Upcastable

Upcastable provides the feature to emulate upcasting in Ruby.

Duck typing sometimes results in NoMethodError unexpectedly by calling methods some classes don't have even if the code pass a test using other classes which have the methods. We can avoid such situations by upcasting. All we have to do is implementing methods defined in the super class or module and we don't have to care about whether or not methods defined only in some subclasses are called.

Add this line to your application's Gemfile:

gem 'upcastable'

And then execute:

$ bundle

Or install it yourself as:

$ gem install upcastable

We define Animal module and Cat class to explain the usage like below:

module Animal
  def talk; end
end

class Cat
  include Animal

  def talk
    'Meow!'
  end

  def run
    'Running...'
  end
end

Upcastable defines Object#upcast_to and you can use Animal like a interface by upcasting Cat object to Animal.

cat = Cat.new
animal = cat.upcast_to(Animal)
animal.class #=> Cat
animal.talk  #=> "Meow!"
animal.run   #=> NoMethodError: `run' is not defined in Animal

Internally, the upcasted object is a Upcastable::UpcastedObject instance which delegates almost all methods to the original object. Therefore the value of #object_id is not changed.

cat.object_id == animal.object_id #=> true

You can also downcast the upcasted object to the original class by calling Object#downcast.

animal.downcast.run #=> "Running..."

There are two additional methods, Object#upcasting and Object#upcasted?. Object#upcasting returns the ancestor to which the object have been upcasted and Object#upcasted? returns true if the object is upcasted.

cat.upcasted?    #=> false
cat.upcasting    #=> nil
animal.upcasted? #=> true
animal.upcasting #=> Animal

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.

Bug reports and pull requests are welcome on GitHub at https://github.com/abicky/upcastable. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

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