如何定義循環類型定義？

Question

這不是一個有效的類型定義：

scala> type Addable = { def +(subject: Addable) } 
<console>:4: error: illegal cyclic reference involving type Addable 
     type Addable = { def +(subject: Addable) } 

可這在Scala中表達？

Answer 1

不，它不能。

在Scala語言規範版本2.7的40頁：

However, it is a static error if a type alias refers recursively to the defined type constructor itself. That is, the type T in a type alias type t[tps] = T may not refer directly or indirectly to the name t.

Answer 2

似乎在斯卡拉2.8，這可能是 「固定」：

http://lampsvn.epfl.ch/trac/scala/ticket/1291

Answer 3

這是我在圖書館做了我寫的，HTH：

trait Addable { 
    type AddableType <: Addable 
    def + (subject: AddableType): AddableType 
    } 
    trait Rational extends Addable { 
    type AddableType = Rational 
    override def + (subject: Rational): Rational 
    }