Interfaces

An interface names behavior without choosing the object’s fields or implementation:

public interface Named
    function name(): String
end

A class opts in with implements and provides a matching public instance method:

public class User implements Named
    private value: String

    public function User:name(): String
        return self.value
    end
end

The class name before : marks the implementation as an instance method. Its explicit parameter and result types must exactly match the interface declaration; the receiver itself is not written in the interface signature. Interface methods are public instance signatures: they do not contain bodies, fields, or independent visibility modifiers.

Programming to behavior

A function can receive the interface instead of one concrete class:

function greet(value: Named)
    print("Hello")
    print(value:name())
end

Any class that explicitly implements Named can be used at that boundary. The compiler verifies the implementation and dispatches the receiver call through the interface’s method slot.

Pop interfaces are nominal, not duck typed. Merely defining a method called name is not enough; the class declaration must say implements Named. This explicit relationship prevents an accidental method-name match from becoming part of a public contract.

Default interface methods, fields, and structural conformance are not implemented in 0.1.0-rc.2.