DelimR

For background information, see this blog post.

Implements delimited continuations in Ruby. This is a direct port of Oleg Kselyov's Scheme implementation.

This is an experiment. Ruby continuations are considered harmful. By the way, I want to take a minute to point out that this talk was at something called "Continuation Fest 2008", and calls Matz out for being a "criminal."

TL;DR: Don't use this for anything "serious."

Installation

Add this line to your application's Gemfile:

gem 'delimr'

And then execute:

$ bundle

Or install it yourself as:

$ gem install delimr

Usage

TODO: make this clearer

Delimited continuations can be a bit tricky. Basically, DelimR.prompt takes a block and marks the boundary of the continuation. Inside DelimR.prompt, you can call DelimR.control which takes a block with a single argument. That argument is a function which represents the rest of the computation around the DelimR.control block. The results of the DelimR.control block, or of calling the continuation, will be returned to where the DelimR.prompt was declared.

Examples

# k represents the computation outside of it
DelimR.prompt { 1 + DelimR.control { |k| k.call(3) } + 7}
# => 11

# If we don't call k, the computation is lost, and simply the value is returned
DelimR.prompt { 1 + DelimR.control { |k| 3 } + 7}
# => 3

# Delmited computations are composable! We can keep applying k to itself
DelimR.prompt { 1 + DelimR.control { |k| k.call(k.call(3)) } + 7}
# (1 + (1 + 3 + 7) + 7)
# => 19

A more involved example

Back in 1.8.7, Ruby core had an implementation of generators using plain old continuations. You can see it here.

Below, we implement Python style generators using delimited continuations.

class Generator
  def initialize(&block)
    @results = []
    DelimR.prompt do
      block.call(self)
    end
  end
  
  def next
    r = @results.pop
    if r.nil?
      raise "Iterator finished"
    end
    @k.call(nil)
    r
  end
  
  def yield(result)
    @results << result
    DelimR.control do |k| 
      @k = k
    end
  end
end

and use it:

counter = Generator.new do |g| 
  ctr = 0
  while true
    g.yield(ctr)
    ctr += 1
  end
end

counter.next
# => 0
counter.next
# => 1
counter.next
# => 2
# ...

For more information on delimited continuations

1. http://community.schemewiki.org/?composable-continuations-tutorial

Contributing

1. Fork it ( https://github.com/[my-github-username]/delimr/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