connectors.scm

;; SICP 3.3.4 - Constraint system

(define (for-each-except exception procedure list)
  (define (loop items)
    (cond ((null? items) 'done)
	  ((eq? (car items) exception) (loop (cdr items)))
	  (else (procedure (car items))
		(loop (cdr items)))))
  (loop list))

;; constraint syntax interface 

(define (inform-about-value constraint)
  (constraint 'I-have-a-value))
(define (inform-about-no-value constraint)
  (constraint 'I-lost-my-value))

;; constraints
(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)

(define (multiplier m1 m2 product)
  (define (process-new-value)
    (cond ((or (and (has-value? m1) (= (get-value m1) 0))
	       (and (has-value? m2) (= (get-value m2) 0)))
	   (set-value! product 0 me))
	  ((and (has-value? m1) (has-value? m2))
	   (set-value! product
		       (* (get-value m1) (get-value m2))
		       me))
	  ((and (has-value? product) (has-value? m1))
	   (set-value! m2
		       (/ (get-value product) (get-value m1))
		       me))
	  ((and (has-value? product) (has-value? m2))
	   (set-value! m1
		       (/ (get-value product) (get-value m2))
		       me))))
  (define (process-forget-value)
    (forget-value! product me)
    (forget-value! m1 me)
    (forget-value! m2 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 -- MULTIPLIER" request))))
  (connect m1 me)
  (connect m2 me)
  (connect product me)
  me)

(define (constant value connector)
  (define (me request)
    (error "Unknown request -- CONSTANT" request))
  (connect connector me)
  (set-value! connector value me)
  me)

(define (probe name connector)
  (define (print-probe value)
    (newline)
    (display "Probe: ")
    (display name)
    (display " - ")
    (display value))
  (define (process-new-value)
    (print-probe (get-value connector)))
  (define (process-forget-value)
    (print-probe "?"))
  (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 -- PROBE" request))))
  (connect connector me)
  me)

;; connector syntax interface

(define (has-value? connector)
  (connector 'has-value?))
(define (get-value connector)
  (connector 'value))
(define (set-value! connector new-value informant)
  ((connector 'set-value!) new-value informant))
(define (forget-value! connector retractor)
  ((connector 'forget) retractor))
(define (connect connector new-constraint)
  ((connector 'connect) new-constraint))

;; connector object
    
(define (make-connector)
  (let ((value false)
	(informant false)
	(constraints '()))
    (define (set-my-value newval setter)
      (cond ((not (has-value? me))
	     (set! value newval)
	     (set! informant setter)
	     (for-each-except setter
			      inform-about-value
			      constraints))
	    ((not (= value newval))
	     (error "Contradiction" (list value newval)))
	    (else 'ignored)))
    (define (forget-my-value retractor)
      (if (eq? retractor informant)
	  (begin (set! informant false)
		 (for-each-except retractor
				  inform-about-no-value
				  constraints))
	  'ignored))
    (define (connect new-constraint)
      (if (not (memq new-constraint constraints))
	  (set! constraints (cons new-constraint constraints)))
      (if (has-value? me)
	  (inform-about-value new-constraint))
      'done)
    (define (me request)
      (cond ((eq? request 'has-value?) (if informant true false))
	    ((eq? request 'value) value)
	    ((eq? request 'set-value!) set-my-value)
	    ((eq? request 'forget) forget-my-value)
	    ((eq? request 'connect) connect)
	    (else
	     (error "Unknown operation -- CONNECTOR" request))))
    me))

(define (celsius-fahrenheit-converter c f)
  (let ((u (make-connector))
        (v (make-connector))
        (w (make-connector))
        (x (make-connector))
        (y (make-connector)))
    (multiplier c w u)
    (multiplier v x u)
    (adder v y f)
    (constant 9 w)
    (constant 5 x)
    (constant 32 y)
    'ok))

(define C (make-connector))
(define F (make-connector))
(celsius-fahrenheit-converter C F)
(probe "Celsius temp" C)
(probe "Fahrenheit temp" F)
(set-value! C 25 'user)
;; Probe: Celsius temp - 25
;; Probe: Fahrenheit temp - 77
;; ;Value: done
(set-value! F 212 'user)
(forget-value! C 'user)
;; Probe: Celsius temp - ?
;; Probe: Fahrenheit temp - ?
;; ;Value: done
(set-value! F 212 'user)
;; Probe: Fahrenheit temp - 212
;; Probe: Celsius temp - 100
;; ;Value: done