Problem Statement

Write a generic swap function or operator which exchanges the values of two variables (or, more generally, any two storage places that can be assigned), regardless of their types. If your solution language is statically typed please describe the way your language provides genericity. If variables are typed in the given language, it is permissible that the two variables be constrained to having a mutually compatible type, such that each is permitted to hold the value previously stored in the other without a type violation. That is to say, solutions do not have to be capable of exchanging, say, a string and integer value, if the underlying storage locations are not attributed with types that permit such an exchange. Generic swap is a task which brings together a few separate issues in programming language semantics. Dynamically typed languages deal with values in a generic way quite readily, but do not necessarily make it easy to write a function to destructively swap two variables, because this requires indirection upon storage places or upon the syntax designating storage places. Functional languages, whether static or dynamic, do not necessarily allow a destructive operation such as swapping two variables regardless of their generic capabilities. Some static languages have difficulties with generic programming due to a lack of support for (Parametric Polymorphism). Do your best!

Let's start with the solution:

(defun swap (a-sym b-sym)
  "Swap values of the variables given by A-SYM and B-SYM."
  (let ((a-val (symbol-value a-sym)))
    (set a-sym (symbol-value b-sym))
    (set b-sym a-val)))
(swap 'a 'b)


(defmacro swap (a b)
  `(setq ,b (prog1 ,a (setq ,a ,b))))


(require 'cl-lib)
(defmacro swap (a b)
  `(cl-psetf ,a ,b
             ,b ,a))

(setq lst (list 123 456))
(swap (car lst) (cadr lst))
;; now lst is '(456 123)