gistfile1.clj

craps.core=> (def my-list '(1 2 3 4))
#'craps.core/my-list
craps.core=> (map println my-list)
(1
2
nil 3
nil 4
nil nil)
craps.core=> (map print my-list)
(12nil 3nil 4nil nil)
craps.core=> (def my-list [1 2 3 4])
#'craps.core/my-list
craps.core=> (map print my-list)
(1234nil nil nil nil)
craps.core=> 

So, the nils show up b/c map is lazy. The output is being interspersed with the resulting sequence. Since println doesn't return anything, you get '(nil nil nil nil).