`Enumerable#sum`

module Enumerable
  def sum(*args)
    inject(*args) { |sum, x| sum + x }
  end
end

Demo

  [1, 2, 3].sum # => 6
   (1..100).sum # => 5050
%w(foo bar).sum # => "foobar" 

[   ].sum(0) # => 0
[ 1 ].sum(2) # => 3

Ah, the joy of dynamic typing. Quack! What?