module Enumerable

[].sum(Payment.new(0)) { |i| i.amount } # => Payment.new(0)

The default sum of an empty list is zero. You can override this default:

[[1, 2], [3, 1, 5]].sum => [1, 2, 3, 1, 5]
['foo', 'bar'].sum # => "foobar"
[5, 15, 10].sum # => 30

It can also calculate the sum without the use of a block.

payments.inject(0) { |sum, p| sum + p.price }

The latter is a shortcut for:

payments.sum(&:price)
payments.sum { |p| p.price * p.tax_rate }

Calculates a sum from the elements.

def sum(identity = 0, &block)
  if block_given?
    map(&block).sum(identity)
  else
    inject { |sum, element| sum + element } || identity
  end
end