loxxy

A Ruby implementation of the Lox programming language, a simple language defined in Bob Nystrom's excellent online book Crafting Interpreters.

Although Lox is fairly simple, it is far from being a toy language:

• Dynamically typed,
• Provides data types such as booleans, number, strings,
• Supports arithmetic operations (+, -, *, / ) and comparison ( >, >= , <, <=)
• Implements equality operators (==, !=) and the logical connectors and and or.
• Control flow statements if, for and while
• Functions and closures
• Object-orientation (classes, methods, inheritance).

• Complete tree-walking interpreter including lexer, parser and resolver
• 100% pure Ruby with clean design (not a port from some other language)
• Passes the jlox (THE reference Lox implementation) test suite
• Can run a Lox interpreter implemented in ... Lox LoxLox,
• Minimal runtime dependency (Rley gem). Won't drag a bunch of gems...
• Ruby API for integrating a Lox interpreter with your code.
• A command-line interpreter loxxy
• Open for your language extensions...

• If programming languages are one of your subject interest...
• ... and you wanted learn how to implement one in Ruby...
• ... then Loxxy can help to understand and experiment in this rewarding craft.

... in less than 3 minutes...

Loxxy's installation is pretty standard:

$ gem install loxxy

Alternatively, you can install loxxy with Bundler.
Add this line to your application's Gemfile:

gem 'loxxy'

And then execute:

$ bundle

Create a text file and enter the following lines:

// hello.lox
// Your first Lox program
print "Hello, world.";

Assuming that you named the file hello.lox, launch the Loxxy interpreter in same directory:

$ loxxy hello.lox

Lo and behold! The output device displays the famous greeting:

Hello, world.

Congrats! You ran your first Lox program thanks Loxxy gem. For a less superficial encounter with the language jump to the next section.

Let's admit it, the hello world example was unimpressive.
To a get a taste of Lox object-oriented capabilities, let's try another Hello world variant:

// oo_hello.lox
// Object-oriented hello world
class Greeter {
  // in Lox, initializers/constructors are named `init`
  init(who) {
    this.interjection = "Hello";
    this.subject = who;
  }

  greeting() {
    this.interjection + ", " + this.subject + ".";
  }
}

var greeter = Greeter("world"); // an instance is created here...
print greeter.greeting();

Running this version will result in the same famous greeting.

Our next assignment: compute the first 20 elements of the Fibbonacci sequence.
Here's an answer using the while loop construct:

// fibbonacci.lox
// Compute the first 20 elements from the Fibbonacci sequence

var a = 0;  // Use the var keyword to declare a new variable
var b = 1;
var count = 20;

while (count > 0) {
  print a;
  print " ";
  var tmp = a;
  a = b;
  b = tmp + b;
  count = count - 1;
}

Assuming, that this source code was put in a file named fibbonacci.lox, then the command line

$ loxxy fibbonacci.lox

Results in:

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181

Fans of for loops will be pleased to find their favorite looping construct.
Here again, the Fibbonacci sequence refactored with a for loop:

// fibbonacci_v2.lox
// Fibbonacci sequence - version 2
var a = 0;
var b = 1;
var count = 20;

for (var i = 0; i < count; i = i + 1) {
  print a;
  print " ";
  var tmp = a;
  a = b;
  b = tmp + b;
}

Lets's call this file fibbonacci_v2.lox and execute it thanks loxxy CLI:

$ loxxy fibbonacci_v2.lox

We see again the same sequence:

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181

To complete our quick tour of Lox language, let's calculate the sequence with a recursive function:

// Fibbonacci sequence - version 3
var count = 20;

// Let's define a recursive function
fun fib(n) {
  if (n < 2) return n;
  return fib(n - 1) + fib(n - 2);
}

// For demo purposes, let's assign the function to a variable
var fib_fun = fib;

for (var i = 0; i < count; i = i + 1) {
  print fib_fun(i);
  print " ";
}

The Loxxy is able to run the LoxLox interpreter. LoxLox is a Lox interpreter written in Lox by Ben Hoyt.
This interpreter with over 1900 lines long is (one of) the longest Lox pragram. As such, it is a good testbed for any Lox interpreter.

Executing a lox program with the LoxLox interpreter that is itself running on top of Loxxy.

Download the LoxLox source file in Github.

