A Kotlin implementation of lox, the language from Crafting Interpreters, with a JVM backend built with ProGuardCORE.

The klox language is a superset of lox and includes features not implemented in the Crafting Interpreters lox implementation.

Building

./gradlew build

The build task will execute all tests and create an output jar lib/klox.jar`.

Executing

A wrapper script bin/klox is provided for convenience in the bin/ directory:

$ bin/klox --help
Usage: klox options_list
Arguments: 
    script -> Lox Script (optional) { String }
Options: 
    --outJar, -o -> output jar { String }
    --useInterpreter, -i -> use interpreter instead of JVM compiler when executing
    --debug -> enable debugging 
    --dumpClasses, -d -> dump textual representation of classes (instead of executing) 
    --args, -arg -> additional arguments to pass to the klox program { String }
    --help, -h -> Usage info

Execute without a script for a REPL, otherwise the provided Lox script will be executed. If a Lox script is provided, by default, the script will be executed by compiling it for the JVM and executing the compiled code. The interpreter can be used instead to execute the script by passing the --useInterpreter option (useful for comparing interpreted vs compiled runtime!).

The compiler can generate a jar for the given script by passing the --outJar option (in this case the script will not be executed by klox`) e.g.

$ bin/klox myScript.lox --outJar myScript.jar
$ java -jar myScript.jar

Example Lox program

fun hello() {
    print "Hello World";
}

hello();

Lox has control flow:

if (1 == 2 and false or true) {
    print "true";
} else {
    print "false";
}

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

while (true) {
    print "Looping forever";
}

Lox is object-oriented:

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

    fullname() {
        return this.name + " " + this.surname + ".";
    }

    greet() {
        print "Hello, " + this.fullname();
    }
}

class Employee < Person {
    greet() {
        super.greet();
        print "Keep up the good work!";
    }
}

Employee("John", "Smith").greet(); 
// Hello, John Smith.
// Keep up the good work!

klox features

Do-while loops

do {
    print "Do-while loop";
} while (false);

Arrays

Arrays can be created with the [] syntax

var arr = [1, 2, 3];

The [] syntax is also used to access array elements:

print arr[0]; // 1

Python-style slices are also supported:

print [1, 2, 3][0:2]; // [1, 2]
print [1, 2, 3][:]; // [1, 2, 3]
print [1, 2, 3][::-1]; // [3, 2, 1]

These are actually syntactic sugar for the init, get, set and slice methods on the Array object:

// var arr = [1, 2, 3];
var arr = Array(3);
arr.set(0, 1);
arr.set(1, 2);
arr.set(2, 3);

print arr.get(0); // print arr[0]; // 1
arr.set(0, 1); // arr[0] = 1;

get`, set and slice methods

Like Array, any class can define get, set and slice methods to take advantage of the [] syntax:

class Foo {
    init(value) {
        this.value = value;
    }

    get(index) {
        if (index == 0) {
            return this.value;
        } else {
            return null;
        }
    }

    set(index, value) {
        this.value = index + ": " + value;
    }
}

var foo = Foo("foo");
print foo[0]; // foo
foo[0] = "bar";
print foo[0]; // bar

Destructuring declarations

var (foo, bar) = ["foo", "bar"];
print foo; // foo
print bar; // bar

Destructuring declarations are syntactic sugar for get methods - any class can implement get(index) to take advantage of destructuring declarations.

Underscore can be used to ignore a value:

var (_, bar) = ["foo", "bar"];

For-in loops

A class implementing a method iterator that returns an Iterator object can be used as a for-in loop. Array and ArrayList are examples of classes that implement this method.

for (var x in [1, 2, 3]) {
    print x;
}

You can also use destructuring declarations with for-in loops:

for (var (a, b) in [[1, 2], [3, 4]]) {
    print a + " " + b;
}
// 1 2
// 3 4

The Iterator class is defined as the following:

class Iterator {
    hasNext(): boolean
    next(): value | nil
}

Range operator ..

