Enforcing an interface in Ruby

create_post.rb

class CreatePost
  # An interactor to create a post.
  
  # Initialize with a request object that implements the request interface for this interactor.
  def initialize(request)
    RequestInterface.required_on! request
    @user = {:title => request.title}
  end
  
  def result
    @user
  end
end

interface.rb

module Interface
  # General basis for an interface
  
  NotImplemented = Class.new ArgumentError
  # Indicates a target object does not implement the checked interface
  MissingMethod = Class.new ArgumentError
  # Raised when a method from the interface is not supported by the implementation
  InvalidReturn = Class.new ArgumentError
  # Raised when the target return value does not meet the expectaions on it.

  # Check if an interface is implemented on a target
  # Checked by seeing if the interface module is fronting the ancesor chain
  def implemented_on?(target)
    target.class.ancestors.first == self
  end

  # Enforce a target implements the interface
  def required_on!(target)
    raise NotImplemented unless self.implemented_on? target
  end

end

main.rb

# successul usecase
request = CreatePost::RubyRequest.new 'Building a ruby interface'
puts CreatePost.new request

# failed usecase
begin
  request = CreatePost::RubyRequest.new 5
  CreatePost.new request
rescue ArgumentError => err
  puts err
end

request_interface.rb

class CreatePost
  module RequestInterface
    # Interface specification for request objects that are used to initialize the create_post interactor
    
    # build on the general interface
    extend Interface

    # any request object must have a title method and it must return an instance of String
    def title
      if defined?(super)
        value = super
        raise Interface::InvalidReturn unless value.is_a? String
        value
      else
        raise Interface::MissingMethod
      end
    end
  end
end

ruby_request.rb

class CreatePost
  class RubyRequest
    # A simpe implementation of the request interface for a create post interactor
    
    # The request interface fronts this implementation.
    prepend RequestInterface
    def initialize(title)
      @title = title
    end

    def title
      @title
    end
  end
end

In the meantime there is a ruby gem class_interface to use the interface pattern in ruby easily:
https://github.com/magynhard/class_interface