Crafting-Interpreters

Implements a simple, half functional half object-oriented language Lox in Java.

Language Features

Dynamic typing

Variables can store values of any type, even functions and classes. Primitive data types include Boolean, Numbers, Strings, Nil

Automatic memory management

JLox takes advantage of Java's GC system, while CLox implements its own GC algorithm.

Arithmetic

Lox supports +,-,*,/ and no more. If arithmetic operations are performed on variables with different types or variables not available for the operation, an error will occur.

Comparison and equality

Comparison operators include <,>,==,!=,>=,<=. Comparisons return a Boolean result. Comparisons between two different types will return a false.

Logical operators

Logical operators include !,and,or. They follow short circuit rules, and return a Boolean value.

Precedence and grouping

The same as C.

Variables

Variables can be declared using var keyword. For example,

var imAVariable = "abc"

Referencing a variable is no different from that in C.

Control Flow

if statement

if (condition) {
    print "yes";
} else {
    print "no";
}

while loop

var a = 1;
while (a < 10) {
    print a;
    a = a + 1;
}

for loop

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

break statement. The same as C.

Functions

A function can be declared using fun keyword. It's important to remember that parameters have no types. For example,

fun printSum(a, b) {
    print a + b;
}

Function call is the same as that in C.
Functions are first class in Lox, meaning they can be passed as variables.

Closures

Since function declarations are statements, you can declare local functions inside another function:

fun returnFunction() {
    var outside = "outside";

    fun inner() {
        print outside;
    }

    return inner;
}

var fn = returnFunction();
fn();

Here, inner() accesses a local variable declared outside of its body in the surrounding function. For that to work, inner() has to “hold on” to references to any surrounding variables that it uses so that they stay around even after the outer function has returned. We call functions that do this closures.

Classes

A class can be declared using class keyword.

class Breakfast {
    cook() {
        print "Eggs a-fryin'!";
    }

    serve(who) {
        print "Enjoy your breakfast, " + who + ".";
    }
}

Declaration of classes in Lox only contains methods.
Classes are first class in Lox.
To instantiate a class, call a class like a function and it produces a new instance of itself.

var breakfast = Breakfast();

To initialize a class, add a init() method in the class declaration and it will be called automatically when the object is constructed.

class Breakfast {
    init(meat, bread) {
        this.meat = meat;
        this.bread = bread;
    }

To add fields to a class, use the . operator.

breakfast.meat = "sausage";
breakfast.bread = "sourdough";

Inheritance

Lox supports single inheritance by introducing < operator.

class Brunch < Breakfast {
    drink() {
        print "How about a Bloody Mary?";
    }
}

Only methods are inherited, not the fields.

Standard library

There are only 2 functions in the standard library.

print
clock(), which returns the nuber of seconds since the program started.

Gun9niR/crafting-interpreters