[rbehrends]

> It may not be a problem in a language like Rust, where the compiler understands the concept of immutability. But most (if not all) mainstream languages except Rust don’t. Your object may be immutable in January but in February someone makes a change in a class used 10 layers below and suddenly your class is no longer immutable. Add invisible sharing to that and your code explodes.

No offense, but this strikes me as a strawman argument. What software design methodology leads to immutable types suddenly being made mutable? Outside of dynamic languages, where is support for immutability not there?

Note that any language with proper information hiding can expressly do immutability (literally going back to the days of Modula-2), plus of course several mainstream languages that have first-class language features to represent immutability for added convenience.

Finally, this is only one example. One underlying problem is that if all reference counts need to be capped at one, you have to either switch to full reference counting or to copy the underlying data.

You can see this play out in the std::collections::HashSet interface. Operations like intersection, union, difference, and symmetric difference return iterators rather than sets. While there are also operators that do return sets, such as bitor, that's implemented as follows.

    fn bitor(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
        self.union(rhs).cloned().collect()
    }
    

Because the result and the argument can't in general share references to elements, you end up essentially doing a deep copy for e.g. a set of strings, which you want to minimize as much as possible. Thus, limitations on the sharing of references dictate aspects of the API.

> Abstractions understood as „I can change only X without changing Y” (aka GoF OOP patterns or most Clean Code OOP patterns) are overrated anyways. Readability and understandability of code is more important than ability to add something without changing something else. If code is readable and constraints are enforced by the compiler, it is easy and safe to change.

So, you never work with third-party libraries where you do not control the API and have never written libraries to be consumed by other teams/third parties?

[pkolaczk]

Java cannot express immutability. `final` is not transitive, so nothing stops an unrelated change in code to break something that was immutable earlier. Same with Golang.

[rbehrends]

You can express immutability simply by making all instance variables private and not providing any methods to modify them.