执行Summable时出现问题

Question

我试图实现类DistanceCM和DistanceMM，我希望它们可以互换，只要它们都从Distance继承。

不过，我得到这个错误：

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

...我不能破译......该错误消息指这条线下面的代码：

shared actual Distance plus(Distance other) 

这是当前代码：

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> 

不冰约束（given子句）。您声称DistanceMM满足Summable<Distance>，但Distance不符合Other（Distance不符合Summable<Distance>）的限制条件。试试这个：

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; 
}