Test {
  def main(args: Array[String]): Unit = {
    M.accept(M.a)
  }
}

// too old
% /scala/inst/scala-2.9.0.r23619/bin/scalac ./1130.scala 
./1130.scala:15: error: type mismatch;
 found   : Obj[object M]
 required: Obj[M.type]
    M.accept(M.a)
               ^
one error found

// fresh enough
% /scala/inst/scala-2.9.0.r23624/bin/scalac ./1130.scala 
%

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