@niwinz: thanks for help with lentes. Nikita answered on github and actually implemented derived-atom https://github.com/tonsky/rum/issues/75. Just yesterday I realised that lenses are indeed two-way, while reactions (and this new derived-atom) is one-way (changing the reaction doesn't change the atoms). Not really sure which one I needed for the case I used as an example, but I'm sure it's nice to have both in my toolkit! 🙂