Hi all, After learning some Haskell recently, I decided to revisit a book about functional programming techniques for Perl: Higher Order Perl. I didn't fully understand the book at the time but now my Haskell experience has proved to be very insightful. Towards the end of the book the author implements a local propagation network. Here is the Perl source code: http://hop.perl.plover.com/Examples/Chap9/Local-Propagation/ The PDF of the specific chapter is here: http://hop.perl.plover.com/book/pdf/09DeclarativeProgramming.pdf I would like to experiment with something similar in Haskell, but the way this network is designed is all about state and references: - Wires have a values that can change over time; - Wires have references to nodes; - Nodes have references to wires; I'm a bit stuck as to how to approach the "object has a list references to other objects" situation from Haskell. I tried this: type Name = String data Node = Node Name [Wire] data Wire = Wire Name Node Double [Node] But that doesn't seem like it would work since when I change a Wire I must find all "copies" of it (in the Node objects) and update them also. Perhaps I should just refer to Wires/Nodes by name and use an association list to lookup them up, but that seems cumbersome. Anybody have any suggestions? Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada