How do I code a tree of objects in Haskell with pointers to parent and children?

Question

I\'ve got the following problem: I have a tree of objects of different classes where an action in the child class invalidates the parent. In imperative languages, it is trivial

Answer 1

I don't have much experience with Haskell, but as far as I know it's not possible to have circles in the reference graph in pure functional languages. That means that:

1. You can't have a 2-way lists, children in trees pointing to their parents, etc.*
2. It is usually not enough to change just one node. Any node that is changed requires changes in every node starting from the "root" of the data structures all the way to the node you wish to change.

The bottom line is, I wouldn't try to take a Java (or any other imperative language) algorithm and try to convert it to Haskell. Instead, try to find a more functional algorithm (and maybe even a different data structure) to solve the problem.

EDIT:

From your clarification it's not entirely clear whether or not you need to invalidate only the direct parent of the object that changed or all its ancestors in the hierarchy, but that doesn't actually matter that much. Since invalidating an object basically means changing it and that's not possible, you basically have to create a modified duplicate of that object, and then you have to make its parent point to it to, so you have to create a new object for that as well. This goes on until you get to the root. If you have some recursion to traverse the tree in order to "modify" your object, then you can recreate the path from that object to the root on your way out of the recursion.

Hope that made sense. :s

*As pointed out in the comments by jberryman and in other answers, it is possible to create circular reference graphs in Haskell using lazy evaluation.