add ex 57 unions 3

build.zig

@@ -288,6 +288,10 @@ const exercises = [_]Exercise{
         .main_file = "56_unions2.zig",
         .output = "Insect report! Ant alive is: true. Bee visited 16 flowers.",
     },
+    .{
+        .main_file = "57_unions3.zig",
+        .output = "Insect report! Ant alive is: true. Bee visited 17 flowers.",
+    },
 };
 
 /// Check the zig version to make sure it can compile the examples properly.

exercises/57_unions3.zig

@@ -0,0 +1,54 @@
+//
+// With tagged unions, it gets EVEN BETTER! If you don't have a
+// need for a separate enum, you can define an inferred enum with
+// your union all in one place. Just use the 'enum' keyword in
+// place of the tag type:
+//
+//     const Foo = union(enum) {
+//         small: u8,
+//         medium: u32,
+//         large: u64,
+//     };
+//
+// Let's convert Insect. Doctor Zoraptera has already deleted the
+// explicit InsectStat enum for you!
+//
+const std = @import("std");
+
+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 = 17 };
+
+    std.debug.print("Insect report! ", .{});
+
+    printInsect(ant);
+    printInsect(bee);
+
+    std.debug.print("\n", .{});
+}
+
+fn printInsect(insect: Insect) void {
+    switch (insect) {
+        .still_alive => |a| std.debug.print("Ant alive is: {}. ", .{a}),
+        .flowers_visited => |f| std.debug.print("Bee visited {} flowers. ", .{f}),
+    }
+}
+
+// Inferred enums are neat, representing the tip of the iceberg
+// in the relationship between enums and unions. You can actually
+// coerce a union TO an enum (which gives you the active field
+// from the union as an enum). What's even wilder is that you can
+// coerce an enum to a union! But don't get too excited, that
+// only works when the union type is one of those weird zero-bit
+// types like void!
+//
+// Tagged unions, as with most ideas in computer science, have a
+// long history going back to the 1960s. However, they're only
+// recently becoming mainstream, particularly in system-level
+// programming languages. You might have also seen them called
+// "variants", "sum types", or even "enums"!

patches/patches/57_unions3.patch

@@ -0,0 +1,4 @@
+18c18
+< const Insect = union(InsectStat) {
+---
+> const Insect = union(enum) {