command_list.zig

const std = @import("std");

const VoidList = std.SinglyLinkedList(void);

const Command = struct {
    tag: Tag,
    node: VoidList.Node = .{ .data = {} },

    const Tag = enum {
        fill,
        text,
    };

    const Fill = struct {
        base: Command = .{ .tag = .fill },
        color: u32,
    };

    const Text = struct {
        base: Command = .{ .tag = .text },
        text: []const u8,
    };
};

pub fn main() void {
    var list = VoidList{};
    var fill = Command.Fill{ .color = 0 };
    var text = Command.Text{ .text = "hello" };
    list.prepend(&fill.base.node);
    list.prepend(&text.base.node);
    {
        var it = list.first;
        while (it) |node| : (it = node.next) {
            const command = @fieldParentPtr(Command, "node", node);
            switch (command.tag) {
                .fill => std.debug.print("fill: {}\n", .{@fieldParentPtr(Command.Fill, "base", command).color}),
                .text => std.debug.print("text: {s}\n", .{@fieldParentPtr(Command.Text, "base", command).text}),
            }
        }
    }
}