ceylon - 实施 Summable 时遇到问题

Question

我正在尝试实现类DistanceCM和DistanceMM，并且我希望它们可以互换地求和，只要它们都继承自Distance.

但是，我收到此错误：

“错误：（46、76）锡兰：声明'Summable'的类型参数'Other'有参数'Distance'，它不能分配给'Other'的上限'Summable'”

...我无法破译...错误消息指的是下面代码中的这一行：

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>

注意约束（given子句）。您声称DistanceMM满足Summable<Distance>，但不满足（不满足）Distance的约束。尝试这个：OtherDistanceSummable<Distance>

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;
}