6.001 Recitation 20

6.001 Structure and Interpretation of Computer Programs
Recitation #23
Friday, December 3, 2004

Review

lazy evaluation
concurrency

Practice

Give all possible values of x that can result from the following code. (As a help, the important atomic operations are labeled by letters.)

(define x 10)
                                 C      A B
(parallel-execute (lambda () (set! x (* x x)))
                  (lambda () (set! x (* x x x)))
                                 G      D E F

What possible values of x remain if the procedures are serialized, as below?

(define x 10)
(define s (make-serializer))
                                    C      A B
(parallel-execute (s (lambda () (set! x (* x x))))
                  (s (lambda () (set! x (* x x x))))
                                    G      D E F

What if the procedures are serialized in a different way, as shown below?

(define x 10)
(define s (make-serializer))
(define t (make-serializer))
                                    C      A B
(parallel-execute (s (lambda () (set! x (* x x))))
                  (t (lambda () (set! x (* x x x))))
                                    G      D E F