将特征限制为对象? [英] Restricting a trait to objects?
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问题描述
有没有办法限制一个特征,使其只能混合到对象中?例如
Is there a way to restrict a trait so that it can only be mixed into objects? E.g.
trait OnlyForObjects {
this: ... =>
}
object Foo extends OnlyForObjects // --> OK
class Bar extends OnlyForObjects // --> compile error
推荐答案
是的!有一个晦涩难懂且几乎没有记录的 scala.Singleton
:
Yes! There's the obscure and mostly undocumented 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
^
在语言规范中多次提到,但在API 文档.
It's mentioned a few times in the language specification, but doesn't even appear in the API documentation.
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