Classes as Unions

Slice does not offer a dedicated union construct because it is redundant. By deriving classes from a common base class, you can create the same effect as with a union:

module M
{
    class Shape
    {
        // Definitions for shapes, such as size, center, etc.
    }

    class Circle extends Shape
    {
        // Definitions for circles, such as radius...
    }

    class Rectangle extends Shape
    {
        // Definitions for rectangles, such as width and length...
    }

    interface ShapeShifter
    {
        Shape translate(Shape s, long xDistance, long yDistance);
    }
}

The parameter s of the translate operation can be viewed as a union of two members: a Circle and a Rectangle. The receiver of a Shape instance can use the type ID of the instance to decide whether it received a Circle or a Rectangle. Alternatively, if you want something more along the lines of a conventional discriminated union, you can use the following approach:

class UnionDiscriminator
{
    int d;
}

class Member1 extends UnionDiscriminator
{
    // d == 1
    string s;
    float f;
}

class Member2 extends UnionDiscriminator
{
    // d == 2
    byte b;
    int i;
}

With this approach, the UnionDiscriminator class provides a discriminator value. The "members" of the union are the classes that are derived from UnionDiscriminator. For each derived class, the discriminator takes on a distinct value. The receiver of such a union uses the discriminator value in a switch statement to select the active union member.

See Also

• Type IDs