You're calling the
square function slightly wrong (you're passing a vector containing a number, and not a number; watch the brackets), but I actually get a different error than you (
java.lang.ClassCastException: clojure.lang.PersistentVector cannot be cast to java.lang.Number).
It sounds like your TryClj session might be hosed, or the site was having issues at the time? Maybe try again.
FWIW, here's a session copy/pasted from TryClj:
--
(coauthor of
Clojure Programming from O'Reilly; creator of
Clojure Atlas)