Classes & Instances

Classes are templates for creating objects. They bundle data (fields) and behavior (methods) into a single unit. Adding classes to our language gives users a powerful organizational tool.

Class Declarations

class Dog {
    init(name, breed) {
        this.name = name;
        this.breed = breed;
    }

    bark() {
        print this.name + " says Woof!";
    }
}

A class contains methods — functions bound to the class. The special init method is the constructor, called automatically when creating an instance.

Instances and Properties

var rex = Dog("Rex", "Labrador");
rex.bark();           // "Rex says Woof!"
print rex.name;       // "Rex"
rex.tricks = 3;       // set a new property
print rex.tricks;     // 3

Instances are created by calling the class like a function. Properties can be accessed with dot notation and are stored per-instance. Unlike Java or C++, our language allows adding properties dynamically — like Python and JavaScript.

The this Keyword

Inside a method, this refers to the instance the method was called on. this is implemented as a hidden variable bound in the method's environment:

callMethod(instance, method, args):
    env = new Environment(enclosing=method.closure)
    env.define("this", instance)
    // bind parameters and execute body

Implementation

A class at runtime is an object with:

• A name
• A map of methods (name to function)

An instance at runtime is an object with:

• A reference to its class (for method lookup)
• A map of fields (name to value)

Property access follows a specific order: first check the instance's fields, then the class's methods. This means a field can shadow a method.

How Python Does It

Python classes are far more dynamic — classes themselves are objects, metaclasses control class creation, and the MRO (Method Resolution Order) handles multiple inheritance. Our system is simpler: single inheritance, no metaclasses, no multiple inheritance. This matches the design in Crafting Interpreters.

Your Task

Implement class declarations, instances, properties (get and set), methods, and the this keyword.