Sample code using the Operation + Result pattern

calculate_shipping_cost.rb

class CalculateShippingCost < Operation
  def initialize(order)
    @order = order
  end

  def perform
    validation_result = validate_address
    return validation_result if validation_result.failure?

    calculate_shipping_cost
  end

  private

  def calculate_shipping_cost
    correios_result = calculate_with_correios
    return correios_result if correios_result.success?

    calculate_internally
  end

  def validate_address
    if @order.address.blank?
      Result.error("Address is required")
    end

    Result.ok
  end

  def calculate_with_correios
    Shipping::Correios.perform(@order)
  end

  def calculate_internally
    Shipping::Simulation.perform(@order)
  end
end

operation.rb

# Classe abstrata para operações
class Operation
  class << self
    def perform(*params)
      new(*params).perform
    end
  end

  #: -> Result
  def perform
    raise NotImplementedError
  end
end

result.rb

class Result
  attr_reader :data, :errors

  def initialize(data, errors)
    @data = data
    @errors = errors
  end

  def success?
    @errors.empty?
  end

  def failure?
    !success?
  end

  class << self
    def ok(data = nil)
      new(data, [])
    end

    def error(errors)
      new(nil, Array(errors))
    end
  end
end