Functional-ish version of stack machine

stack_machine_functional.rb

$stack = []

def solution(s)
	s.each_char do |c|
		case c
		when /[0-9]/
			$stack.push c
			puts c
		when '+'
			return -1 unless op_nums(lambda { |left, right| left + right })
		when '*'
			return -1 unless op_nums(lambda { |left, right| left * right })
		end
	end

	return $stack.pop
end

def op_nums operation
	left = $stack.pop
	right = $stack.pop
	if left and right
		$stack.push operation.call(left.to_i, right.to_i)
	else
		false
	end
end