A big one is persistent data structures. Where 'persistent' may not mean what you think it means. In Clojure it is impossible to surgically modify a data stru

A big one is persistent data structures. Where 'persistent' may not mean what you think it means. In Clojure it is impossible to surgically modify a data structure. That is, you can't do something like: (SETF (CAR (CDR x)) 'foo) which would alter a data structure. You can modify a data structure, but it returns a new data structure, yet the old one remains if it is not GC'able. All of the common data structures have this property. Sequences (eg, lists), arrays, maps and sets. If you change the 50 thousandth element of an array, this returns a new array. The old array is unaffected. Yet it gives the performance you expect of an array. (Meaning no apparent cost of copying.) If you're writing a search procedure, it is trivial to transform one chessboard into a different chessboard, but without concern about the cost of copying (close to zero), or having altered the original value (you haven't). Other variables that have a pointer to that first chessboard don't see any changes. There are other things such as a great story about concurrency. Hope that helps.