Question: what’s the difference between #() and (fn []) ? user=> (#()) () user=> ((fn [])) nil why the first case returns () but the second returns nil?

http://clojure.org/reader#The%20Reader--Macro%20characters describese #(...) like so

#(...) => (fn [args] (...))

which would be consistent with what you're seeing.

i see. thank you @jaen

Is membership in this channel mandatory?

nah

Huh, or is it

I don't see "Leave" when you click on #C03RZGPG1 on top.