Moved exercises to exercises because exercises

exercises/29_errdefer.zig

@@ -0,0 +1,60 @@
+//
+// Another common problem is a block of code that could exit in multiple
+// places due to an error - but that needs to run do something before it
+// exits (typically to clean up after itself).
+//
+// An "errdefer" is a defer that only runs if the block exits with an error:
+//
+//     {
+//         errdefer cleanup();
+//         try canFail();
+//     }
+//
+// The cleanup() function is called ONLY if the "try" statement returns an
+// error produced by canFail().
+//
+const std = @import("std");
+
+//
+var counter: u32 = 0;
+
+const MyErr = error{ GetFail, IncFail };
+
+pub fn main() void {
+    // We simply quit the entire program if we fail to get a number:
+    var a: u32 = makeNumber() catch return;
+    var b: u32 = makeNumber() catch return;
+
+    std.debug.print("Numbers: {}, {}\n", .{a,b});
+}    
+
+fn makeNumber() MyErr!u32 {
+    std.debug.print("Getting number...", .{});
+
+    // Please make the "failed" message print ONLY if the makeNumber()
+    // function exits with an error:
+    std.debug.print("failed!\n", .{});
+
+    var num = try getNumber();     // <-- This could fail!
+
+    num = try increaseNumber(num); // <-- This could ALSO fail!
+
+    std.debug.print("got {}. ", .{num});
+
+    return num;
+}
+
+fn getNumber() MyErr!u32 {
+    // I _could_ fail...but I don't!
+    return 4;
+}
+
+fn increaseNumber(n: u32) MyErr!u32 {
+    // I fail after the first time you run me!
+    if (counter > 0) return MyErr.IncFail;
+    
+    // Sneaky, weird global stuff.
+    counter += 1;
+
+    return n + 1;
+}