revival of the async-io functions

3 files changed, 63 insertions, 25 deletions

diff --git a/build.zig b/build.zig
index fa0dcac..d2ee3aa 100644
--- a/build.zig
+++ b/build.zig
@@ -1116,9 +1116,6 @@ const exercises = [_]Exercise{
         .main_file = "084_async.zig",
         .output = "Current time: <timestamp>s since epoch",
         .timestamp = true,
-        // .hint = "Read the facts. Use the facts.",
-        // .skip = true,
-        // .skip_hint = "async has not been implemented in the current compiler version.",
     },
     .{
         .main_file = "085_async2.zig",
@@ -1126,9 +1123,11 @@ const exercises = [_]Exercise{
     },
     .{
         .main_file = "086_async3.zig",
-        .output = "5 4 3 2 1",
-        .skip = true,
-        .skip_hint = "async has not been implemented in the current compiler version.",
+        .output =
+        \\1 + 2 = 3
+        \\6 * 7 = 42
+        \\Total: 45
+        , // pay attention to the comma
     },
     .{
         .main_file = "087_async4.zig",
diff --git a/exercises/086_async3.zig b/exercises/086_async3.zig
index c8f1113..07221e9 100644
--- a/exercises/086_async3.zig
+++ b/exercises/086_async3.zig
@@ -1,29 +1,50 @@
 //
-// Because they can suspend and resume, async Zig functions are
-// an example of a more general programming concept called
-// "coroutines". One of the neat things about Zig async functions
-// is that they retain their state as they are suspended and
-// resumed.
+// The real power of async shows when you launch MULTIPLE tasks!
 //
-// See if you can make this program print "5 4 3 2 1".
+// With io.async(), you can start several operations, then await
+// them all. The Io backend may run them concurrently:
 //
-const print = @import("std").debug.print;
+//     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;
 
-pub fn main() void {
-    const n = 5;
-    var foo_frame = async foo(n);
+    // 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", .{});
+    print("{} + {} = {}\n", .{ 1, 2, sum });
+    print("{} * {} = {}\n", .{ 6, 7, product });
+    print("Total: {}\n", .{sum + product});
 }
 
-fn foo(countdown: u32) void {
-    var current = countdown;
+fn slowAdd(a: u32, b: u32) u32 {
+    return a + b;
+}
 
-    while (current > 0) {
-        print("{} ", .{current});
-        current -= 1;
-        suspend {}
-    }
+fn slowMul(a: u32, b: u32) u32 {
+    return a * b;
 }
diff --git a/patches/patches/086_async3.patch b/patches/patches/086_async3.patch
new file mode 100644
index 0000000..3a3c2c4
--- /dev/null
+++ b/patches/patches/086_async3.patch
@@ -0,0 +1,18 @@
+--- exercises/086_async3.zig	2026-04-01 22:51:05.540094851 +0200
++++ answers/086_async3.zig	2026-04-01 22:50:44.579669189 +0200
+@@ -29,12 +29,12 @@
+     const io = init.io;
+ 
+     // Launch both tasks asynchronously.
+-    var future_a = io.async(slowAdd, .{ 10, 20 });
+-    var future_b = ???(slowMul, .{ 6, 7 });
++    var future_a = io.async(slowAdd, .{ 1, 2 });
++    var future_b = io.async(slowMul, .{ 6, 7 });
+ 
+     // Await both results.
+     const sum = future_a.await(io);
+-    const product = future_b.???(io);
++    const product = future_b.await(io);
+ 
+     print("{} + {} = {}\n", .{ 1, 2, sum });
+     print("{} * {} = {}\n", .{ 6, 7, product });