Coming from a background mainly involving game development and control systems, this is one of the things that I find myself shaking my head at when reading HN. So many people here forget there's a software world outside web code, where memory and CPU time aren't effectively infinite, you're modeling processes and systems with continuously evolving state, and where trying to make everything immutable is just plain silly.
Well my last job was working on simulations and I've only worked on performance sensitive system....
Making everything immutable maybe is silly, but it's still a very good goal to have in mind. A continuously evolving system can be designed in an mostly immutable manner as well without sacrificing performance.
In a big picture sense you are trying to have your system be a function of time. And while for efficiency you sometimes should preserve intermediate states, you can typically accomplish that with clever caching systems that are not exposed in the interface. So the design remains "immutable" in a sense
It's certainly not realistic to make everything immutable, but it's still worthwhile to move global state into some monadic encapsulation to make your code more testable and composable. This might not be an option on embedded systems, though, or for certain specialized hardware, e.g. GPU's.
