Question

我试图实现类DistanceCM和DistanceMM，我希望这些可以互换，只要它们都继承自Distance。

然而，我收到此错误：

＆＃34;错误：（46,76）锡兰：类型参数＆＃39;其他＆＃39;声明＆＃39;累加型＆＃39;有争论＆＃39;距离＆＃39;这不能分配给鞋帮绑定＆＃39; Summable＆＃39; ＆＃39;其他＆＃39;＆＃34;

...我无法解密...错误消息在下面的代码中引用此行：

shared actual Distance plus(Distance other)

这是当前的代码：

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不符合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;
}

无法实现Summable

1 个答案: