为什么我不能在 Scala 中创建 F 有界对象 [英] Why I can't create F-bounded object in Scala

问题描述

假设我有:

trait A[AA <: A[AA]]
//or even just `
trait A[AA]

这不起作用:

scala> object AAA extends A[AAA.type]
<console>:8: error: illegal cyclic reference involving object AAA
   object AAA extends A[AAA.type]
                        ^

但这有效:

scala> class AAA extends A[AAA]; object AAA extends AAA
defined class AAA
defined module AAA

做几乎(不完全)相同的工作.有什么原因吗?

Doing almost (not exactly) same and this works. Any reason?

附:还有，我能做什么在这样的对象内部强制编译器内部的无限循环?

P.S. And also, what exactly can I do inside such object to force infinte cycle inside the compiler itself?

推荐答案

正如您在标题中提到的，工作案例 class AAA extends A[AAA] 是 F-bounded polymorphism，这是一个递归类型定义，其中定义引用自身.递归在类型中相当普遍，即使是不起眼的 List 也是递归的；这是一个很好理解的领域.

As you allude to in your title, the working case class AAA extends A[AAA] is an example of F-bounded polymorphism, which is a recursive type definition where the definition refers to itself. Recursion is fairly common in types, even the humble List is recursive; it's fairly well understood territory.

但是，object AAA extends A[AAA.type] 不是递归类型.这里 AAA 是一个 value，并且您的声明要求编译器在定义值时解析对值类型的引用，这不是 Scala 设计/预期的功能拥有.

However, object AAA extends A[AAA.type] is not a recursive type. Here AAA is a value, and your declaration asks the compiler to resolve the reference to a value's type while it is being defined, which is not a capability Scala was designed/intended to have.

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