> Curious, I've been hearing a lot lately about functional languages and immutability and wasn't quite sure how it worked. I agree with it in premise but it see

> Curious, I've been hearing a lot lately about functional languages and immutability and wasn't quite sure how it worked. I agree with it in premise but it seems like all these immutable data structures introduce massive overhead. There is some seemingly inherent overhead for certain data structures. On the other hand it allows for structural sharing, and plays more nicely with concurrency and whatnot. Suffice it to say that you'd be wise to design your immutable (persistent, specifically) data structures very differently to how you would design your regular imperative data structures. There was a famous PhD dissertation - now a book - on the topic. You'll find it easily enough if you search for it. Yes, immutable data structures seem very inefficient. But they're implemented in a more clever way than to just take regular imperative data structures and copying for every "update" instead of directly mutating. > What is going on behind the scenes when I modify a single value in an array with 10k members? Surely it's not going to create an entirely new array. It would be unwise to use a huge array (in the usual sense - contiguous memory) like that to begin with. You'd probably have an "array" that is more in the shape of a tree, which facilitates more efficient "updates".