From: ChrisKaelin <ck.stonedragon@...>
Date: 2007-04-21T04:10:09+09:00
Subject: once modifier (pickaxe book page 391)

The following code is from the pickaxe book page 391 (slightly
modified in order to have it available in all classes, that are
defined). It implements a once modifier, that ensures a method's body
is only called, if it's return value (an instance variable) is not
already set.

# Extending Module with the amazing once modifier
class Module
  def once(*ids) # :nodoc:
    for id in ids
      module_eval <<-"EOF"
        alias_method :__#{id.to_i}__, :#{id.to_s}
        private_methods :__#{id.to_i}__
        def #{id.to_s}(*args, &block)
          (@__#{id.to_i}__ ||= [__#{id.to_i}__(*args, &block)])[0]
        end
      EOF
    end
  end
end

It really works very well and is a wonderful example of ruby's power.

But I got some noob questions now:
- Why isn't this cool method available in ruby directly?
- Why can't I use "ids.each" instead of "for id in ids"? Ok, this
question has nothing to do with the code itself ;-)
- I know what "||=" does (evaluate assignment only, if the left part
is false), but what do you call this?

Sorry for my lack of these basics and thanks in advance for any
answers.

Chris