title: Ruby dependency injection libraries
body: <p>I've been looking at some Ruby dependency injection libraries.  In particularly, I checked out  <a href="http://needle.rubyforge.org/" rel="noreferrer">Needle</a> and <a href="http://copland.rubyforge.org/" rel="noreferrer">Copland</a>.  They've been around for quite awhile, yet not a lot of usages.  </p>
  
  <p>What are some of the pros and cons of using these two libraries? It sure seems like a lot of libraries / frameworks out there could make good use of these two libraries, e.g. <a href="http://github.com/sam/extlib/tree/master/lib/extlib/hook.rb" rel="noreferrer">Merb / Datamapper's Hook</a>.</p>