Functional programming: state vs. reassignment

Question

I need help getting my head around the difference between my current OOP notion of state, and the way it would be done in a functional language like Haskell or Clojure.

Answer 1

In a pure functional style, you'll never overwrite any variable.

An analogy would be to spacetime in physics. If you consider the world as 3d, then objects don't have fixed positions - they move over time. To bring math to bear on the physical world, we therefore add a time dimension and consider the values of various properties at particular times. In doing so, we've made the objects of our study into constants. Similarly, in programming, there is a conceptual simplicity to be had by working with immutable values. Objects with an identity in the real world can be modeled as a sequence of immutable values (the states of the object at increasing times) rather than as a single value that changes.

Of course the details of how to associate the sequence of values to an "object identity" can be a little hairy. Haskell has Monads that let you model state. Functional Reactive Programming is a more literal attempt at modeling objects in the world with pure functional updates, that I think is a very promising direction for programming.

I will note that Clojure, unlike Haskell, isn't pure, and you can update variables as you suggested. If you're only updating a few variables at a high level, you'll still probably enjoy many of the conceptual simplicity benefits of functional programming.