It can be done but it's quite expensive to engineer such a solution, both in time spent theorizing and in runtime performance consequences. There are many small

It can be done but it's quite expensive to engineer such a solution, both in time spent theorizing and in runtime performance consequences. There are many small gameplay features that are trivial with mutability and a data-oriented model, but require a major spec change if functional style is applied. The parts that are easy to do functionally are relatively trivial in comparison. The happy medium for gameplay simulations is to write straight-line imperative code and minimize your use of subroutines and indirection - you want "static" synchronization that loops over data in big batches where execution flow reads easily. When it's kept simple like this, major desynchronization challenges come down to compilation behaviors and interactions with other modules(render, network, etc). The main loop gets big as a result, but factoring it out only makes it harder to follow, because the complexity is coming from the high number of sync points, not duplicated code.