Scheme pass-by-reference

Question

How can I pass a variable by reference in scheme?

An example of the functionality I want:

(define foo
  (lambda (&x)
    (set! x 5)))

(define y 2)

         


        

Answer 1

You can affect an outer context from within a function defined in that outer context, which gives you the affect of pass by reference variables, i.e. functions with side effects.

(define (outer-function)
  (define referenced-var 0)
  (define (fun-affects-outer-context) (set! referenced-var 12) (void))
  ;...
  (fun-affects-outer-context)
  (display referenced-var)
)
(outer-function) ; displays 12

This solution limits the scope of the side effects.

Otherwise there is (define x (box 5)), (unbox x), etc. as mentioned in a subcomment by Eli, which is the same as the cons solution suggested by erjiang.