https://github.com/mveytsman/delimr

Delimited continuations for Ruby
https://github.com/mveytsman/delimr

Last synced: 27 days ago
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Delimited continuations for Ruby

• Host: GitHub
• URL: https://github.com/mveytsman/delimr
• Owner: mveytsman
• License: mit
• Created: 2014-07-14T06:39:26.000Z (over 10 years ago)
• Default Branch: master
• Last Pushed: 2014-07-14T20:50:27.000Z (over 10 years ago)
• Last Synced: 2024-04-14T05:51:50.047Z (7 months ago)
• Language: Ruby
• Size: 125 KB
• Stars: 2
• Watchers: 2
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • License: LICENSE.txt

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README

        
# DelimR

*For background information, see this [blog post](http://blog.ontoillogical.com/blog/2014/07/12/delimited-continuations-in-ruby/).*

Implements [delimited continuations](https://en.wikipedia.org/wiki/Delimited_continuation) in Ruby. This is a direct port of Oleg Kselyov's Scheme [implementation](http://okmij.org/ftp/continuations/implementations.html#delimcc-scheme).

This is an experiment. Ruby continuations are [considered harmful](http://www.atdot.net/~ko1/pub/ContinuationFest-ruby.pdf). 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

```ruby

# 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](https://github.com/ruby/ruby/blob/ruby_1_8_7/lib/generator.rb).

Below, we implement Python style generators using delimited continuations.

```ruby

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:

```ruby

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