>For your comment about performance, immutable structure being comparable by reference can provide huge gains in read and re-use heavy environments, thanks to m

>For your comment about performance, immutable structure being comparable by reference can provide huge gains in read and re-use heavy environments, thanks to memoization-like techniques. Fair enough. I'm often doing things to graphics, and the concept of using immutable data when the data is a bitmap with 4Mb of pixels, and you're trying to draw hundreds of sprites or shapes to it...well, let's just say that immutable doesn't make sense for that. Even when I'm just working on a game, though, the more frequent allocations required by copy-on-write structures means more fragmentation of the heap and more deallocations later. Ideally during runtime nothing gets allocated. You may even be right that the amortized speed of using immutable structures is the same as doing it with mutable data. But anything that adds a lot of allocations and therefore adds to the frequency of garbage collection will cause (or increase) jankiness in a game. You have 16.66ms to accomplish all the work for a frame. If a garbage collector comes along and steals even 10ms, if you can't do the remaining work in 6.6ms, it will skip a frame, and users will see it. And my current game needs to run in a browser, at least mostly. So here I am. :)