TCOMethod

The tco_method gem provides a number of different APIs to facilitate evaluating code with tail call optimization enabled in MRI Ruby.

The TCOMethod.with_tco method is perhaps the simplest means of evaluating code with tail call optimization enabled. TCOMethod.with_tco takes a block and compiles all code in that block with tail call optimization enabled.

The TCOMethod::Mixin module extends Classes and Modules with helper methods (kind of like method annotations) to facilitate compiling some types of methods with tail call optimization enabled.

The TCOMethod.tco_eval method provides a direct means to evaluate code strings with tail call optimization enabled. This API is the most cumbersome, but it can be useful for loading full files with tail call optimization enabled (see examples below). It is also the foundation of all of the other TCOMethod APIs.

Be warned, there are a few gotchas. For example, even when using one of the APIs provided by the tco_method gem, require, load, and Kernel#eval still won't evaluate code with tail call optimization enabled without changing the RubyVM settings globally. More on the various limitations of the tco_method gem are outlined in the docs in the Gotchas section.

Installation

Add this line to your application's Gemfile:

gem "tco_method"

And then execute:

$ bundle

Or install it yourself as:

$ gem install tco_method

Usage

Require the TCOMethod library:

require "tco_method"

TCOMethod.with_tco

The fastest road to tail call optimized glory is the TCOMethod.with_tco method. Using TCOMethod.with_tco you can evaluate a block of code with tail call optimization enabled liked so:

TCOMethod.with_tco do
  class MyClass
    def factorial(n, acc = 1)
      n <= 1 ? acc : factorial(n - 1, n * acc)
    end
  end
end

puts MyClass.new.factorial(10_000).to_s.length
# => 35660

It's worth noting that in the example above the actual optimized tail call occurs outside of the TCOMethod.with_tco block. TCOMethod.with_tco is used to compile code in such a way that tail call optimization is enabled. Once compiled, the tail call optimized code can be invoked from anywhere in the program.

TCOMethod::Mixin

Alternatively, you can extend a Class or Module with the TCOMethod::Mixin and let the TCO fun begin using helpers that act like method annotations.

To redefine an instance method with tail call optimization enabled, use tco_method:

class MyClass
  extend TCOMethod::Mixin

  def factorial(n, acc = 1)
    n <= 1 ? acc : factorial(n - 1, n * acc)
  end
  tco_method :factorial
end

puts MyClass.new.factorial(10_000).to_s.length
# => 35660

Or alternatively, use tco_module_method or tco_class_method for a Module or Class method:

module MyFibonacci
  extend TCOMethod::Mixin

  def self.fibonacci(index, back_one = 1, back_two = 0)
    index < 1 ? back_two : fibonacci(index - 1, back_one + back_two, back_one)
  end
  tco_module_method :fibonacci
end

puts MyFibonacci.fibonacci(10_000).to_s.length
# => 2090

TCOMethod.tco_eval

Finally, depending on your needs (and your love for stringified code blocks), you can also use TCOMethod.tco_eval directly. TCOMethod.tco_eval can be useful in situations where the method_source gem is unable to determine the source of a particular block or for loading entire files with tail call optimization enabled.

TCOMethod.tco_eval(<<-CODE)
  class MyClass
    def factorial(n, acc = 1)
      n <= 1 ? acc : factorial(n - 1, n * acc)
    end
  end
CODE

MyClass.new.factorial(10_000).to_s.length
# => 35660

You can kind of get around the need for stringified code blocks by reading the code you want to compile with tail call optimization dynamically at runtime, but this approach also has downsides in that it goes around the standard Ruby require/load process. For example, consider the Fibonacci example broken across two scripts, one script serving as a loader and the other script acting as a more standard library:

# loader.rb

require "tco_method"
fibonacci_lib = File.read(File.expand_path("../fibonacci.rb", __FILE__))
TCOMethod.tco_eval(fibonacci_lib)

puts MyFibonacci.fibonacci(10_000).to_s.length
# => 2090


# fibonacci.rb

module MyFibonacci
  def self.fibonacci(index, back_one = 1, back_two = 0)
    index < 1 ? back_two : fibonacci(index - 1, back_one + back_two, back_one)
  end
end

If you really want to get crazy, you can include the TCOMethod::Mixin module in the Module class to add these behaviors to all Modules and Classes. To quote VIM plugin author extraordinaire, Tim Pope, "I don't like to get crazy." Consider yourself warned.

# Don't say I didn't warn you...

Module.include(TCOMethod::Mixin)

module MyFibonacci
  def self.fibonacci(index, back_one = 1, back_two = 0)
    index < 1 ? back_two : fibonacci(index - 1, back_one + back_two, back_one)
  end
  tco_module_method :fibonacci
end

puts MyFibonacci.fibonacci(10_000).to_s.length
# => 2090

Gotchas

Quirks with the method_source gem:

• Annotations and TCOMethod.with_tco use the method_source gem to retrieve the method source to evaluate. As a result, class annotations and TCOMethod.with_tco can act strangely when used in more dynamic contexts like irb or pry. Additionally, if the code to be evaluated is formatted in unconventional ways, it can make it difficult for method_source and/or tco_method to determine the unambiguous source of the method or code block. Most of these ambiguities can be solved by following standard Ruby formating conventions.

Quirks with TCOMethod.with_tco:

• Because the source code of the specified block is determined using the method_source gem, the given block will be evaluated with a binding different from the one it was defined in. Attempts have been made to get around this, but so far, no dice. Seems like a job for a C extension.
• require, load, and eval will still load code without tail call optimization enabled even when called from within a block given to TCOMethod.with_tco. Each of these methods compiles code using the primary RubyVM::InstructionSequence object which honors the configuration specified by RubyVM::InstructionSequence.compile_option.

Quirks with Module and Class annotations:

• Annotations only work with methods defined using the def keyword.
• Annotations reopen the Module or Class by name to redefine the given method. This process will fail for dynamic Modules and Classes that aren't assigned to constants and, ergo, don't have names that can be used for lookup.

There are almost certainly more gotchas, so check back for more in the future if you run into weirdness while using this gem. Issues are welcome.

Contributing

1. Fork it ( https://github.com/tdg5/tco_method/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

Reference

• Class annotations are based on Nithin Bekal's blog post Tail Call Optimization in Ruby which follows his efforts to create a method decorator to recompile methods with tail call optimization.
• For more background on how tail call optimization is implemented in MRI Ruby, see Danny Guinther's Tail Call Optimization in Ruby: Deep Dive.
• For those on flavors of Ruby other than MRI, check out Magnus Holm's Tailin' Ruby for some insight into how else tail call optimization (or at least tail call optimization like behavior) can be achieved in Ruby.