[Haskell-beginners] Implementing a Local Propagation Network

Patrick LeBoutillier patrick.leboutillier at gmail.com
Mon May 17 20:59:11 EDT 2010

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:
The PDF of the specific chapter is here:

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 LeBoutillier
Rosemère, Québec, Canada

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