Hi, I've been thinking a lot lately about heterogeneous and extensible data structures for which HList certainly offers a solution. While HList is implemented through type-level programming I wonder if I can achieve similar results through value-level programming alone. This is where I was thinking of functional references. I wonder if or rather how one could do this: Let's say there was some clever monad ... someMonad = do h1 <- add "twenty" h2 <- add False h3 <- add 16 ..... modify h2 True and get a ("twenty",(True, 16)) back. And while *in* the monad some accessors available. Now come to think of it I think I actually read about this somewhere so I doubt this is truly my idea. Anybody got some thoughts on this? Günther