add ex56 for real

build.zig

@@ -284,6 +284,10 @@ const exercises = [_]Exercise{
         .main_file = "55_unions.zig",
         .output = "Insect report! Ant alive is: true. Bee visited 15 flowers.",
     },
+    .{
+        .main_file = "56_unions2.zig",
+        .output = "Insect report! Ant alive is: true. Bee visited 16 flowers.",
+    },
 };
 
 /// Check the zig version to make sure it can compile the examples properly.

exercises/56_unions2.zig

@@ -0,0 +1,64 @@
+//
+// It is really quite inconvenient having to manually keep track
+// of the active field in our union, isn't it?
+//
+// Thankfully, Zig also has "tagged unions", which allow us to
+// store an enum value within our union representing which field
+// is active.
+//
+//     const FooTag = enum{ small, medium, large };
+//
+//     const Foo = union(FooTag) {
+//         small: u8,
+//         medium: u32,
+//         large: u64,
+//     };
+//
+// Now we can use a switch directly on the union to act on the
+// active field:
+//
+//     var f = Foo{ .small = 10 };
+//
+//     switch (f) {
+//         .small => |my_small| do_something(my_small),
+//         .medium => |my_medium| do_something(my_medium),
+//         .large => |my_large| do_something(my_large),
+//     }
+//
+// Let's make our Insects use a tagged union (Doctor Zoraptera
+// approves).
+//
+const std = @import("std");
+
+const InsectStat = enum { flowers_visited, still_alive };
+
+const Insect = union(InsectStat) {
+    flowers_visited: u16,
+    still_alive: bool,
+};
+
+pub fn main() void {
+    var ant = Insect{ .still_alive = true };
+    var bee = Insect{ .flowers_visited = 16 };
+
+    std.debug.print("Insect report! ", .{});
+
+    // Could it really be as simple as just passing the union?
+    printInsect(???);
+    printInsect(???);
+
+    std.debug.print("\n", .{});
+}
+
+fn printInsect(insect: Insect) void {
+    switch (???) {
+        .still_alive => |a| std.debug.print("Ant alive is: {}. ", .{a}),
+        .flowers_visited => |f| std.debug.print("Bee visited {} flowers. ", .{f}),
+    }
+}
+
+// By the way, did unions remind you of optional values and errors?
+// Optional values are basically "null unions" and errors use "error
+// union types". Now we can add our own unions to the mix to handle
+// whatever situations we might encounter:
+//          union(Tag) { value: u32, toxic_ooze: void }