If you watch the linked video above you will see how clever persistent data structures mean that nothing is ever cloned or copied.

Good to know. I never got into immutable collections because I presumed they would result in things being allocated all the time (and that that would be expensive).

I'd guess this library clones on modification, but I was talking about cloning the object on every function call.

Persistent data structures allow for the appearance of cloning (you have a new thing) but are much more efficient behind the scenes (you don't actually copy everything). So if you find yourself doing a lot of defensive cloning, then Mori or this library would probably be a lot faster for you.

Absolutely! If anyone is reading this, I recommend Eric Lippert's 11 part series on immutability in C#. The concept apply everywhere, of course.

http://blogs.msdn.com/b/ericlippert/archive/2007/11/13/immut...