How is set! defined in scheme?

Question

How would you implement your own set! function in Scheme? A set! function is a destructive procedure that changes a value that is defined taking into account th

Answer 1

If you are implementing a simple interpreter, then it is not all that hard. Your environment will map from identifiers to their values (or syntactic keywords to their transformers). The identifier->value mapping will need to account for possible value changes. As such:

(define (make-cell value)
  `(CELL ,value))

(define cell-value cadr)
(define (cell-value-set! cell value)
  (set-car! (cdr cell) value))

...
((set-stmt? e)
 (let ((name  (cadr e))
       (value (interpret (caddr e) env)))
   (let ((cell (env-lookup name)))
     (assert (cell? cell))
     (cell-value-set! cell value)
     'return-value-for-set!)))
...

With two other changes being that when you bind an identifier to a value (like in a let or lambda application) you need to extend the environment with something like:

  (env-extend name (cell value) env)

and also when retrieving a value you'll need to use cell-value.

Of course, only mutable identifiers need a cell, but for a simple interpreter allocating a cell for all identifier values is fine.