title: What is the RWS Monad and when is it used
body: <p>I ran across the RWS Monad and its MonadTransformer while looking up something in the mtl library.  There is no real documentation there, and I was wondering what this is and where it gets used.</p>
  
  <p>I've gotten as far as finding that RWS is an acronym fro Reader, Writer, State and that is is a stack of those three monad transformers.  What I cannot figure out is why this is better then State by itself.</p>