> A computation that takes two days instead of half an hour does make a very real difference to me IMHO, such a big difference only arrises when big difference

> A computation that takes two days instead of half an hour does make a very real difference to me IMHO, such a big difference only arrises when big differences in algorithmic complexity is involved. Again I must mention that I'm of the opinion that if you know what you're doing, then it's fine, but you must be able to defend your choice, as an argument like "gosh, heard from the Internet that immutable data-structures are slower" is not acceptable given the extra sanity that it brings. Plus, depending on context (e.g. web services that are dealing with parallelism combined with I/O) immutable data-structures might actually improve throughput. Your use-case is entirely legitimate, therefore that rule on usage of immutable data-structures is a SHOULD (optional), rather than a MUST (required). If you wanna do it, then do it - after all, the ability to get down and dirty is one thing I like about Scala, too bad that it doesn't have a GC-less version :-)