added async-io quiz

exercises/093_async9.zig

@@ -0,0 +1,57 @@
+//
+// We've been using io.async() to launch tasks. But there's a
+// stronger variant: io.concurrent().
+//
+// The difference:
+//
+//   io.async():
+//     * The function MAY run on another thread, or it may run
+//       immediately on the current thread (synchronously).
+//     * Never fails — if no thread is available, it just runs
+//       the function right away.
+//     * More portable, works with all Io backends.
+//
+//   io.concurrent():
+//     * GUARANTEES a separate unit of concurrency (a real thread
+//       in the Threaded backend).
+//     * Can fail with error.ConcurrencyUnavailable if resources
+//       are exhausted or the backend doesn't support it.
+//     * Use when you NEED true parallelism.
+//
+// Because concurrent() can fail, you must handle the error:
+//
+//     var future = try io.concurrent(myFn, .{args});
+//     const result = future.await(io);
+//
+// Notice the 'try' — that's the key difference in usage!
+//
+// Fix this program to launch the computation concurrently.
+//
+const std = @import("std");
+const print = std.debug.print;
+
+pub fn main(init: std.process.Init) !void {
+    const io = init.io;
+
+    // Launch with a guaranteed separate thread.
+    // Which Io method guarantees true concurrency?
+    // (Hint: unlike io.async, this one can fail!)
+    var future = try io.???(compute, .{io});
+
+    print("Main thread continues...\n", .{});
+
+    // Wait 100 millisecond so the output order is deterministic.
+    io.sleep(std.Io.Duration.fromMilliseconds(100), .awake) catch {};
+
+    print("Main thread done waiting.\n", .{});
+
+    const result = future.await(io);
+    print("Result: {}\n", .{result});
+}
+
+fn compute(io: std.Io) u32 {
+    print("Computing on a separate thread!\n", .{});
+    // Simulate some work.
+    io.sleep(std.Io.Duration.fromMilliseconds(200), .awake) catch return 0;
+    return 123;
+}