Chapter 3, Modularity, Objects, and State
Exercise 3.33
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;; Averager
(define a (make-connector))
(define b (make-connector))
(define avg (make-connector))
(define (make-averager a b avg)
(let ((const (make-connector))
(total (make-connector)))
(adder a b total)
(multiplier const avg total)
(constant 2 const)
'ok))
(make-averager a b avg)
(probe "a" a)
(probe "b" b)
(probe "avg" avg)
Output:
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1 ]=> (set-value! a 20 'ovais)
;Value 32: #[compound-procedure 32 me]
1 ]=>
Probe: a = 20
;Value: done
1 ]=> (set-value! b 30 'ovais)
Probe: b = 30
Probe: avg = 25
;Value: done
1 ]=> (set-value! a 50 'ovais)
;Contradiction (20 50)
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.
2 error> (restart 1)
;Abort!
1 ]=> (forget-value! a 'ovais)
Probe: a = ?
Probe: avg = ?
;Value: done
1 ]=> (set-value! a 29 'ovais)
Probe: a = 29
Probe: avg = 59/2
;Value: done
1 ]=>
Note that when some error occurs, MIT scheme keep showing it in prompt unless (restart 1) is called.
There is one important aspect that caused me some time to understand - Ownership
Any one can set a connector if it has no value. But if there is a value set in it then only that user/code is able to forget that value. This is a good idea - one who sets is responsible to remove. It makes easier to debug things if something goes wrong.
Note that in the above example - when (forget-value! a 'ovais) is invoked, the value of b is not forgoten - because it was not set my the squarer procedure! But it was set me user- ‘ovais.
I am also putting the complete code here for future reference:
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;; primitives - adder, multiplier, constant and interfaces for these contstraints(inform-about-value, informa-about-no-value)
(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 (inform-about-value constraint)
(constraint 'I-have-a-value))
(define (inform-about-no-value constraint)
(constraint 'I-lost-my-value))
;; probe
(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)
;; connectors
(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 (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))
(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))
;;Celcius - Ferhenhite sytem implementation
(define C (make-connector))
(define F (make-connector))
(celsius-fahrenheit-converter C F)
(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))
(probe "Celsius temp" C)
(probe "Fahrenheit temp" F)
;; Averager
(define a (make-connector))
(define b (make-connector))
(define avg (make-connector))
(define (make-averager a b avg)
(let ((const (make-connector))
(total (make-connector)))
(adder a b total)
(multiplier const avg total)
(constant 2 const)
'ok))
(make-averager a b avg)
(probe "a" a)
(probe "b" b)
(probe "avg" avg)