Lecture 29: The More Things Change, the More They are the Same

1. readings: TSS ch 18
2. design review -- the final project will require a design review we will accept one project per peer group; if we get two, you lose all credit
   
   
3. set-phone-name!, set-phone-number!

   instead of modifying what a VARIABLE stands for, we can also change the contents of a record

   (define a-rec (make-phone 'Bumpy 555666))
   
   > (phone-number a-rec) 
   5556666
   > (phone-number a-rec) 
   5556666
   > (set-phone-number! a-rec 5557777)
   > (phone-number a-rec) 
   5557777
   

   We can define programs now that modify a record on a list:

   ;; update : symbol number phone-list -> phone-list ;; Purpose: produce a list like a-pl, but with the ;; first record for a-name replaced with ;; (make-phone a-name a-number) (define (update a-name a-number a-pl) (cond ((empty? a-pl) (void)) (else (cond ((eq? (phone-name (first a-pl)) a-name) (cons (make-phone (phone-name (first a-pl)) a-number) (rest a-pl))) (else (cons (first a-pl) (update a-name a-number (rest a-pl))))))))

   ;; update! : symbol number phone-list -> void ;; Effect: modify the first record in phone-list ;; whose name is a-name so that it now contains ;; a-number (define (update! a-name a-number a-pl) (cond ((empty? a-pl) (void)) (else (cond ((eq? (phone-name (first a-pl)) a-name) (set-phone-number! (first a-pl) a-number)) (else (update! a-name a-number (rest a-pl)))))))

   Compare it with the functional version!

   Some expressions have an effect only. Similar to routines that draw stuff on the screen. Since we might want to combine those in a sequence, e.g. do do do do, we introduce a new construct:

   (begin expression ... expression) This is like and, except that the results of the first few expressions are thrown away. The result of the last one is the result of the entire expression.

   We can now also define functions that modify several records on a list:

   ;; update : symbol number phone-list -> phone-list ;; Purpose: produce a list like a-pl, but with all ;; records for a-name replaced with ;; (make-phone a-name a-number) (define (update a-name a-number a-pl) (cond ((empty? a-pl) (void)) (else (cond ((eq? (phone-name (first a-pl)) a-name) (cons (make-phone (phone-name (first a-pl)) a-number) (update a-name a-number (rest a-pl)))) (else (cons (first a-pl) (update a-name a-number (rest a-pl))))))))

   ;; update! : symbol number phone-list -> void ;; Effect: modify the first record in phone-list ;; whose name is a-name so that it now contains ;; a-number (define (update! a-name a-number a-pl) (cond ((empty? a-pl) (void)) (else (cond ((eq? (phone-name (first a-pl)) a-name) (begin (set-phone-number! (first a-pl) a-number) (update! a-name a-number (rest a-pl)))) (else (update! a-name a-number (rest a-pl)))))))

   We could have used local for the left (functional) version and begin in a different manner for the right (destructive) one so that the natural recursions are not repeated textually.