Interface polymorphism

This recipe declares a small interface, implements it explicitly, and calls a method through the interface value.

Project tree

polymorphism/
└── main.km

Source

import go fmt from "fmt";

interface Greeter {
  function greet(): string;
}

class Guest implements Greeter {
  constructor(private name: string) {}

  public function greet(): string {
    return "Welcome, " + this.name;
  }
}

function message(greeter: Greeter): string {
  return greeter.greet();
}

function main(): void {
  const guest = new Guest("Aki");
  fmt.Println(message(guest));
}

Run

keika check main.km
keika run main.km

Expected output:

Welcome, Aki

Contract demonstrated

• Guest explicitly names Greeter in implements.
• The compiler validates its public instance method signature.
• A Guest class value is a reference with identity.
• Passing it as Greeter uses generated Go interface dispatch.
• The private name field is not exposed as a public Go field.

Interfaces are intentionally explicit even when a method set happens to match. Classes can also implement imported Go interfaces when their generated public method set satisfies the original Go contract.

See Classes and structs.