> Right, but then you can't multi-thread merge() Not sure If I understand what you mean. It would be both possible to call "merge" in parallel or to implement

> Right, but then you can't multi-thread merge() Not sure If I understand what you mean. It would be both possible to call "merge" in parallel or to implement merge to do the work in parallel as well. > Or else, you say that it's immutable, and when some thread changes the state, it produces a new map3, and the old one doesn't change. But then you have the problem of getting all the other threads updated to see the new version of map3. Yes, but this "problem" is exactly the beauty of this style of programming. It forces you to make your data-flows explicit and hence easy to discover, understand and manipulate/change by other developers. > And, by the way, I've worked on that router for TV stations. Modeling that matrix in a way that invalid states are unrepresentable is, um, extremely non-trivial... I'm not saying it is trivial. But it is possible and depending on the programming language it is actually surprisingly easy. Not only that, with a good typesystem, you not only prevent invalid state from occurring, you even get a lot of support from your IDE due to the extra information it has. Here's an example how that can look like for matrices and functions to manipulate them (like transpose): https://youtu.be/DRq2NgeFcO0?t=1137 https://youtu.be/DRq2NgeFcO0?t=1137 The syntax might be alien to you, but I hope it still shows what's possible at compile-time before even running the program with modern languages today.