module Module::Concerning

We often find ourselves with a medium-sized chunk of behavior that we’d like to extract, but only mix in to a single class.

We typically choose to leave the implementation directly in the class, perhaps with a comment, because the mental and visual overhead of defining a module, making it a Concern, and including it is just too great.

Using comments as lightweight modularity:

class Todo
  # Other todo implementation
  # ...

  ## Event tracking
  has_many :events

  before_create :track_creation
  after_destroy :track_deletion

  def self.next_by_event
    # ...
  end

  private
    def track_creation
      # ...
    end
end

Trying on a noisy embedded module:

class Todo
  # Other todo implementation
  # ...

  module EventTracking
    extend ActiveSupport::Concern

    included do
      has_many :events
      before_create :track_creation
      after_destroy :track_deletion
    end

    module ClassMethods
      def next_by_event
        # ...
      end
    end

    private
      def track_creation
        # ...
      end
  end
  include EventTracking
end

Once our chunk of behavior starts pushing the scroll-to-understand it boundary, we give in and move it to a separate file. At this size, the overhead feels in good proportion to the size of our extraction, despite diluting our at-a-glance sense of how things really work.

Mix-in noise exiled to its own file:

class Todo
  # Other todo implementation
  # ...

  include TodoEventTracking
end

Introducing Module#concerning.

By quieting the mix-in noise, we arrive at a natural, low-ceremony way to do bite-sized modularity:

class Todo
  # Other todo implementation
  # ...

  concerning :EventTracking do
    included do
      has_many :events
      before_create :track_creation
      after_destroy :track_deletion
    end

    class_methods do
      def next_by_event
        # ...
      end
    end

    private
      def track_creation
        # ...
      end
  end
end

Todo.ancestors
# => Todo, Todo::EventTracking, Object

This small step forward has some wonderful ripple effects. We can:

Public Instance Methods

concern(topic, &module_definition) click to toggle source

A low-cruft shortcut to define a concern.

concern :EventTracking do
  ...
end

module EventTracking
  extend ActiveSupport::Concern

  ...
end
# File lib/concerning/module_concerning.rb, line 134
def concern(topic, &module_definition)
  const_set topic, Module.new {
    extend ActiveSupport::Concern
    module_eval(&module_definition)
  }
end
concerning(topic, &block) click to toggle source

Define a new concern and mix it in.

# File lib/concerning/module_concerning.rb, line 119
def concerning(topic, &block)
  include concern(topic, &block)
end