Clojure loop collection

Question

I want to know if this is the right way to loop through an collection:

(def citrus-list ["lemon" "orange" "grapefruit"])

(defn display-citrus [citruses]
  (loop [[citrus & citruses] citruses]
    (println citrus)
    (if citrus (recur citruses))
    ))

(display-citrus citrus-list)

I have three questions:

1. the final print displays nil, is it ok or how can avoid it?
2. I understand what & is doing in this example but I don´t see it in other cases, maybe you could provide a few examples
3. Any other example to get the same result?

Thanks, R.

Answer 1

First of all your implementation is wrong. It would fail if your list contains nil:

user> (display-citrus [nil "asd" "fgh"])
;;=> nil
nil

And print unneeded nil if the list is empty:

user> (display-citrus [])
;;=> nil
nil

you can fix it this way:

(defn display-citrus [citruses]
  (when (seq citruses)
    (loop [[citrus & citruses] citruses]
      (println citrus)
      (if (seq citruses) (recur citruses)))))

1) it is totally ok: for non-empty collection the last call inside function is println, which returns nil, and for empty collection you don't call anything, meaning nil would be returned (clojure function always returns a value). To avoid nil in your case you should explicitly return some value (like this for example):

(defn display-citrus [citruses]
  (when (seq citruses)
    (loop [[citrus & citruses] citruses]
      (println citrus)
      (if (seq citruses) (recur citruses))))
  citruses)

user> (display-citrus citrus-list)
;;=> lemon
;;=> orange
;;=> grapefruit
["lemon" "orange" "grapefruit"]

2) some articles about destructuring should help you

3) yes, there are some ways to do this. The simplest would be:

(run! println citrus-list)