Add implementation of exercise 3-55

Author: ramsayleung

README.org

@@ -25,7 +25,7 @@
   习题完成情况:
   - 章节一: 43/46
   - 章节二: 88/97
-  - 章节三: 54/82
+  - 章节三: 55/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-54.rkt

@@ -3,18 +3,18 @@
 (require "exercise3-50.rkt")
 (require "infinite-stream.rkt")
 
-(define (mul-stream s1 s2)
+(define (mul-streams s1 s2)
   (map-stream * s1 s2))
 
-(define factorials (cons-stream 1 (mul-stream (stream-cdr integers) factorials)))
+(define factorials (cons-stream 1 (mul-streams (stream-cdr integers) factorials)))
 
 (module+ test
   (require rackunit)
 
   (test-case "Test for mul-stream"
              (define s1 (list-to-stream '(2 3 4 5)))
              (define s2 (list-to-stream '(3 4 5 6)))
-             (define s3 (mul-stream s1 s2))
+             (define s3 (mul-streams s1 s2))
              (check-equal? (stream-ref s3 2) 20)
              (check-equal? (stream-to-list s3 4) '(6 12 20 30))
              )

chapter3/exercise3-55.rkt

@@ -0,0 +1,16 @@
+#lang racket
+(require "stream.rkt")
+(require "infinite-stream.rkt")
+
+(define (partial-sums s1)
+  (cons-stream (stream-car s1) (add-streams (partial-sums s1) (stream-cdr s1)))
+  )
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for partial-sums"
+             (define s1 (partial-sums integers))
+             (check-equal? (stream-to-list s1 5) '(1 3 6 10 15))
+  )
+)