Interfaces

Interfaces are abstract types that define a set of method requirements. Other types can implement an interface, and the compiler checks that all the required methods are provided by the implementing type.

Interfaces can provide default implementations of methods.

interface Fooable {
    int foo();

    int fooSquared() {
        return foo() * foo();
    }
}

struct X: Fooable {
    int foo() {
        return 2;
    }
}

void callFoo<T: Fooable>(T& f) {
    println(f.foo());
}

void main() {
    var x = X();
    println(x.fooSquared()); // prints 4
    callFoo(x); // prints 2
}

Enums can implement interfaces as well; see Enum types.