add way to check preregistered id creations after world creation

CHANGELOG.md

@@ -2,14 +2,6 @@
 
 ## Unreleased
 
-### Added
-- Added signals that allow listening to relation part of pairs in signals.
-
-### Changed
-- `OnRemove` hooks so that they are allowed to move entity's archetype even during deletion.
-
-## 0.8.0
-
 ### Added
 - `jecs.Exclusive` trait for making exclusive relationships.
 

addons/ob.luau

@@ -160,10 +160,6 @@ local function monitors_new<T...>(
 	end
 
 	local function removed(entity: jecs.Entity, component: jecs.Id)
-		local r = jecs.record(world, entity)
-		if not archetypes[r.archetype.id] then
-			return
-		end
 		local EcsOnRemove = jecs.OnRemove :: jecs.Id
 		if callback ~= nil then
 			callback(entity, EcsOnRemove)

benches/general.luau

@@ -199,7 +199,6 @@ do
 			end
 
 			local q = world:query(A, B, C, D)
-			q:archetypes()
 			START()
 			for id in q do
 			end

benches/query.luau

@@ -11,22 +11,15 @@ end
 local jecs = require("@jecs")
 local mirror = require("@mirror")
 
+type i53 = number
+
 do
 	TITLE(testkit.color.white_underline("Jecs query"))
-	local ecs = jecs.world() :: jecs.World
+	local ecs = jecs.world()
 	do
 		TITLE("one component in common")
 
-		local function view_bench(world: jecs.World,
+		local function view_bench(world: jecs.World, A: i53, B: i53, C: i53, D: i53, E: i53, F: i53, G: i53, H: i53)
-			A: jecs.Id,
-			B: jecs.Id,
-			C: jecs.Id,
-			D: jecs.Id,
-			E: jecs.Id,
-			F: jecs.Id,
-			G: jecs.Id,
-			H: jecs.Id
-		)
 			BENCH("1 component", function()
 				for _ in world:query(A) do
 				end
@@ -138,21 +131,11 @@ end
 
 do
 	TITLE(testkit.color.white_underline("Mirror query"))
-	local ecs = mirror.World.new() :: jecs.World
+	local ecs = mirror.World.new()
 	do
 		TITLE("one component in common")
 
-		local function view_bench(
+		local function view_bench(world: jecs.World, A: i53, B: i53, C: i53, D: i53, E: i53, F: i53, G: i53, H: i53)
-			world: mirror.World,
-			A: jecs.Id,
-			B: jecs.Id,
-			C: jecs.Id,
-			D: jecs.Id,
-			E: jecs.Id,
-			F: jecs.Id,
-			G: jecs.Id,
-			H: jecs.Id
-		)
 			BENCH("1 component", function()
 				for _ in world:query(A) do
 				end

docs/api/world.md

@@ -355,8 +355,45 @@ This operation is the same as calling:
 world:target(entity, jecs.ChildOf, 0)
 ```
 
-:::
+## contains
 
+Checks if an entity or component (id) exists in the world.
+
+```luau
+function World:contains(
+    entity: Entity,
+): boolean
+```
+
+Example:
+
+::: code-group
+
+```luau [luau]
+local entity = world:entity()
+print(world:contains(entity))
+print(world:contains(1))
+print(world:contains(2))
+
+-- Outputs:
+-- true
+-- true
+-- false
+```
+
+```ts [typescript]
+const entity = world.entity();
+print(world.contains(entity));
+print(world.contains(1));
+print(world.contains(2));
+
+// Outputs:
+// true
+// true
+// false
+```
+
+:::
 
 ## remove
 
@@ -423,11 +460,11 @@ Example:
 
 ```luau [luau]
 local entity = world:entity()
-print(world:contains(entity))
+print(world:has(entity))
 
 world:delete(entity)
 
-print(world:contains(entity))
+print(world:has(entity))
 
 -- Outputs:
 -- true
@@ -564,7 +601,7 @@ print(retrievedParent === parent) // true
 
 ## contains
 
-Checks if an entity or component (id) exists and is alive in the world.
+Checks if an entity exists and is alive in the world.
 
 ```luau
 function World:contains(
@@ -603,26 +640,12 @@ function World:exists(
 
 ## cleanup
 
-Cleans up empty archetypes.
+Cleans up deleted entities and their associated data. This is automatically called by jecs, but can be called manually if needed.
 
