That is not hidden by immutable data either - the state is just global in the latter case, in a way.

There is no getting away from essential state, from a theoretical point of view. In my opinion the often leaky partial encapsulation of state is still one of the better ways to deal with it. And immutability is another axis so of course the frequent case where it makes sense should be adhered to, but not religiously.

It is much easier to reason about immutable state than mutable state, both for humans and compilers, especially in a distributed setting (any modern piece of hardware).

The problem with "leaky" encapsulation as you put it, is the combinational explosion of the state space as many stateful objects are composed.

Most mainatream programming languages are still unfortunately not well geared up for working with immutable data, they lack even persistent collections. Functional languages are of course ideal for this.

> The problem with "leaky" encapsulation as you put it, is the combinational explosion of the state space as many stateful objects are composed.

I don’t think it has to be necessarily more than with an immutable approach. Like, there is an essential amount of state you will have to have either case and it is not clear to me that immutability is always the better choice.

But don’t get me wrong, I also default to immutable data structures, I’m just saying that 1) OOP is not incompatible with FP 2) not every problem is solved better with FP-idioms. It’s not accidental that haskell has state monads as well.

Ah, I see what you mean, though I do think that immutability purists like yourself have their own monsters to contend with (even apart from the obvious performance hit).

Absolutely we do! For example, garbage collectors are complex beasts that are hard to tune and tough to scale. But of course some OOP languages have started using GC anyway, in which case, I honestly believe they might as well be functional programmers :)