; Define a function that takes a single parameter (which needs to be a list-like thing)
(defn size [_vec]
    ; Loop over _list with starting value of our parameter and _size with starting value of 0
    (loop [_list _vec, _size 0]
        ; Code to execute is just an "if" block
        (if (empty? _list)
            ; if our list is empty, return the size
            _size
            ; else call loop's body again (the "if" block) but this time pass in
            ; the tail of the list and the incremented size counter 
            (recur (rest _list) (inc _size)))))

(println (size [1 2 3 5 7 9]))
(println (size (list 1 2 3)))
(println (size #{}))
(println (size #{1}))