Chapter 1 jlox

jlox is my implementation of the Lox programming language, written in Java. It uses the tree-walk interpreter from the Crafting Interpreters book.

My jlox is currently still a work in progress.

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1.1 Motivation

Over time, programming languages have become more and more abstract. As such, the code we write has gotten closer and closer to spoken language.

This is a trend that I believe will only continue in future years, especially with the development of Artificial Intelligence (AI) that is able to process inputs as high-level as natural language. Even the practice of designing and refining the prompts provided to LLMs is somewhat akin to programming (prompt engineering).

It can be easy to take for granted the amount of work that high-level programming languages perform for us behind the scenes. Wanting to know more about how they really work, I took an interest in language interpreters and compilers.

1.2 Language Features

1.2.1 Data Types

Lox features several built-in data types.

1.2.1.1 Booleans

true;
false;

1.2.1.2 Numbers

Lox uses only floating-point numbers.

wholeNumber = 34;           // 34.0
fractionalNumber = 12.5;    // 12.5

1.2.1.3 Strings

"Hello, world!";
"";     // An empty string.

1.2.1.4 nil

nil represents no value, similar to null in other languages. It shows up in several places such as void functions and uninitialised variables.

1.2.2 Expressions

1.2.2.1 Mathematical Expressions

jlox features common numeric operations and negation:

a + b - a;
a * b / c;
-value;

1.2.2.2 Comparisons

Numbers can be compared with the usual comparison operators < > <= >=:

1 <= 2;             // true

We can test for equality or inequality with == and !=:

1 == 1;             // true
"foo" != "bar";     // true
"42" == 42;         // false

Different types are never equivalent.

1.2.2.3 Logical Expressions

!true;              // false
true or false;      // true
true and false;     // false

1.2.2.4 Variables

Declare variables with var:

var message = "Hello";
var uninitialisedVariable;      // set to nil

1.2.3 Control Flow

if and else statements:

if (condition) {
    // ...
} else {
    // ...
}

while and for loops:

while (num > 0) {
    num = num - 1;
}
for (var i = 0; i < 5; i = i + 1) {
    print i;
}

1.2.4 Functions

Functions are first class in Lox, and return nil by default.

fun sum(first, second) {
    return first + second;
}
sum(1, 2);

1.2.5 Classes

Class methods are declared without the fun keyword.

class Entity {
    move() {
        print "Entity moved!";
    }
}

var myEntity = Entity();      // Created instance of Entity class

We can assign fields, or create them if they don’t already exist:

myEntity.age = 57;
myEntity.name = "Bob";

We can also do this in the init method which is called when the object is constructed:

class Entity {
    init(age, name) {
        this.age = age;
        this.name = name;
    }

    // ...
}

var myEntity = Entity(57, "Bob");

The arguments passed to the class are forwarded to the init method.

1.2.5.1 Inheritance

Lox supports single inheritance using the < operator.

class Alien < Entity {
    init(age, name, planet) {
        super.init(age, name);
        this.planet = planet;
    }
}

We can access methods of the superclass using super.

1.2.6 Comments

jlox allows single-line comments with // or C-style block comments with /**/.