But mm... why you would want to mutable state like current directory in first place? Or even mutable filesystem? Why aren't those just immutable parameters of your program?
Because that's how just about any OS, application, library, filesystem, device driver, database, nearly everything related to computing at all, works. Reinventing the universe has never, ever lead to success. If you want even the slightest hope for non-negligible adoption of your operating system, you need to be able to interface with the rest of the world. And that's why you don't want the filesystem to be an "immuatble parameter of your program".
> any OS, application, library, filesystem, device driver, database, nearly everything related to computing at all
-is trying to actually do to your files and operation like reverting changes is trivial. Sure it's not performance optimal and effects what kind of software is ideal to work within such a system. It's just a different approach with different trade-offs.
Anyway if I one day want to build new OS it's for trying different interesting approaches and not yet another 'successful any-OS'.
You can actually consider current directory as immutable already.
It became "badly mutable" for example if you manipulate the current directory and this change is observable from a program point of view which is already running. Would that be really useful?
To clarify I am not trying to say that absolutely everything should be immutable but it's fun thought experiment how far you can go.
