to_proc

class Integer
  def sum_of_digits
    #why the following inject method call works?
    #[1,2].inject(0) {|object, *args| object.send('+', *args)}
    to_s.split('').map(&:to_i).inject(0, &:+)
  end
end

# why the following doesn't work
[12, 34].inject(0, &:sum_of_digits) 

# The reason is code above will be converted to something like
[12, 34].inject(0) {|obj, *args| obj.send(:sum_of_digits, *args)}

#this works
[12, 34].inject(0){|sum, n| sum += n.sum_of_digits}