side-effect-free-approach-1.rb

# This is a possible approach to baking in side-effect free support into
# the context object for DCI. The #on_call would support a declarative way of
# knowing which roles to mixin/unmix on a given object.  This could
# use something like mixology or another implementation under the covers.

class AddToCartContext < Context
  attr_reader :user, :book

  def self.call(user, book)
    AddToCartContext.new(user, book).call
  end

  def initialize(user, book)
    @user, @book = user, book
  end
  
  on_call :user => becomes(Customer) do
    @user.add_to_cart, @book
  end
end