memcached.rb

class Memcached
  def fetch(key)
    if value = get(key)
      value
    elsif block_given?
      begin
        add(key, value = yield)
      rescue Memcached::NotStored => e
        value = get(key) 
      end
      value
    end
  end
end

Looking at it again...the rescue clause is useless since #get should return nil since nothing was stored. Here's another stab:

  def fetch(key)
    value = get(key)
    if value.nil? && block_given?
      value = yield
      add(key, value)
    end

    value
  rescue Memcached::NotStored
    nil # or do we want value, the result of the yield? change the above to ensure?
  end

And now to curse FriendFeed. :-)

I don't know that it's necessarily completely useless. If, for some reason, the add to Memcache is slow, and a key of the same name gets added between the check for a value and the completion of our add, we'll get the NotStored error. If that happens, we want to return the value of the key that was stored while we were trying to store something else. Haha.

Maybe it's overly defensive? I don't know. I left it out initially, but I can see its purpose.

  def fetch(key)
      get(key) || add(key, yield) rescue get(key) if block_given?
  end

Is that legal? Ruby ♥ moment if it is. Where's the spec?