fixes #638

Author: martin-henz

xml/chapter3/section5/subsection1.xml

@@ -1384,6 +1384,39 @@ function stream_map_2(f, s1, s2) {
 	modifying your
 	<JAVASCRIPTINLINE>stream_map_2</JAVASCRIPTINLINE>
 	such that the result stream employs memoization.
+	<SOLUTION>
+	  <SNIPPET HIDE="yes">
+	    <NAME>stream_map_2_optimized_example</NAME>
+	    <JAVASCRIPT>
+const ints1 = integers_from(1);
+const ints0 = integers_from(0);
+const adds = (x, y) => x + y;
+
+display(eval_stream(stream_map_2(adds, ints1, ints0), 5));
+display(eval_stream(stream_map_2_optimized(adds, ints1, ints0), 5));	      
+	    </JAVASCRIPT>
+	  </SNIPPET>
+	  <SNIPPET>
+	    <NAME>stream_map_2_optimized</NAME>
+	    <REQUIRES>memo</REQUIRES>
+	    <EXAMPLE>stream_map_2_optimized_example</EXAMPLE>
+	    <JAVASCRIPT>
+function stream_map_2(f, s1, s2) {
+    return is_null(s1) || is_null(s2)
+        ? null
+        : pair(f(head(s1), head(s2)),
+                () => stream_map_2(f, stream_tail(s1), stream_tail(s2)));
+}
+
+function stream_map_2_optimized(f, s1, s2) {
+    return is_null(s1) || is_null(s2)
+        ? null
+        : pair(f(head(s1), head(s2)),
+                memo(() => stream_map_2(f, stream_tail(s1), stream_tail(s2))));
+}
+	    </JAVASCRIPT>
+	  </SNIPPET>
+	</SOLUTION>
       </EXERCISE>
 
       <EXERCISE>