added async-io quiz

exercises/087_async3.zig

@@ -0,0 +1,50 @@
+//
+// The real power of async shows when you launch MULTIPLE tasks!
+//
+// With io.async(), you can start several operations, then await
+// them all. The Io backend may run them concurrently:
+//
+//     var f1 = io.async(taskA, .{});
+//     var f2 = io.async(taskB, .{});
+//
+//     // Both tasks may be running now!
+//     const a = f1.await(io);
+//     const b = f2.await(io);
+//
+// There's also io.concurrent() which provides a STRONGER guarantee:
+// it ensures the function gets its own unit of concurrency (e.g. a
+// real OS thread). But it can fail with error.ConcurrencyUnavailable
+// if resources are exhausted.
+//
+// io.async() is more portable: if no thread is available, it simply
+// runs the function synchronously. This makes it the right default
+// for most code.
+//
+// Fix this program to launch both tasks and collect their results.
+//
+const std = @import("std");
+const print = std.debug.print;
+
+pub fn main(init: std.process.Init) !void {
+    const io = init.io;
+
+    // Launch both tasks asynchronously.
+    var future_a = io.async(slowAdd, .{ 10, 20 });
+    var future_b = ???(slowMul, .{ 6, 7 });
+
+    // Await both results.
+    const sum = future_a.await(io);
+    const product = future_b.???(io);
+
+    print("{} + {} = {}\n", .{ 1, 2, sum });
+    print("{} * {} = {}\n", .{ 6, 7, product });
+    print("Total: {}\n", .{sum + product});
+}
+
+fn slowAdd(a: u32, b: u32) u32 {
+    return a + b;
+}
+
+fn slowMul(a: u32, b: u32) u32 {
+    return a * b;
+}