example_low_coupling.rb

class Invoice
  def initialize *lines
    @lines = lines
  end
  
  def total
    @lines.inject(0){ |subtotal, line| subtotal += line_subtotal(line) }
  end

  def line_subtotal line
    line[:quantity] * line[:price]
  end
end


invoice = Invoice.new({quantity: 3, product_name: "ice tea", price: 300}, {quantity: 2, product_name: "Coca Cola", price: 1500})
print invoice.total