Hi, I have a problem that, it seems, I can not solve cleanly without resorting to imperative programming (via ST): Assume you have a tree (and you can think of a real tree here), defined by something like this: data Tree a = Bud | Branch a Double Tree Tree -- | ` Lenght of this branch -- ` General storage field for additional information Now, I have a nice algorithm that calulates something for each branch, but it only works on lists of branches, so I have to cut them apart first, remembering their position in space, and then work on these, well, logs. data Point = Point Double Double data Log = Log Point Point type Info = ... noInfo :: Info cutTreeApart :: Tree a -> [(Log, a)] someAlgorithm :: [(Log,a)] -> [(a, Info)] Conveniently, the algorithm allows me to tag the logs with something, to be able to keep track at least somewhat of the logs. Unfortunately, I need this information in the storage field in my Tree, as the list of logs is not sufficient for later calculations. Idea: Using ST ============== annotateTree :: Tree a -> Tree Info annotateTree tree = runSt $ do -- Put an STRef in each node treeWithPointer <- mapM const (newSTRef noInfo) tree -- Cut this tree apart let logsWithPointers = cutTreeApart treeWithPointer -- Run the algorithm let informations = someAlgorithm logsWithPointers -- Put the information back, via the ST ref mapM (\(stRef, info) -> writeSTRef stRef info) informations -- Read the ST refs mapM readIORef tree Note that I assume a instance Traversable Tree here, and mapM is Data.Traversable.mapM. Now while this works, and while ST is still somewhat pure, I’m wondering if there is no better way of expressing "This piece of information came from the point in a data structure, so something else can be put here easily". Some ideas where numbering the Nodes and then using this number as the tag on the log, but this is not much different from using STRefs, it seems. Thanks, Joachim -- Joachim "nomeata" Breitner mail: mail@joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata@joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata@debian.org