Question

I am new to Clojure and have found that when I loop over this vector in clojure using a list comprehension I get some nils at the end.

(def myVec [1,2,3])

user=> (for [x myVec] (println x))
(1
2
3
nil nil nil)

I get the same thing using map

user=> (map println myVec)
(1
2
3
nil nil nil)

What causes the nill to be printed in these cases?

Answer 1

Those nil are the return value of println. Every time you call

(println "something")

the println function prints something on the standard output, and then returns nil. The overall effect in your code is that you see all the side effects (I/O) of all println invocations, then the REPL prints the return value coming from each and every invocation (e.g. three times nil).