This actually seems like a very good collection of strategies. The only one I use that I see missing is converting a closure capture into an argument. If you de

This actually seems like a very good collection of strategies. The only one I use that I see missing is converting a closure capture into an argument. If you design something like: zebra.onmove(|| log(barn.contains(zebra))), then you will find everything locks up due to the references of the closure. Instead you convert the data to args: zebra.onmove(|world, zebra| log(world.barn.contains(zebra))). Obviously with cheap data which you can freeze and copy like a BarnId it's fine to do that. In general, "stop, drop, reacquire" is a good motto. ie finish figuring out what you want to happen, release the resources that you needed to figure that out, reacquire exactly the resources you need to make the thing happen, do it. That's basically the premise of 'mutation-as-data'.