Question

In lisp, I can pass an argument to a function and have it altered within the function. (AKA destructive functions). However, in Clojure, I’ve read somewhere that it is not permissible to alter the given arguments within that same function. For example:

(defn add-two-lists [list1 list2]
  (for [n (range (count list1))]
    (+ (nth list1 n) (nth list2 n))))

This is a normal function and its output is the addition of the two identical lists. However, I want something like this:

(defn add-two-lists [list1 list2 added_list]
  (set! added_list 
       (for [n (range (count list1))]  
          (+ (nth list1 n) (nth list2 n)))))

Perhaps my use of set! is wrong or misused, and I still get errors. Is there a elegant way to destructively modify arguments in Clojure?

Answer 1

The with-local-vars macro lets you create thread-locally bound vars that you can modify with var-set. You also have to access the var’s value with var-get, which can be shortened to just @.

(defn add-two-lists [list1 list2 added-list]
  (var-set added-list 
           (for [n (range (count list1))]  
             (+ (nth list1 n) (nth list2 n)))))

(with-local-vars [my-list nil]
  (add-two-lists '(1 2 3) '(3 4 5) my-list)
  @my-list)

EDIT:

On a stylistic note, you could use map to add the two lists without using the nth function to random-access each index in each list:

(defn add-two-lists [list1 list2 added-list]
  (var-set added-list (map + list1 list2)))