Why evaluated list returns only number and error in other case?

Question

In Clojure REPL:

('+ 1 2) ; 2
('+ 1 2 3) ; (err) clojure.lang.ArityException: Wrong number of args (3) passed to: clojure.lang.Symbol

Why it happens and how it works? I tried to inspect the code, but I can't figure out what is calling in function position

Answer 1

Clojure symbols are functions too and are implemented like get. So what you are calling here is (get 1 '+ 2), which is OK (tries to look up '+ in 1, which can not work and therefor returns the fallback of 2). And the arity just does not work; only 1-2 arguments allowed.