限制对象的特征？

Question

有没有办法限制特征，使它只能混合到对象中？ E.g。

trait OnlyForObjects {
  this: ... =>
}

object Foo extends OnlyForObjects  // --> OK

class Bar extends OnlyForObjects   // --> compile error

Answer 1

是的！有晦涩难懂且大多没有记载的scala.Singleton：

scala> trait OnlyForObjects { this: Singleton => }
defined trait OnlyForObjects

scala> object Foo extends OnlyForObjects
defined module Foo

scala> class Bar extends OnlyForObjects
<console>:15: error: illegal inheritance;
 self-type Bar does not conform to OnlyForObjects's selftype OnlyForObjects
 with Singleton
       class Bar extends OnlyForObjects
                         ^

在language specification中有几次提及，但在API文档中甚至没有出现。