執行Summable時出現問題

Question

我試圖實現類DistanceCM和DistanceMM，我希望它們可以互換，只要它們都從Distance繼承。

不過，我得到這個錯誤：

"Error:(46, 76) ceylon: type parameter 'Other' of declaration 'Summable' has argument 'Distance' which is not assignable to upper bound 'Summable' of 'Other'"

...我不能破譯......該錯誤消息指這條線下面的代碼：

shared actual Distance plus(Distance other) 

這是當前代碼：

abstract class Distance() of DistanceMM | DistanceCM { 
    shared formal Distance multiplyScalar(Float scalar); 
} 


class DistanceMM(variable Float val) extends Distance() satisfies Summable<Distance> 
{ 
    shared Float distanceInMillimeters; 
    shared Float distanceInCentimeters; 

    switch (unit) 
    case (millimeter) { 
     distanceInMillimeters => val; 
     distanceInCentimeters => val/10; 
    } 
    case (centimeter) { 
     distanceInMillimeters => val * 10; 
     distanceInCentimeters => val; 
    } 


    shared actual DistanceMM multiplyScalar(Float scalar) { 
     val = val * scalar; 
     return this; 
    } 


    shared actual Distance plus(Distance other) { 
      switch (other) 
      case (DistanceMM) { 
       return DistanceMM(val + other.distanceInMillimeters(), unit); 
      } 
      case (DistanceCM) { 
       return DistanceMM(val + other.distanceInCentimeters(), unit); 
      } 
    } 
} 


class DistanceCM(variable Float val) extends Distance() satisfies Summable<Distance> 
{ 
    shared Float distanceInMillimeters; 
    shared Float distanceInCentimeters; 

    switch (unit) 
    case (millimeter) { 
     distanceInMillimeters => val; 
     distanceInCentimeters => val/10; 
    } 
    case (centimeter) { 
     distanceInMillimeters => val * 10; 
     distanceInCentimeters => val; 
    } 

    shared actual DistanceCM multiplyScalar(Float scalar) { 
     val = val * scalar; 
     return this; 
    } 
    // implementation missing 
} 

Answer 1

interface Summable<Other> of Other​ given Other satisfies Summable<Other> 

不冰約束（given子句）。您聲稱DistanceMM滿足Summable<Distance>，但Distance不符合Other（Distance不符合Summable<Distance>）的限制條件。試試這個：

interface Distance of Centimeter | Millimeter satisfies Summable<Distance> {} 

class Centimeter() satisfies Distance { 
    shared actual Distance plus(Distance other) => nothing; 
} 

class Millimeter() satisfies Distance { 
    shared actual Distance plus(Distance other) => nothing; 
}