# Allows methods to be modified so that they accept continuation-passing
# style, passing their return value to a block instead of returning it
# directly

module Continuations
  METHODS = {}

  def accept_continuation(*args)
    args.each do |method_name|
      func = instance_method(method_name)
      id = func.object_id
      METHODS[id] = func

      module_eval <<-EOS
        def #{method_name}(*params, &block)
          value = Continuations::METHODS[#{id}].bind(self).call(*params)
          block.call(value) if block
          value
        end
      EOS
    end
  end
end


# Example

class String
  extend Continuations
  accept_continuation :gsub, :downcase
end

"FOO".downcase do |str|
  puts str + "  !!"
end