1 ;; The first three lines of this file were inserted by DrScheme. They record metadata
2 ;; about the language level of this file in a form that our tools can easily process.
3 #reader(lib "htdp-intermediate-lambda-reader.ss" "lang")((modname |27.2|) (read-case-sensitive #t) (teachpacks ((lib "draw.ss" "teachpack" "htdp") (lib "arrow.ss" "teachpack" "htdp") (lib "gui.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "draw.ss" "teachpack" "htdp") (lib "arrow.ss" "teachpack" "htdp") (lib "gui.ss" "teachpack" "htdp")))))
4 ;Exercise 27.2.1.   Determine what the list-of-lines representation for empty, (list 'NL), and (list 'NL 'NL) should be. Why are these examples important test cases?
5 
6 ;A file is either
7 ;1. empty or
8 ;2. (cons s f)
9 ;where s is a symbol and f is a file.
10 
11 ;Examples
12 ;
13 ;(file->list-of-lines empty)
14 ;empty
15 ;(file->list-of-lines (list 'NL))
16 ;(list empty)
17 ;(file->list-of-lines (list 'NL 'NL))
18 ;(list empty empty)
19 
20 ;The NEWLINE character is represented by 'NL
21 (define NEWLINE 'NL)
22 
23 ;file->list-of-lines : file -> (listof (listof symbols))
24 ;Converts a-file into a (listof (listof symbols)) representing the file.  Each new list starts right at the end of a newline symbol and ends at the first occurrence of a newline symbol.  file->list-of-lines uses generative recursion: the first line is appended to the front of the remainder of the file that has already been converted into a list of lines.
25 ;Termination Argument: Since each application of file->list-of-lines necessarily shortens the argument for the new generative recursion, the paramater must get smaller over time and eventually become the empty list, for which a trivial solution is available.
26 
27 ;first-line : file -> (listof symbols)
28 ;Returns the first line of a-file.
29 
30 ;remove-first-line : file -> file
31 ;Given a-file, remove the first line.
32 
33 (define (file->list-of-lines a-file)
34   (local ((define (first-line a-file)
35             (cond
36               [(symbol=? NEWLINE (first a-file)) empty]
37               [else (cons (first a-file)
38                           (first-line (rest a-file)))]))
39           (define (remove-first-line a-file)
40             (cond
41               [(symbol=? NEWLINE (first a-file)) (rest a-file)]
42               [else (remove-first-line (rest a-file))])))
43     (cond
44       [(empty? a-file) empty]
45       [else (cons (first-line a-file)
46                   (file->list-of-lines (remove-first-line a-file)))])))
47 
48 
49 (file->list-of-lines '(Hi my name is Joe NL
50                           and I live in a button factory NL
51                           I have a wife NL
52                           and three kids NL))