Designing a Language

Building a language and an interpreter for it from scratch

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• Language Design
  • First Look
  • Data Types
  • Expressions
  • Statements
  • Variables
  • Control Flow
  • Functions
  • Classes
    • Class-based
    • Prototype-based
    • Lox Approach
    • Inheritance in Lox
  • Standard Library
    • TODO
  • PyLox our Interpreter
  • References

Language Design

📝 Note
Currently in early stage all language design decisions will follow the tutorial and examples from here.

First Look

// This is a comment
print "Hello, World!";

Current Lox will follow syntax of C-like languages, but with some differences. It will have the following features:

• Dynamic typing
• Automatic memory management (garbage collection)

Data Types

Lox will have the following data types:

• Booleans

    true;
    false;
  

• Numbers

    123;
    123.456;
  

• Strings

    "Hello, World!";
    ""; // Empty string
  

• Nil

    nil;
  

Expressions

Lox will have the following expressions:

• Arithmetic

    1 + 2;
    3 - 4;
    5 * 6;
    7 / 8;
    -9;  // prefix
  

• Comparison

    1 > 2;
    3 < 4;
    5 >= 6;
    7 <= 8;
    123 == 456;
    123 != 456;
  

• Logical

    true and false;
    true or false;
    !true;
  

• Grouping

    (1 + 2) * 3;
  

Statements

A statement’s job is to produce some effect. They are executed for their side effects such as output, input, or assignment and modification of variables.

Variables

Variables are used to store values. They are created with the var keyword.

var a = 1;
var b;  // Uninitialized variable defaults to `nil`

Control Flow

Lox will have the following control flow statements:

• if

    if (true) {
        print "true";
    } else {
        print "false";
    }
  

• while

    while (true) {
        print "true";
    }
  

• for

    for (var i = 0; i < 10; i = i + 1) {
        print i;
    }
  

Functions

Functions are used to group statements together to perform some task. They are created with the fun keyword.

fun add(a, b) {
    return a + b;
}


fun main() {
    print add(1, 2);  // returns `nil` since main() doesn't return anything
}

Functions are considered first-class citizens in Lox. This means that they can be passed around like any other value.

fun add(a, b) {
    return a + b;
}

fun main() {
    var adder = add;
    print adder(1, 2);
}

fun wrapper()
{
	fun inner()
	{
		print "Hello, World!";
	}
}

Classes

There are two ways to approach objects in OOP:

• Class-based: Objects are instances of classes, and classes inherit from other classes.
• Prototype-based: Objects are instances of other objects, and objects inherit from other objects.

Class-based

In class-based languages, classes are the blueprints for objects. They define what properties and methods an object will have. Objects are instances of classes. Instances store state in fields, and behavior is defined by methods.

Prototype-based

In prototype-based languages, objects are the blueprints for objects. They define what properties and methods an object will have. Objects are instances of other objects.

Lox Approach

Lox will follow the class-based approach.

class Person {
    init(name) {
        this.name = name;
    }

    greet() {
        print "Hello, my name is " + this.name + ".";
    }
}


var person = Person("John");
person.greet();

Inheritance in Lox

class Person {
    init(name) {
        this.name = name;
    }

    greet() {
        print "Hello, my name is " + this.name + ".";
    }
}


class Student < Person {
    init(name, id) {
        super.init(name);
        this.id = id;
    }

    greet() {
        super.greet();
        print "My student ID is " + this.id + ".";
    }
}

Standard Library

For now standard libraries in Lox will be limited to the default clock() method. I will slowly add other methods to give it basic functionality.

TODO

Look for the complete list of methods here

PyLox our Interpreter

PyLox is a tree walk interpreter written in Python. It will be used as the default interpreter for Lox.

References

• Crafting Interpreters

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