added async-io quiz

exercises/086_async2.zig

@@ -0,0 +1,48 @@
+//
+// Now that we know how to get an Io value, let's use it for
+// asynchronous execution!
+//
+// io.async() launches a function and returns a Future. The result
+// won't necessarily be available until you call .await() on it:
+//
+//     var future = io.async(someFunction, .{ arg1, arg2 });
+//     // ... do other work here ...
+//     const result = future.await(io);
+//
+// The function *may* run immediately or on another thread -
+// your code doesn't need to care! That's the beauty of the
+// Io abstraction. (In the Threaded backend, if no thread is
+// available, the function runs synchronously right away and
+// .await() just returns the already-computed result.)
+//
+// io.async() returns a Future(T) where T is the return type
+// of the function you passed in. Future has two key methods:
+//
+//     .await(io)  - block until the result is ready, return it
+//     .cancel(io) - request cancellation, then return the result
+//
+// Fix this program so that computeAnswer runs asynchronously
+// and its result is properly awaited.
+//
+const std = @import("std");
+
+pub fn main(init: std.process.Init) !void {
+    const io = init.io;
+
+    // Launch computeAnswer asynchronously.
+    // io.async() takes a function and a tuple of its arguments.
+    var future = io.async(computeAnswer, .{ 6, 7 });
+
+    // Meanwhile, print something to show we're not blocked.
+    std.debug.print("Computing... ", .{});
+
+    // Now collect the result. What method on Future gives us
+    // the value, blocking if it isn't ready yet?
+    const answer = future.???(io);
+
+    std.debug.print("The answer is: {}\n", .{answer});
+}
+
+fn computeAnswer(a: u32, b: u32) u32 {
+    return a * b;
+}