while_with_index.rb

def while_with_index(proc, &block)
  index = 0
  loop do
    break unless proc.call
    yield index
    index += 1
  end 
end

a = "woot"
while_with_index(Proc.new {a != "foobar" }) do |index|
  puts "index = #{index}"
  a = "foobar" if index == 10
end