6.001 Recitation #7                                        Fall 1997
Prof. Albert R. Meyer

         5-minute-problem #11: Environment Diagram for LEARNER's
     (posed in email, 10/22/97, 12:45pm for presentation Fri, 10/24)

Show the step-by-step construction of the environment diagram which
displays the environment resulting from the evaluation of the following
sequence of expressions:

(define (make-learner)
  (let ((learned-set the-empty-set))
    (lambda (n)
      (or (element-of-set? n learned-set)
          (begin
            (set! learned-set (adjoin-set n learned-set))
            #f)))))
;Value: make-learner

(define learner1 (make-learner))
;Value: learner1

(define learner2 (make-learner))
;Value: learner2

(learner1 7)
;Value: #f

(learner1 7)
;Value: #t         ;LEARNER1 has learned 7 

(learner2 7)
;Value: #f         ;LEARNER2 has not learned 7 before

(learner2 7)
;Value: #t         ;NOW LEARNER2 has also learned 7

(learner2 9)
;Value: #f

(learner2 9)       ;LEARNER2 has learned 9
;Value: #t

(learner1 9)       ;LEARNER1 has not learned 9 before
;Value: #f

(learner1 6)
;Value: #f         ;LEARNER1 has not learned 6 before