$ loxxy lox.lox

Once loxxy CLI starts its interpreter that, in turn, executes the LoxLox interpreter. This may take a couple of seconds. Don't be surprised, if the program seems unresponsive: it is waiting for you input. Enter a line like this:

  print "Hello, world!";

Then terminate with an end of file (crtl-D on Linuxes, crtl-z on Windows) followed by an enter key. You should see the famous greeting.

The following snippet executes the LoxLox interpreter and feeds to it the input text. That input text is made available through a StringIO that replaces the $stdio device.

require 'stringio'
require 'loxxy'

# Place your Lox pragram within the heredoc
program = <<-LOX_END
  print "Hello, world!";
LOX_END

lox_filename = 'lox.lox'
File.open(lox_filename, 'r') do |f|
  source = f.read
  cfg = { istream: StringIO.new(program, 'r')}
  lox = Loxxy::Interpreter.new(cfg)
  lox.evaluate(source)
end

Save this snippet as a Ruby file, launch Ruby with this file in command line. After a couple of seconds, you'll see the Ruby interpreter that executes the Loxxy interpreter that itself executes the LoxLox interpreter written in Lox. That last interpreter is the one that run the hello world line.

That's definitively meta...

This completes our quick tour of Lox, to learn more about the language, check the online book Crafting Interpreters

... Nothing...
Bob Nystrom designed a language simple enough so that he could present two interpreter implementations (a tree-walking one, then a bytecode one) in one single book.

In other words, Lox contains interesting features found in most general-purpose languages. In addition to that, there are numerous implementations in different languages

Lox interpreters, like Loxxy give the unique opportunity for the curious to learn the internals of reasonably-sized language.

While Lox blends interesting features from the two mainstream paradigms (OO and functional), it doesn't pretend to be used in real-life projects.

In fact, the language was designed to be simple to implement at the expense of missing advanced features.
Here are features I'll like to put on my wish list:

• Collection classes (e.g. Arrays, Maps (Hash))
• Modules
• Standard library
• Concurrency / parallelism constructs

That Lox cannot be compared to a full-featured language, is both a drawback and and an opportunity. Indeed, an open-source language that misses some features is an invitation for the curious to tinker and experiment with extensions. There are already a number of programming languages derived from Lox...

• To deliver an open source example of a programming language fully implemented in Ruby
  (from the scanner and parser to an interpreter).

• Extend the test suite
• Improve the error handling
• Improve the documentation

The next examples show how to use the interpreter directly from Ruby code.

require 'loxxy'

lox_program = <<LOX_END
  // Your first Lox program!
  print "Hello, world!";
LOX_END

lox = Loxxy::Interpreter.new
lox.evaluate(lox_program) # Output: Hello, world!

require 'loxxy'

lox_program = <<LOX_END
  fun add4(n) {
    n + 4;
  }

  print add4(6); // Output: 10
LOX_END

lox = Loxxy::Interpreter.new
lox.evaluate(lox_program) # Output 10

The Loxxy interpreter returns the value of the last evaluated expression.

require 'loxxy'

lox = Loxxy::Interpreter.new

lox_program = '47 - 5; // THE answer'
result = lox.evaluate(lox_program) # => Loxxy::Datatype::Number

# `result` is a Ruby object, so let's use it...
puts result.value # Output: 42

require 'loxxy'

lox_input = <<-LOX_END
  // Your first Lox program!
  print "Hello, world!";
LOX_END

lox = Loxxy::Interpreter.new
lox.evaluate(lox_program) # => Hello, world!

Here are the language features currently supported by the interpreter:

• Comments
• Keywords
• Datatypes
• Statements
  -Expressions
• Variable declarations
• For statement
• If Statement
• Print Statement
• While Statement
• Block Statement
• Function declaration

Loxxy supports single line C-style comments.

// single line comment

Loxxy implements the following Lox reserved keywords:

and, class, else, false, for, fun, if,
nil, or, print, return, this, true, var, while

loxxy supports all the standard Lox datatypes:

• Boolean: Can be true or false
• Number: Can be an integer or a floating-point numbers. For example: 123, 12.34, -45.67
• String: Sequence of characters surrounded by ". For example: "Hello!"
• Nil: Used to define a null value, denoted by the nil keyword

Loxxy supports the following statements:

• Expressions -Arithmetic expressions
  -String concatenation
  -Comparison expressions
  -Logical expressions
  -Grouping expressions
  -Variable expressions and assignments
  -Function call

