Add implementation of exercise 3-50

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: 49/82
+  - 章节三: 50/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-50.rkt

@@ -0,0 +1,28 @@
+#lang racket
+(require "stream.rkt")
+
+(define (stream-map proc . argstreams)
+  (if (stream-null? (car argstreams))
+      the-empty-stream
+      (cons-stream
+       (apply proc (map stream-car argstreams))
+       (apply stream-map
+              (cons proc (map stream-cdr argstreams))))))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for two streams mapping"
+             (define s1 (list-to-stream '(1 2 3 4)))
+             (define s2 (list-to-stream '(10 20 30 40)))
+             (define added (stream-map + s1 s2))
+             (check-equal? (stream-to-list added 4) '(11 22 33 44))
+             )
+
+  (test-case "Test for different length streams(stop at shortest)"
+             (define s1 (list-to-stream '(1 2)))
+             (define s2 (list-to-stream '(10 20 30 40)))
+             (define added (stream-map + s1 s2))
+             (check-equal? (stream-to-list added 4) '(11 22))
+             )
+  )