1 12687dd9 2023-08-04 jrmu ;; The first three lines of this file were inserted by DrScheme. They record metadata
 2 12687dd9 2023-08-04 jrmu ;; about the language level of this file in a form that our tools can easily process.
 3 12687dd9 2023-08-04 jrmu #reader(lib "htdp-intermediate-reader.ss" "lang")((modname |22.2|) (read-case-sensitive #t) (teachpacks ((lib "draw.ss" "teachpack" "htdp") (lib "arrow.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "draw.ss" "teachpack" "htdp") (lib "arrow.ss" "teachpack" "htdp")))))
 4 12687dd9 2023-08-04 jrmu ;Exercise 22.2.2.   Define an abstract version of sort (see exercise 19.1.6) using the new recipe for abstraction.    Solution
 5 12687dd9 2023-08-04 jrmu 
 6 12687dd9 2023-08-04 jrmu ;abs-sort : (number number -> boolean) -> ((listof number) -> (listof number))
 7 12687dd9 2023-08-04 jrmu ;Given the operator, which determines whether to sort ascending or descending, abs-sort returns a sort function that sorts a (listof numbers).
 8 12687dd9 2023-08-04 jrmu 
 9 12687dd9 2023-08-04 jrmu (define (abs-sort operator)
10 12687dd9 2023-08-04 jrmu   (local ((define (sort alon)
11 12687dd9 2023-08-04 jrmu             (local ((define (sort alon)
12 12687dd9 2023-08-04 jrmu                       (cond
13 12687dd9 2023-08-04 jrmu                         [(empty? alon) empty]
14 12687dd9 2023-08-04 jrmu                         [else (insert (first alon) (sort (rest alon)))]))
15 12687dd9 2023-08-04 jrmu                     (define (insert an alon)
16 12687dd9 2023-08-04 jrmu                       (cond
17 12687dd9 2023-08-04 jrmu                         [(empty? alon) (list an)]
18 12687dd9 2023-08-04 jrmu                         [else (cond
19 12687dd9 2023-08-04 jrmu                                 [(operator an (first alon)) (cons an alon)]
20 12687dd9 2023-08-04 jrmu                                 [else (cons (first alon) (insert an (rest alon)))])])))
21 12687dd9 2023-08-04 jrmu               (sort alon))))
22 12687dd9 2023-08-04 jrmu     sort))
23 12687dd9 2023-08-04 jrmu 
24 12687dd9 2023-08-04 jrmu ;sort-ascend : (listof numbers) -> (listof numbers)
25 12687dd9 2023-08-04 jrmu ;Given a-lon, return a list-of-numbers sorted in ascending order.
26 12687dd9 2023-08-04 jrmu (define sort-ascend (abs-sort <))
27 12687dd9 2023-08-04 jrmu 
28 12687dd9 2023-08-04 jrmu ;sort-descend : (listof numbers) -> (listof numbers)
29 12687dd9 2023-08-04 jrmu ;Given a-lon, return a list-of-numbers sorted in descending order.
30 12687dd9 2023-08-04 jrmu (define sort-descend (abs-sort >))
31 12687dd9 2023-08-04 jrmu 
32 12687dd9 2023-08-04 jrmu ;abs-fold : (X Y -> Y) -> ((listof X) -> Y)
33 12687dd9 2023-08-04 jrmu ;Given operator and base, return the fold function with operator filled in.
34 12687dd9 2023-08-04 jrmu 
35 12687dd9 2023-08-04 jrmu (define (abs-fold operator base)
36 12687dd9 2023-08-04 jrmu   (local ((define (fold alon)
37 12687dd9 2023-08-04 jrmu            (cond
38 12687dd9 2023-08-04 jrmu              [(empty? alon) base]
39 12687dd9 2023-08-04 jrmu              [else (operator (first alon)
40 12687dd9 2023-08-04 jrmu                              (fold (rest alon)))])))
41 12687dd9 2023-08-04 jrmu     fold))
42 12687dd9 2023-08-04 jrmu 
43 12687dd9 2023-08-04 jrmu (define sum (abs-fold + 0))
44 12687dd9 2023-08-04 jrmu (define product (abs-fold * 1))