Insert space for additional async exercises

exercises/111_labeled_switch.zig

@@ -0,0 +1,79 @@
+//
+// You've heard of while loops in exercises 011,012,013 and 014
+// You've also heard of switch expressions in exercises 030 and 31.
+// You've also seen how labels can be used in exercise 063.
+//
+// By combining while loops and switch statements with continue and break statements
+// one can create very concise State Machines.
+//
+// One such example would be:
+//
+//      pub fn main() void {
+//          var op: u8 = 1;
+//          while (true) {
+//              switch (op) {
+//                  1 => { op = 2; continue; },
+//                  2 => { op = 3; continue; },
+//                  3 => return,
+//                  else => {},
+//              }
+//              break;
+//          }
+//          std.debug.print("This statement cannot be reached\n", .{});
+//      }
+//
+// By combining all we've learned so far, we can now proceed with a labeled switch.
+//
+// A labeled switch is some extra syntactic sugar, which comes with all sorts of
+// candy (performance benefits). Don't believe me? Directly to source https://github.com/ziglang/zig/pull/21367
+//
+// Here is the previous excerpt implemented as a labeled switch instead:
+//
+//      pub fn main() void {
+//          foo: switch (@as(u8, 1)) {
+//              1 => continue :foo 2,
+//              2 => continue :foo 3,
+//              3 => return,
+//              else => {},
+//          }
+//          std.debug.print("This statement cannot be reached\n", .{});
+//      }
+//
+// The flow of execution on this second case is:
+//  1. The switch starts with value '1';
+//  2. The switch evaluates to case '1' which in turn uses the continue statement
+//     to re-evaluate the labeled switch again, now providing the value '2';
+//  3. In the case '2' we repeat the same pattern as case '1'
+//     but instead the value to be evaluated is now '3';
+//  4. Finally we get to case '3', where we return from the function as a whole,
+//     so the debug statement is never executed.
+//  5. In this example, since the input does not have clear, exhaustive patterns and
+//     can essentially be any 'u8' integer, we need to handle all cases not explicitly
+//     covered by using the 'else => {}' branch as the default case.
+//
+//
+const std = @import("std");
+
+const PullRequestState = enum(u8) {
+    Draft,
+    InReview,
+    Approved,
+    Rejected,
+    Merged,
+};
+
+pub fn main() void {
+    // Oh no, your pull request keeps being rejected,
+    // how would you fix it?
+    pr: switch (PullRequestState.Draft) {
+        PullRequestState.Draft => continue :pr PullRequestState.InReview,
+        PullRequestState.InReview => continue :pr PullRequestState.Rejected,
+        PullRequestState.Approved => continue :pr PullRequestState.Merged,
+        PullRequestState.Rejected => {
+            std.debug.print("The pull request has been rejected.\n", .{});
+            return;
+        },
+        PullRequestState.Merged => break, // Would you know where to break to?
+    }
+    std.debug.print("The pull request has been merged.\n", .{});
+}