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

Semantics are wrong. Hash#fetch doesn't store if there's no value. So you should yield and do nothing else but return.

So: def fetch(key); get(key) || yield if block_given?; end

This is a copy of ActiveSupport::Cache::Store#fetch, more or less. http://apidock.com/rails/ActiveSupport/Cache/Store/fetch

The goal is, if there's no value from the #get, store the return value of the block as the value for the given key.

@avdi recently highlighted Hash#fetch. idk, place the yield as an argument to #add. One liner bliss?

Haha, there's that. It's a difficult method to make elegant, because it seems necessary to write it very defensively.

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?