[marxama]

With regards to performance, sometimes taking the penalty of immutability in the small leads to gains in the large. The React wrappers in ClojureScript are a great example of this - since all data is immutable, shouldComponentUpdate is trivially and automatically implemented for you, meaning React not having to diff parts of the virtual DOM that are guaranteed to be identical.

There are other cases where immutability helps, even with regards of performance. At my day job, we use C#, with little focus on immutability. Performance has been a huge problem lately. One of the things we did was to change lots of constructors which initialized empty lists. In most cases, these lists remain empty, so we created a whole bunch of objects unnecessarily, which put a lot of toll on the GC. The "solution" was to initialize them to null instead, which made the code a lot more brittle and cumbersome. Had we used immutable lists instead, we could've shared one single, empty object instead (per type, of course).

We also have lots of copy constructors which we use often - these would be completely unnecessary with immutable data.