Sum digits to 100

sums_100.rb

def sum_digits_to(n)
  enumerator = Enumerator.new do |yielder|
    # recursive proc (backtracking) which iterates over all possible
    # combinations of digits and operators.
    p = -> prev_lhs = "", digit = 1 do
      lhs = "#{ prev_lhs }#{ digit }"
      if digit == 9
        yielder << lhs if eval(lhs) == n
      else
        [" + ", " - ", nil].each do |op|
          p.call("#{ lhs }#{ op }", digit.next)
        end
      end
    end
    p.call
  end
  enumerator.to_a
end

puts sum_digits_to(100)