Jak zdefiniować definicję typu cyklicznego?

Question

To nie jest ważna definicja typu:

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

Może to być wyrażone w Scala?

Answer 1

Nie, to nie może .

Na stronie 40 Scala Language Specification Version 2.7:

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

Wydaje się to może być "stałe" w Scala 2.8:

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

Answer 3

Oto co zrobiłem w bibliotece pisałem, HTH:

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