Whether circular or not, sharing values by using a "back pointer" is problematic for any update. Why not use a zipper instead? I looked into zippers before and the problem I had was that they never really matched the structure which I needed and which led me to think about this whole knot tying thing again.[1] The things I read about them always assumed either a list like (i.e. linear) or a tree like (i.e. existence of a root) structure on the type to be plugged into the zipper. Now I know that this is not necessary and there are references to a generic zipper implementation but the thing is that I only found the source code and decided to first look into the knot tying thing before opening another can of worms with delimited continuations since I already spend too muchtime thinking about that stuff.[2] So if anyone has pointers to a good generic zipper explanation I would be thankful. Note that I don't need full blown error handling. I would first like to understand the basics. Now I think I came to the conclusion that locally updating a tied knot in a pure way really is hard and that it's not me just not seeing some obvious solution. So I just have to decide whether to use IORefs/Vars (clunky) or to implement zippers for the structure I need (probably toohard for me). Anyways thanks for all the answers from which I learned a few unexpected things and which assured me that my instincts aren't that far off.
Cheers and warm regards (and a happy new year) Stephan [1]: "Again" means that I already looked into both tying the knot and zippers about a year ago out of curiosity. This time it was out of necessity although the original question I posed was more a necessity diverging into curiosity again. [2]: Note that "too much time" rather refers to the quantity I have at my disposal than to personal preferences. If I could I would like nothing more than to think about this "stuff" for the rest of my life.