3_35.rkt

#lang racket
#| Solution for exercise 3_35. |#

(require "../solutions/constraints.rkt")
(provide squarer)

(define (squarer a b)
  (define (process-new-value)
    (cond ((has-value? b)
           (if (< (get-value b) 0)
             (error "square less than 0 -- SQUARER" (get-value b))
             (set-value! a (sqrt (get-value b)) me)))
          ((has-value? a) (set-value! b (* (get-value a) (get-value a)) me))))
  (define (process-forget-value)
    (forget-value! a me)
    (forget-value! b me)
    (process-new-value))
  (define (me request)
    (cond ((eq? request 'I-have-a-value)
           (process-new-value))
          ((eq? request 'I-lost-my-value)
           (process-forget-value))
          (else
           (error "Unknown request -- SQUARER" request))))
  (connect a me)
  (connect b me)
  me)


(define (adder a1 a2 sum)
  (define (process-new-value)
    (cond ((and (has-value? a1) (has-value? a2))
           (set-value! sum
                       (+ (get-value a1) (get-value a2))
                       me))
          ((and (has-value? a1) (has-value? sum))
           (set-value! a2
                       (- (get-value sum) (get-value a1))
                       me))
          ((and (has-value? a2) (has-value? sum))
           (set-value! a1
                       (- (get-value sum) (get-value a2))
                       me))))
  (define (process-forget-value)
    (forget-value! sum me)
    (forget-value! a1 me)
    (forget-value! a2 me)
    (process-new-value))
  (define (me request)
    (cond ((eq? request 'I-have-a-value)
           (process-new-value))
          ((eq? request 'I-lost-my-value)
           (process-forget-value))
          (else
           (error "Unknown request -- ADDER" request))))
  (connect a1 me)
  (connect a2 me)
  (connect sum me)
  me)