參考自類型參數化性狀

Question

我有以下的問題：我想有一個「實例」（參數性狀）和「配置」，而後者是由實例類型參數化：

trait Kind 
trait Kind1 extends Kind 
trait Kind2 extends Kind 

trait Instance[T <: Kind] { 
    def configuration: InstanceConfiguration[Instance[_]] 
} 

trait InstanceConfiguration[+T <: Instance[_]] { 
} 

class AInstance extends Instance[Kind1] { 
    override def configuration: AConfiguration = ??? 
} 

class AConfiguration extends InstanceConfiguration[AInstance] { 
} 

class BInstance extends Instance[Kind1] { 
    override def configuration: AConfiguration = ??? 
} 

因此，問題是InstanceConfiguration[Instance[Kind2]]不能用作Instance[Kind1]的配置，但是現在它太泛化了，我希望編譯器在BInstance.configuration上發出錯誤。

那麼如何更改實例trait中配置的定義以引用具體類型的具體InstanceConfiguration？

Answer 1

你需要使用F-界限的多態。我建議你閱讀the section在Twitter的Scala的學校，但簡單，Instance需要採取具體類型爲類型參數：

trait Kind 
trait Kind1 extends Kind 
trait Kind2 extends Kind 

trait Instance[A <: Instance[A, T], T <: Kind] { 
    self: A => 
    def configuration: InstanceConfiguration[A] 
} 

trait InstanceConfiguration[+T <: Instance[_, _]] { 
} 

class AInstance extends Instance[AInstance, Kind1] { 
    override def configuration: AConfiguration = ??? 
} 

class AConfiguration extends InstanceConfiguration[AInstance] { 
} 

class BInstance extends Instance[BInstance, Kind1] { 
    override def configuration: AConfiguration = ??? 
}