The range operator invokes leftHandSide.rangeTo(rightHandSide) on an object, which should return an Range object. Numbers and characters are built-in types that work with .. but custom classes can also implement rangeTo.

for (var x in 0..10) { // equivilant: for (var x in NumberRange(0, 10))
    print x;
}

for (var c in "A".."Z") { // equivilant: for (var c in CharacterRange(0, 10))
    print c;
}

class Foo {
    rangeTo(x) = NumberRange(0, x)
}

var foo = Foo();
for (var x in foo..10) {
    print x;
}

Multiple declarations in the same statement

var a = 1, b = 2;

(Post/pre)fix increment and decrement operators

Numerical values can be incremented and decremented:

var i = 0;
i++;
++i;
i--;
--i;

Anonymous functions

var foo = fun (x) {
    return x + 1;
};

print foo(1); // 2

Single-expression functions

fun foo() = 1;
fun bar() = 2 // optional ;
fun baz() = foo() + bar()

Classes without bodies

class Foo
class Bar < Foo
print Bar is Foo;

Class header inline initializers

class Foo(x)
var foo = Foo(1);
print foo.x; // 1

class Base(foo)
class Bar(x) < Base(x)
var bar = Bar(1);
print bar.foo; // 1

Data classes

Data classes implement boilerplate code for get, equals, hashCode and toString:

data class Foo(x)
print Foo(1); // Foo(x = 1)

Operator overloading

The following binary operators can be overloaded by declaring a method with a particular name:

Null-safe operator

The null-safe operator ? can be used to chain property accesses without needing to explicitly check for null and without the programming exiting if the property is not found:

var x = foo?.bar?.baz;

// equivalent to:

var x = foo.bar;
if (x.baz != nil) x = x.baz;

Early-return operator

The early-return operator !? can be used to return from a function if an error is thrown:

fun foo() {
    // if bar returns Error, then the function will return early with that Error.
    var x = bar()!?;
    print "Bar success"; // control-flow will only reach this line if bar() returns successfully
    return Ok(x); 
}

var (result, error) = foo();

comma operator

The comma operator is used to chain multiple expressions together, the left-hand side of the expression is evaluated & its result discarded then right-hand side is evaluated and returned.

print "foo", "bar"; // prints bar

modulo operator, power operator

print 5 % 2; // prints 1

print 3 ** 2; // prints 9

bitwise operators

& (and), | (or), ^ (xor) and ~ (complement), << (left shift), >> (right shift) and >>> (unsigned right shift) bitwise operators are supported.

break and continue statements

for (var i = 0; i < 10; i = i + 1) {
    if (i == 2) {
        continue;
    } else if (i == 5) {
        break;
    }
}

property getters

class Circle {
    init(radius) {
        this.radius = radius;
    }

    area {
        return 3.14159265359 * this.radius * this.radius;
    }
}

var circle = Circle(4);
print circle.area; // approx 50.266

static class methods

class Math {
    static square(n) {
        return n * n;
    }
}

print Math.square(3); // prints 9

static class property getters

class Math {
    static PI {
        return 3.14159265358979323846;
    }
}

print Math.PI; // approx. 50.265

multi-line comments (nestable)

/*
* Does something. /* nested comment */
*
* returns: nothing
*/
fun foo() {
    print "bar";
}

is instance check

class Super { }
class Foo < Super { }
class Bar { }

var foo = Foo();

print foo is Foo; // true
print foo is Super; // true
print foo is Bar; // false

toString method

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

    toString() {
        return "Hello " + this.name;
    }
}

print Greeter("James"); // Hello James

equals method

The root class Object implements equals(other) which returns true if the two objects are the same instance. The == operator calls obj.equals(other) if obj is an instance of Object.

Klox standard library

Klox comes with a set of standard library functions and classes.

Lox built-in

For compatibility with lox the built-in, top-level clock() returns the current time in milliseconds.

