scala - Scala将递归有界类型参数（F有界）转换为类型成员

Question

我将如何转换：

trait Foo[A <: Foo[A]]

类型成员？

即，我想要以下内容：

trait Foo {
  type A <: Foo {type A = ???}
}

但我遇到了困难，因为名称 A 已经在类型细化中采用。这个问题是相似的（并且产生自）：F-bounded quantification through type member instead of type parameter?

Answer 1

使用自我类型：

scala> trait Foo { self => type A <: Foo {type A = self.A}}
defined trait Foo

scala> class Bar extends Foo { type A = Bar }
defined class Bar

scala> class Bar extends Foo { type A = Int }
<console>:10: error: overriding type A in trait Foo with bounds <: Foo{type A = Bar.this.A};
 type A has incompatible type
       class Bar extends Foo { type A = Int }
                                    ^