Adds :fact to Fixnum

factorial.rb

module WhiteHat101
  module Factorial
    # returns 1 for input < 1
    def fact
      (1..self).inject 1, :*
    end
    alias_method :!, :fact
  end
end

Fixnum.send :include, WhiteHat101::Factorial

# 6.fact => 720
# 0.fact => 1
# 3.!    => 6