[noblethrasher]

Immutability was never incompatible with OOP, just the opposite in fact. Even Alan Kay often criticized languages like C++ and Java for encouraging the use of setters and, thus, “turning objects back into data structures”.

C# is still one of my favorite languages (even though I use F# most of the time now), but I do admire Java for making it significantly more painful to write mutable rather than immutable classes; it's too bad that fact was lost on so many programmers.

Kudos for sharing the Rich Hickey video; it's one of my favorites of all time.

[azth]

> but I do admire Java for making it significantly more painful to write mutable rather than immutable classes;

Out of curiosity, how does it do that? As far as I know, everything in Java is mutable by default.

[noblethrasher]

You have to go through the extra ceremony of writing a setter.

[azth]

The same applies to C# though, correct? Plus, I was thinking more of the lines of something like:

    class Foo {
      private int x = 0;

      public void bar() {
        this.x += 1; // Whoops!
      }
    }

    Foo x = new Foo();
    x.bar(); // Mutating call.

Which Java does not prevent.

[noblethrasher]

Yes, the same applies to my beloved C#, but that language was much less hostile to immutability. Indeed, the prettier mutator syntax was even positioned as a feature once upon a time.

To be clear, I'm the guy that insists on defining classes as either abstract or sealed, and almost always marks fields as readonly. But, I'm okay with the kind bounded mutability that you mentioned; clients of a `Foo` instance have to treat it as immutable.

Here is how I do OOP:

* I make classes to hide state, and hidden state is the same as being stateless.

* As I learn more about the problem, I start subdividing classes into smaller classes (not necessarily via inheritance).

* So, as my understanding of the problem increases, the number of class division increases, and by the pigeonhole principle, the amount of state approaches zero.