Properties (getters and setters) in Ceylon

How do I define a property in Ceylon? I know I can write getName and setName functions to get and set a backing variable:

class Circle(shared variable Float radius) {
    shared Float getArea() {
        return pi * radius ^ 2;
    }

    shared void setArea(Float area) {
        radius = sqrt(area / pi);
    }
}

value circle = Circle(4.0);
circle.setArea(10.0);
print(circle.getArea());

But I would like to be able to provide attribute-like access to the property:

value circle = Circle(4.0);
circle.area = 10.0;
print(circle.area);

How do I do this in Ceylon?

Solution

Getters are declared like defining a function with no parameter list. The getter body then behaves like a normal function and must return the calculated value of the property.

variable String local_var = "Hello world!";

// Getter
String name1 {
    return local_var;
}

// Also getter, using functional syntax for body
String name2 => local_var;

Setters are declared using the assign keyword. The setter body then behaves like a void function, performing whatever mutation of the local environment is appropriate and must not return a value. The incoming value being assigned can be referred to in the body via the name of the property.

// Setter
assign name1 {
    local_var = name1; // name1 here is value being assigned
}

// Also setter, using modified functional syntax
assign name2 => local_var = name2;

Unlike most programming languages, properties can be top level members of the package, not just members of a class:

class Circle(shared variable Float radius) {
    // Class-level getter
    shared Float area {
        return pi * radius ^ 2;
    }

    // Class-level setter
    assign area {
        radius = sqrt(area / pi);
    }
}

Circle window = Circle(1.0);

// Top-level getter
Circle outer_window => Circle(window.radius * 2);

// Top-level setter
assign outer_window => window.radius = outer_window.radius / 2;