Very unfinished idea for a vertex format wrapper struct

VertexFormat.gml

/// 

#macro POSITION_2D vertex_format_add_position
#macro POSITION_3D vertex_format_add_position_3d
#macro COLOR       vertex_format_add_color
#macro UV          vertex_format_add_texcoord
#macro NORMAL      vertex_format_add_normal

/*
vertex_format_add_position_3d();
vertex_position_3d();
buffer_write(buffer, buffer_f32, 4);
*/

function VertexFormat(attributes) constructor {
	vertex_format_begin();
	var i = 0;
	repeat(array_length(attributes)) {
		attributes[i++]();
	}
	format = vertex_format_end();
	
	descriptor = attributes;
	
	static _add = {
		POSITION_2D: vertex_position,
		POSITION_3D: vertex_position_3d,
		COLOR:       vertex_color,
		UV:          vertex_texcoord,
		NORMAL:      vertex_normal,
	}
	
	attribute_list = function() {
		var index = ds_map_create();
		index[?vertex_format_add_position] = 0;
		index[?vertex_format_add_position_3d] = 0;
		index[?vertex_format_add_color] = 0;
		index[?vertex_format_add_texcoord] = 0;
		index[?vertex_format_add_normal] = 0;
		var str_list = "";
		for(var i = 0;i < array_length(descriptor);i++) {
			var desc = descriptor[i];
			switch(desc) {
				case vertex_format_add_position:
					str_list += "attribute vec2 in_Position";
					break;
				case vertex_format_add_position_3d:
					str_list += "attribute vec3 in_Position";
					break;
				case vertex_format_add_color:
					str_list += "attribute vec4 in_Colour";
					break;
				case vertex_format_add_texcoord:
					str_list += "attribute vec2 in_TextureCoord";
					break;
				case vertex_format_add_normal:
					str_list += "attribute vec3 in_Normal";
					break;
			}
			str_list += (index[?desc] == 0) ? "" : string(index[?desc]+1);
			str_list += ";\n";
			index[?desc]++;
		}
		ds_map_destroy(index);
		return str_list;
	}
	
	_begin = function() {
		vb = vertex_create_buffer();
		vertex_begin(vb, format);
	}
	
	vertex = function(params) {
		for(var i = 0;i < array_length(params);i++) {
			array_insert(params, 0, vb);
			_add[$ descriptor[i]](params);
		}
	}
	
	_end = function(freeze) {
		vertex_end(vb);
		vertex_freeze(vb);
	}
	
	vb = undefined;
	
	bytesize = function() {
	    static buff = buffer_create(256, buffer_fixed, 1);
	    var vb = vertex_create_buffer_from_buffer_ext(
	        buff, format, 0, 1
	    );
	    var size = vertex_get_buffer_size(vb);
	    vertex_delete_buffer(vb);
	    return size;
	}
	
	toString = function() {
		var str = "{";
		var names = variable_struct_get_names(self);
		for(var i = 0;i < array_length(names);i++) {
			var name = names[i];
			if is_method(self[$name]) { continue; }
			str += " " + name + " : " + string(self[$name]) + ",";
		}
		str += "}";
		return str;
	}
	
	size = bytesize(format);
}