[spyrosg]

Better than that, you can "mutate" if you're not invalidating information about the variable. Oz (not incidentally, developed by Prolog people) lets you add constraints to a variable as long as they don't contradict the previous ones. Binding to a specific value is just a very strong constraint. Re-binding to the same value is okay.

This is less strict than pure functional programming, but still feels declarative and makes concurrent programming easy: no piece of code that looked previously at your variable will have its assumptions about it broken.