loxxy

What is loxxy?

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

Purpose of this project:

• To deliver an open source example of a programming language fully implemented in Ruby
  (from the scanner and parser to an interpreter).
• The implementation should be mature enough to run LoxLox,
  a Lox interpreter written in Lox.

Current status

The interpreter currently can execute all allowed Lox expressions and statements except
object-oriented feaures (classes and objects). The goal is to implement these missing features in Q2 2021.

What's the fuss about Lox?

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

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

• Dynamically typed,
• Provides datatypes 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).

In other words, Lox contains interesting features found in most general-purpose languages.

What's missing in Lox?

Lox was constrained by design and was therefore not aimed to be a language used in real-world applications. Here are some missing parts to make it a practical language:

• Collections (arrays, maps, ...)
• Modules (importing stuff from other packages/files)
• Error handling (e.g. exceptions)
• Support for concurrency (e.g. threads, coroutines)

Also a decent standard library for IO, networking,... is lacking.

For sure, the language has shortcomings but on the other hand, it exhibits the essential features to cover in an introduction to language implementation.

That's already fun... and if all this gives you the inspiration for creating your own language, that might be even funnier...

Last point: what's makes Lox interesting is the fact that there are implementations in many languages

Hello world example

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!

A function definition example

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

Retrieving the result from a Lox program

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

Example using RawParser class

require 'loxxy'

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

# Show that the raw parser accepts the above program
base_parser = Loxxy::FrontEnd::RawParser.new

# Now parse the input into a concrete parse tree...
ptree = base_parser.parse(lox_input)

# Display the parse tree thanks to Rley formatters...
visitor = Rley::ParseTreeVisitor.new(ptree)
tree_formatter = Rley::Formatter::Asciitree.new($stdout)
tree_formatter.render(visitor)

This is the output produced by the above example:

program
+-- declaration_plus
|   +-- declaration
|       +-- statement
|           +-- printStmt
|               +-- PRINT: 'print'
|               +-- expression
|               |   +-- assignment
|               |       +-- logic_or
|               |           +-- logic_and
|               |               +-- equality
|               |                   +-- comparison
|               |                       +-- term
|               |                           +-- factor
|               |                               +-- unary
|               |                                   +-- call
|               |                                       +-- primary
|               |                                           +-- STRING: '"Hello, world!"'
|               +-- SEMICOLON: ';'
+-- EOF: ''

Suppported Lox language features

On one hand, the parser covers the complete Lox grammar and should therefore, in principle, parse any valid Lox program.

On the other hand, the interpreter is under development and currently it can evaluate only a subset of Lox. But the situation is changing almost daily, stay tuned...

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

Comments

Loxxy supports single line C-style comments.

// single line comment

Keywords

Loxxy implements the following Lox reserved keywords:

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

Datatypes

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

Statements

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

Expressions

Arithmetic expressions

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

String concatenation

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

Comparison expressions

• ==: 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

Logical expressions

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

Grouping expressions

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

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

Variable expressions and assignments

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

Variable declarations

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

Function call

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

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

For statement

Similar to the for statement in C language

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

If statement

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.

Warning: nested if...else

Call it a bug ... Nested if else control flow structure aren't yet supported by Loxxy.
The culprit has a name: the dangling else.

The problem in a nutshell: in a nested if ... else ... statement like this:

'if (true) if (false) print "bad"; else print "good";

... there is an ambiguity.
Indeed, according to the Lox grammar, the else could be bound either to the first if or to the second one. This ambiguity is usually lifted by applying an ad-hoc rule: an else is aways bound to the most recent (rightmost) if.
Being a generic parsing library, Rley doesn't apply any of these supplemental rules.
As a consequence,it complains about the found ambiguity and stops the parsing... Although Rley can cope with ambiguities, this requires the use of an advanced data structure called Shared Packed Parse Forest (SPPF). SPPF are much more complex to handle than the "common" parse trees present in most compiler or interpreter books. Therefore, a future version of Rley will incorporate the capability to define disambuiguation rules.

In the meantime, the Loxxy will progress on other Lox features like:

• Block structures...
• Iteration structures (for and while loops)

Print Statement

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

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

While Statement

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

Block Statement

Lox has code blocks.

var a = "outer";

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

print a; // output: outer

Function Declaration

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

Installation

Add this line to your application's Gemfile:

gem 'loxxy'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install loxxy

Usage

TODO: Write usage instructions here

Other Lox implementations in Ruby

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

Development

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.

Contributing

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

License

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