Patrick, If you want to implement it in a functional style, you have to use an association map of some sort. Haskell only has values, but not any concept of a reference (unless you count things like IORef, but I am not counting those). Generally speaking this is needed whenever you are dealing with a data structure that has cycles. (Generally speaking because it's possible to make data structures lazily refer to themselves.) People usually use IntMap, but there's a new package EnumMap on Hackage which is really powerful. It's like IntMap only typesafe. You will need a counter in your data structure as a source of unique ids. You can also use value-supply (from Hackage), which is a great bit of code. On the face of it, this seems cumbersome, but the way to do it is to create a data structure and access it through accessor functions like "add node", "delete node", "follow wire", etc. This way you can abstract those details away. People have done various directed/undirect graph packages and so on on Hackage - I can't recommend anything. Stick with it - this approach does work. I've done things like conversion of 3D models into triangle strips using this method, with very satisfying results. Steve On 18/05/10 12:59, Patrick LeBoutillier wrote:
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