实施 Summable 时遇到问题

Question

我正在尝试实现类 DistanceCM 和 DistanceMM，我希望它们可以互换求和，只要它们都继承自 Distance。

但是，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 Distance 不满足 Other 的约束（Distance 不满足 Summable<Distance> ).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;
}

实施 Summable 时遇到问题

Trouble implementing Summable

ceylon