Functional State Encapsulation

Today I came across an interesting example in Scheme of functional state encapsulation and the utility of procedures as first-class language constructs. It’s interesting to consider procedures as vehicles for state encapsulation in the same way that objects are described to be.

The following is an example of a procedure make-monitored, which takes in a procedure and produces a version of the procedure that maintains a count of how many times it has been invoked since being monitored. Passing a special symbol as an argument causes the internal dispatch method to return the count or reset the counter.

(define (make-monitored procedure)
  (define counter 0)
  (define (dispatch . m)
    (cond [(eq? (car m) 'how-many-calls?) counter]
          [(eq? (car m) 'reset-counter) (begin
                                          (set! counter 0)
                                          counter)]
          [else (begin
                  (set! counter (+ 1 counter))
                  (apply procedure m))]))
  dispatch)


> (define monitored-sqrt (make-monitored sqrt))
> (monitored-sqrt 14)
3.7416573867739413
> (monitored-sqrt 'how-many-calls?)
1
> (monitored-sqrt 'reset-counter)
0