zig/doc/langref/test_union_method.zig

const std = @import("std");
const expect = std.testing.expect;

const Variant = union(enum) {
    int: i32,
    boolean: bool,

    // void can be omitted when inferring enum tag type.
    none,

    fn truthy(self: Variant) bool {
        return switch (self) {
            Variant.int => |x_int| x_int != 0,
            Variant.boolean => |x_bool| x_bool,
            Variant.none => false,
        };
    }
};

test "union method" {
    var v1 = Variant{ .int = 1 };
    var v2 = Variant{ .boolean = false };

    try expect(v1.truthy());
    try expect(!v2.truthy());
}

// test