[eru]

Here's a challenge: Design a zipper that works for cyclic graphs. There will be 10 GBP for the winner's charity of choice. (jacquesm may give another 10 Euro in addition.)

(Hint: "Purely Functional Datastructures" solves this problem for graphs shaped like a sigle ring. Perhaps this solution can be generalized?)

[eru]

Please post entries either to HN or send me an email. My address is in my profile.

I can also give more details about rules and conditions, if needed.

P.S. Might as well make it 100 GBP for the first challenge. I am really interested to see whether there's a solution that does it in O(1), like zippers do. (Or alternatively a proof that O(1) to move from node to node is not possible.)

[copper]

Wouldn't the Data.Graph.Inductive representation of a graph satisfy this (It might not be O(1), but it's pretty close)? I must admit that I find it hard to see what a zipper for a graph should look like, though.

[eru]

I know how to get a zipper for a static graph by Tying the Knot (http://haskell.org/haskellwiki/Tying_the_Knot). But writing one where you can add and remove nodes and edges seems much harder.

You already know how a zipper for a tree looks like. And you have probably seen a functional queue. Generalizing from both of those, you can imagine how a zipper would work.

I can write down the types and all, even if I don't know of any implementation. But that's probably a topic for a blog post, or so.

[copper]

I think I understand that, but without actually writing it out, it's going to be impossible to test.

The types I was talking about go as: type Context a b = (Adj b, Node, a, Adj b) type Adj b = [(b, Node)]

It definitely allows traversal, and adding nodes seems to be possible, too (judging from the api, of course, it ought to be :)

[eru]

From the types you give, I can't see how it works. Would you please elaborate?