module Enumerable

def sum(identity = nil, &block)

[].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 = nil, &block)
  if identity
    _original_sum_with_required_identity(identity, &block)
  elsif block_given?
    map(&block).sum
  else
    first = true
    reduce(nil) do |sum, value|
      if first
        first = false
        unless value.is_a?(Numeric) || value.respond_to?(:coerce)
          ActiveSupport::Deprecation.warn(<<-MSG.squish)
            Rails 7.0 has deprecated Enumerable.sum in favor of Ruby's native implementation available since 2.4.
            Sum of non-numeric elements requires an initial argument.
          MSG
        end
        value
      else
        sum + value
      end
    end || 0
  end
end