Monar

This gem is implementation of Monad and Monad syntax in Ruby.

Installation

Add this line to your application's Gemfile:

gem 'monar'

And then execute:

$ bundle

Or install it yourself as:

$ gem install monar

Usage

This usage is unstable. It may be changed.

At first, Define flat_map in any object And include Monad module.

class Just
  include Monad
  include MonadPlus
  include Monar::Maybe

  def initialize(value)
    @value = value
  end

  def fmap(&pr)
    self.pure(pr.call(@value))
  end

  def flat_map(&pr)
    pr.call(@value)
  end

  # If this monad may returns more than 2 kinds object
  # Please indicate parent class by `monad_class` method.
  def monad_class
    Monar::Maybe
  end
end

class Nothing
  include Monad
  include MonadPlus
  include Monar::Maybe

  def initialize(*value)
  end

  def fmap(&pr)
    self
  end

  def flat_map(&pr)
    self
  end

  def mzero
    self
  end

  def mplus(_)
    self
  end

  def monad_class
    Monar::Maybe
  end
end

Use monadic_eval.

Just.new(val).monadic_eval do |x|
  a = x
  y <<= pure(a + 14)
  raise "error"
  z <<= case y
        when :prime?.to_proc
          Just.new(y)
        when 20
          Just.new(y)
        else
          Nothing.new
        end
end

The block given for monadic_eval must return same monad_class object.

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/joker1007/monar.

License

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