Object

Unlike lox, all klox classes extend from the root Object`. Object is the only class with no super class.

Array

The Array class represents a fixed-size array.

class Array {
    init(size);
    get(index);
    set(index, value);
    length();
    slice(start, stop, step);
    map(function);
    reduce(initial, function);
    filter(function);
    forEach(function);
    forEachIndexed(function);
}

var array = Array(2);
array.set(0, "foo");
array.set(1, 123);
print array.get(0); // foo
print array.length(); // 2

The static method System.arraycopy can be used to efficiently copy one array to another.

System

• System.arg(number): string | nil returns the nth argument passed to the program or nil if the argument is out of range.
• System.exit(code) exits the program with the given exit code.
• System.fail(message) exits the program with a non-zero exit code and the given message.
• System.arraycopy(src, srcPos, dest, destPos, length)`

Strings

• String.length(string): number returns the length of string`.
• String.substring(string, start, end): string | Error`
• String.indexOf(string, substring, start): number` returns the substring of string between start (inclusive) and end (exclusive). Returns an Error on failure.
• String.toNumber(string): number | Error converts a string to a number e.g. "5" -> 5.

Characters

• Character.toCharCode(c): number returns the numeric value of c e.g. "A" -> 65.
• Character.fromCharCode(n): c returns the character value of n e.g. 65 -> "A".

Files

class File {
    init(path);
    readText();
    writeText(string);
    delete();
}

Input/Output

Input/output is handled by sub-classes of InputStream / OutputStream.

class FileInputStream < InputStream {
    init(file);
    /**
    * Returns the next byte from the stream or -1 if the end of the stream is reached.
    *
    * Returns an Error if there is an error.
    */
    readByte();

    /**
    * Returns the next character from the stream or nil if the end of the stream is reached.
    *
    * Returns an Error if there is an error.
    */
    readChar();

    /**
    * Closes the stream.
    *
    * Returns true or an Error if there is an error.
    */
    close();
}

class FileOutputStream < OutputStream {
    init(file);
    /**
    * Writes a byte to the outputstream.
    *
    * Returns true or an Error if there is an error.
    */
    writeByte(b);

    /**
    * Writes a character to the outputstream.
    *
    * Returns true or an Error if there is an error.
    */
    writeChar(c);

    /**
    * Closes the stream.
    *
    * Returns true or an Error if there is an error.
    */
    close();
}

Math

• Math.PI 3.141592653589793
• Math.sqrt(number): number`
• Math.ceil(number): number`
• Math.floor(number): number`
• Math.round(number): number`
• Math.min(number): number`
• Math.max(number): number`
• Math.abs(number): number`

Error handling

Functions can return a Result object of which there are two variants Ok and Error (inspired by Rust).

fun foo(a, b) {
    if (b == 0) return Error("Cannot divide by zero");
    else return Ok(a / b);
}

var (result, error) = foo(1, 0);

print result; // expect: nil
print error; // expect: Cannot divide by zero

Result provides convenience functions for working with Ok or Error results, e.g.

foo(1, 0).orFail(); // will exit the program with the error message if there is an error.
foo(1, 0).orNil(); // returns nil if there is an error.

These can be chained e.g.

var err = file.writeText("Hello World").andThen(fun (x) {
    print "F ile written"; // expect: File written
    return file.readText().andThen(fun (text) {
        print "File read"; // expect: File read
        print text; // expect: Hello World
        return file.delete().andThen(fun (x) {
            print "File deleted"; // expect: File deleted
        });
    });
});

print err; // expect: nil

The !? operator can be used with functions that return Result to return early if there is an error. If the Result is an Error then the calling function returns early with the Error instance. Otherwise, the value of an `Ok result is unwrapped.

fun foo() {
    var a = doSomething()!?;
    var b = doAnotherThing()!?;
    return Ok(a + b);
}

This allows chaining of potentially error throwing functions:

fun foo(x, y, z) {
    return A().a(x)!?.b(y)!?.c(z)!?;
}