This is a continuation of a conversation started in the following two links. https://groups.google.com/forum/#!searchin/haskell-cafe/michael$20litchard|sort:date/haskell-cafe/n0Tc29UUgoQ/iitt3z3PCwAJ https://groups.google.com/forum/#!searchin/haskell-cafe/michael$20litchard|sort:date/haskell-cafe/qD2kaZ9qpEA/jTDAp8KoCgAJ I am trying to model a 3-D space for a mudlike. Here's the criteria I have so far: (1) Objects in space will be either ships that can move or stationary things like non-moving game-controlled ships, space-stations , moons and planets. (2) Objects will not have spatial extent (3) Space is non-continuous (4) Collision happens only when one object is stationary or each moving objects are moving directly toward each other, on the same line. (5) Movement happens by setting a destination vector and a speed. There's no steering exactly, but you can change destination while moving, slow down or stop suddenly. (6) Octree looks like the data structure I want to use for modeling. I'm looking at http://hackage.haskell.org/package/Octree as best library for my application, if I am right about how to go about updating. I'm not sure I am. I think I can use the octree to track things in space, rather than having a node for each point. At most there will be say, 200 objects in space, therefore at most 200 nodes in the tree. So I am visualizing the following steps: (1) Begin with a [(Vector3 http://hackage.haskell.org/package/Octree-0.5.4.2/docs/Data-Octree.html#t:Ve..., a)]. This includes stationary objects and ships already in space. (2) If a new ship entered this space, add it to end of list. (3) make Octree a from above list (4) ships move by setting destination vector and then speed, calculate next Vector3 they will be at next tick (5) Make a new [(Vector3 http://hackage.haskell.org/package/Octree-0.5.4.2/docs/Data-Octree.html#t:Ve..., a)] (6) Go to (1) The first problem with this is in creating a new Octree from a list, when I would rather just recreate the new tree with a subtree of the nodes that changed. The library I am considering does not offer that operation. Maybe this is not a real problem considering the maximum objects in space will be 200. There may be other problems with my thinking here. I look to haskell-cafe for any insight it may offer.