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Interfaces

Interfaces define a contract that classes can implement.

Definition

interface Printable {
    function print_me(): None;
}

Implementation

Classes implement interfaces explicitly via implements. The compiler validates that required methods exist with compatible signatures.

class Book implements Printable {
    title: String;
    
    constructor(title: String) {
        this.title = title;
    }
    
    function print_me(): None {
        println("Book: {this.title}");
        return None;
    }
}

Interface Inheritance

Interfaces can extend other interfaces.

interface Named extends Printable {
    function get_name(): String;
}

Generic interfaces can also be referenced directly in implements and extends clauses.

interface Reader<T> {
    function read(): T;
}

interface StringReader extends Reader<String> {}

class ConfigReader implements Reader<String> {
    function read(): String {
        return "ok";
    }
}

Project builds also rewrite alias-qualified generic interface types in ordinary type positions, so declarations such as reader: api.Reader<String> and reader: ReaderAlias<String> work the same way as single-file check.

Polymorphism

You can use interfaces as types.

function display(item: Printable): None {
    item.print_me();
    return None;
}

See examples/37_interfaces_contracts.arden for a full contract + interface-typed-parameter example.

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