An example of hook class/module

hooks.rb

class ShippingOption
  @children = []
  def self.inherited(child)
    @children << child
  end

  def self.for(weight, international)
    @children.select do |child|
      child.can_ship?(weight, international)
    end
  end
end

# ../shipping_priority_flat_rate
class PriorityFlatRate < ShippingOption
  def self.can_ship?(weight, international)
    !international
  end
end

# ../shipping_madia_mail.rb
class MediaMail < ShippingOption
  def self.can_ship?(weight, international)
    !international and weight < 4*16
  end
end

# If the files are on diferent directory
# Dir.glob("shipping_*.rb") { |name| require name }
puts ShippingOption.for(3*16, false)
# => PriorityFlatRate
# => MediaMail
puts ShippingOption.for(5*100, false)
# => PriorityFlatRate