DSL for calculating various fees on an eCommerce purchase. Developed for @Ticketbud. Not our actual equation, but an example of how you could use it...

calculator_dsl.rb

# ASSUMPTIONS
# Assumes we respond_to #fee_schedule, which has various details, 
# like processor_rate, merchant_custom_handling_fee, or the like.  
module CalculatorDSL
  extend ActiveSupport::Concern

  included do
    singleton_class.send :alias_method, :method_missing, :define_method
  end

  def method_missing(name, &block)
    unless defined?(@@calculated)
                              # voodoo. Sends each method defined on a calculator.
      @@calculated = true and self.class.instance_methods(false).each(&method(:send))
    end

    self.class.send(name, &block) if block_given?

    [self, fee_schedule].find { |obj| obj.respond_to?(name) }.try(name) || 0.0
  end
end

example_calculator.rb

class ExampleLineItemCalculator < Struct.new(:price, :fee_schedule) # lazy example, don't mind the Struct
  include CalculatorDSL

  # You can nest blocks as many times as you want, 
  # for example, if you want to wrap net and our_revenue in 
  # a sans_processor_fees { ... } block, go for it...
  gross {
                  net {                               price    +
                          merchant_custom_item_handling_fee }  +

          our_revenue {                      net * our_rate    +
                                           our_per_item_fee }

    processor_revenue {                 ( net + our_revenue )  *
                                             processor_rate }
  } # end gross
end

example_usage.rb

schedule = Struct.new(:merchant_custom_item_handling_fee, :our_rate, :our_per_item_fee, :processor_rate).
                  new(                              1.00,      0.02,              0.99,           0.029)
                  
calculator = ExampleLineCalculator.new(10.0, schedule)
calculator.net         # => 11
calculator.our_revenue # => 1.21
calculator.gross       # => 12.56