-Variable declarations
-If Statement
-Print Statement
-While Statement
-Block Statement
-Function Declaration

Loxxy supports the following operators for arithmetic expressions:

• +: Adds of two numbers. Both operands must be of the type Number
  E.g. 37 + 5; // => 42
7 + -3; // => 4
• -: (Binary) Subtracts right operand from left operand. Both operands must be numbers.
  E.g. 47 - 5; // => 42
• -: (Unary) Negates (= changes the sign) of the given number operand.
  E.g. - -3; // => 3
• *: Multiplies two numbers
  E.g. 2 * 3; // => 6
• /: Divides two numbers
  E.g. 8 / 2; // => 4 5 / 2; // => 2.5

• +: Concatenates two strings. Both operands must be of the String type.
  E.g. "Hello" + ", " + "world! // => "Hello, world!"

• ==: Returns true if left operand is equal to right operand, otherwise false
  E.g. false == false; // => true
5 + 2 == 3 + 4; // => true
"" == ""; // => true
• !=: Returns true if left operand is not equal to right operand, otherwise false
  E.g. false != "false"; // => true
5 + 2 != 4 + 3; // => false
• <: Returns true if left operand is less than right operand, otherwise false. Both operands must be numbers
  E.g. 1 < 3; // => true
1 < 0; // => false
2 < 2; // => false
• <=: Returns true if left operand is equal to right operand, otherwise false. Both operands must be numbers
  E.g. 1 <= 3; // => true
1 <= 0; // => false
2 <= 2; // => true
• >: Returns true if left operand is equal to right operand, otherwise false. Both operands must be numbers
  E.g. 1 > 3; // => false
1 > 0; // => true
2 > 2; // => false
• >=: Returns true if left operand is equal to right operand, otherwise false. Both operands must be numbers
  E.g. 1 > 3; // => false
1 > 0; // => true
2 > 2; // => false

REMINDER: In Lox, false and nil are considered falsey, everything else is truthy.

• and: When both operands are booleans, then returns true if both left and right operands are truthy, otherwise false.
  If at least one operand isn't a boolean then returns first falsey operand else (both operands are truthy) returns the second operand. truthy returns the second operand.
  E.g. false and true; // => false
true and nil; // => nil
0 and true and ""; // => ""
• or: When both operands are booleans, then returns true if left or right operands are truthy, otherwise false.
  If at least one operand isn't a boolean then returns first truthy operand else (both operands are truthy) returns the second operand. E.g. false or true; // => true
true or nil; // => nil
false or nil; // => nil
0 or true or ""; // => 0
• !: Performs a logical negation on its operand
  E.g. !false; // => true
!!true; // => true
!0; // => false

Use parentheses ( ) for a better control in expression/operator precedence.

print 3 + 4 * 5;  // => 23
print (3 + 4) * 5; // => 35

In Lox, a variable expression is nothing than retrieving the value of a variable.

var foo = "bar;" // Variable declaration
print foo;  // Variable expression (= use its value)
foo = "baz"; // Variable assignment
print foo; // Output: baz

var iAmAVariable = "my-initial-value";
var iAmNil; // __Lox__ initializes variables to nil by default;
print iAmNil; // output: nil

// Calling a function without argument
print clock();

// Assumption: there exists a function `add` that takes two arguments
print add(2, 3);

Similar to the for statement in C language

for (var a = 1; a < 10; a = a + 1) {
  print a; // Output: 123456789
}

Based on a given condition, an if statement executes one of two statements:

if (condition) {
print "then-branch";
} else {
print "else-branch";
}

As for other languages, the else part is optional.

The statement print + expression + ; prints the result of the expression to stdout.

print "Hello, world!";  // Output: Hello, world!

  var a = 1;
  while (a < 10) {
    print a;
    a = a + 1;
  } // Output: 123456789

Lox has code blocks.

var a = "outer";

{
  var a = "inner";
  print a; // output: inner
}

print a; // output: outer

The keyword fun is used to begin a function declaration. In Lox a function has a name and a body (which may be empty).

fun add4(n) // `add4` will be the name of the function
{
  n + 4;
}

print add4(6); // output: 10

For Ruby, there is the lox gem.
There are other Ruby-based projects as well:

• SlowLox, described as a "1-to-1 conversion of JLox to Ruby"
• rulox

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/famished_tiger/loxxy.

The gem is available as open source under the terms of the MIT License.