The Jack Programming Language

Compiler and Virtual Machine

The Jack Language

Jack is an educational programming language created by Shimon Schocken and Noam Nisan for the course The Elements of Computing Systems, also known as Nand2Tetris, offered by The Hebrew University of Jerusalem. Students are asked to build their own compilers for this object-oriented language.

Compilation

The Jack language is compiled in a two-tier process:

The code is first translated to an intermediate program (like Java's bytecode and C#'s Intermediate Language) and then a Virtual Machine Translator finishes the compilation process either by interpreting the VM code or by translating to the target's machine assembly language.

Build (Linux Only)

On the parent directory:

xxxxxxxxxx
$ sudo ./install.sh 

Compilation

Compile from Jack to VM

xxxxxxxxxx
$ ./dcc <path>

Interpret the VM code

xxxxxxxxxx
$./jack <path>

Hello, World

Hello world program in Jack

xxxxxxxxxx
class Main {
    function int main() {
        do Output.printf("Hello, World!\n");
        return 0;
    }
}

Save the file as Main.jack and run

xxxxxxxxxx
$ dcc .

This will compile all source files in the current folder. Run

xxxxxxxxxx
$ jack .

To interpret the VM files produced in the current folder

Output:

xxxxxxxxxx
Hello, World!

Features

Jack is a Turing-complete, object-oriented language, with most of the features that any modern language has, such as branching, loops, assignments, arrays, classes, methods, arithmetic operations and more.

Syntax

Jack has a C-like syntax. Reserved words in Jack are:

Arrays

xxxxxxxxxx

class Main {
    function int main() {
        var Array a;

        let a = Array.new(392, 32, 29, 3, 9);
        do a.sort();

        do Output.printf("%s is %i elements long.\n", a.to_s(), a.length());
        return 0;
    }
}

Output

xxxxxxxxxx
[3,9,29,32,392] is 5 elements long

Object Oriented features

In Dog.jack:

xxxxxxxxxx
class Dog {

    field int weight;
    field String name;

    constructor Dog new(int w, String n) {
        let weight = w;
        let name = n;  
        return this;
    }

    method void bark() {
        var String bark;

        if (weight < 20) {
            let bark = "woof, woof";
        } else {
            let bark = "WOOF, WOOF";
        }

        do Output.printf("%s says %s!\n", name, bark); 

        return;

    }
}

In Main.jack:

xxxxxxxxxx
class Main {

    function void main() {
        var Dog codie, fido;

        let codie = Dog.new(24, "Codie");
        let fido = Dog.new(12, "Fido");
        do codie.bark();
        do fido.bark();
        return;
    }
}

Compile && Run

xxxxxxxxxx
$./dcc .
$./jack .

Output

xxxxxxxxxx
Codie says WOOF, WOOF!
Fido says woof, woof!