 ```luau
 function World:cleanup(): void
 ```
 
-:::info
-It is recommended to profile the optimal interval you should cleanup because it varies completely from game to game.
-
-Here are a couple of reasons from Sander Mertens:
-	- some applications are memory constrained, so any wasted memory on empty
-	  archetypes has to get cleaned up
-	- many archetypes can get created during game startup but aren't used later
-	  on, so it would be wasteful to keep them around
-	- empty archetypes can slow queries down, especially if there are many more
-	  empty ones than non-empty ones
-	- if the total number of component permutations (/relationships) is too
-	  high, you have no choice but to periodically cleanup empty archetypes
-:::
-
 Example:
 
 ::: code-group

examples/luau/entities/basics.luau

@@ -1,19 +1,15 @@
 local jecs = require("@jecs")
-local world = jecs.world()
+local world = jecs.World.new()
 
-local Position = world:component() :: jecs.Id<vector>
+local Position = world:component()
 local Walking = world:component()
-local Name = world:component() :: jecs.Id<string>
+local Name = world:component()
-
-local function name(e: jecs.Entity): string
-	return assert(world:get(e, Name))
-end
 
 -- Create an entity with name Bob
 local bob = world:entity()
 
 -- The set operation finds or creates a component, and sets it.
-world:set(bob, Position, vector.create(10, 20, 30))
+world:set(bob, Position, Vector3.new(10, 20, 30))
 -- Name the entity Bob
 world:set(bob, Name, "Bob")
 -- The add operation adds a component without setting a value. This is
@@ -22,16 +18,15 @@ world:add(bob, Walking)
 
 -- Get the value for the Position component
 local pos = world:get(bob, Position)
-assert(pos)
+print(`\{{pos.X}, {pos.Y}, {pos.Z}\}`)
-print(`\{{pos.x}, {pos.y}, {pos.z}\}`)
 
 -- Overwrite the value of the Position component
-world:set(bob, Position, vector.create(40, 50, 60))
+world:set(bob, Position, Vector3.new(40, 50, 60))
 
 local alice = world:entity()
 -- Create another named entity
 world:set(alice, Name, "Alice")
-world:set(alice, Position, vector.create(10, 20, 30))
+world:set(alice, Position, Vector3.new(10, 20, 30))
 world:add(alice, Walking)
 
 -- Remove tag
@@ -39,7 +34,7 @@ world:remove(alice, Walking)
 
 -- Iterate all entities with Position
 for entity, p in world:query(Position) do
-	print(`{name(entity)}: \{{p.x}, {p.y}, {p.z}\}`)
+	print(`{entity}: \{{p.X}, {p.Y}, {p.Z}\}`)
 end
 
 -- Output:

examples/luau/queries/archetypes/targets.luau

@@ -1,60 +0,0 @@
--- Using world:target is the recommended way to grab the target in a wildcard
--- query. However the random access can add up in very hot paths. Accessing its
--- column can drastically improve performance, especially when there are
--- multiple adjacent pairs.
-
-local jecs = require("@jecs")
-local pair = jecs.pair
-local __ = jecs.Wildcard
-
-local world = jecs.world()
-
-local Likes = world:entity()
-local function name(e, name: string): string
-	if name then
-		world:set(e, jecs.Name, name)
-		return name
-	end
-	return assert(world:get(e, jecs.Name))
-end
-
-local e1 = world:entity()
-name(e1, "e1")
-local e2 = world:entity()
-name(e2, "e2")
-local e3 = world:entity()
-name(e3, "e3")
-
-world:add(e1, pair(Likes, e2))
-world:add(e1, pair(Likes, e3))
-
-local likes = jecs.component_record(world, pair(Likes, __))
-assert(likes)
-
-local likes_cr = likes.records
-local likes_counts = likes.counts
-
-local archetypes = world:query(pair(Likes, __)):archetypes()
-
-for _, archetype in archetypes do
-	local types = archetype.types
-
-	-- Get the starting index which is what the (R, *) alias is at
-	local wc = likes_cr[archetype.id]
-	local count = likes_counts[archetype.id]
-
-	local entities = archetype.entities
-	for i = #entities, 1, -1 do
-		-- It is generally a good idea to iterate backwards on arrays if you
-		-- need to delete the iterated entity to prevent iterator invalidation
-		local entity = entities[i]
-		for cr = wc, wc + count - 1 do
-			local person = jecs.pair_second(world, types[cr])
-			print(`entity ${entity} likes ${person}`)
-		end
-	end
-end
-
--- Output:
---  entity $273 likes $275
---  entity $273 likes $274

examples/luau/queries/archetypes/visibility_cascades.luau

@@ -1,44 +0,0 @@
--- To get the most out of performance, you can lift the inner loop of queries to
--- the system in which you can do archetype-specific optimizations like finding
--- the parent once per archetype rather than per entity.
-
-local jecs = require("@jecs")
-local pair = jecs.pair
-local ChildOf = jecs.ChildOf
-local __ = jecs.Wildcard
-
-local world = jecs.world()
-
-local Position = world:component() :: jecs.Id<vector>
-local Visible = world:entity()
-
-local parent = world:entity()
-world:set(parent, Position, vector.zero)
-world:add(parent, Visible)
-
-local child = world:entity()
-world:set(child, Position, vector.one)
-world:add(child, pair(ChildOf, parent))
-
-local parents = jecs.component_record(world, pair(ChildOf, __))
-assert(parents)
-
-local parent_cr = parents.records
-
-local archetypes = world:query(Position, pair(ChildOf, __)):archetypes()
-
-for _, archetype in archetypes do
-	local types = archetype.types
-	local p = jecs.pair_second(world, types[parent_cr[archetype.id]])
-	if world:has(p, Visible) then
-		local columns = archetype.columns_map
-		local positions = columns[Position]
-	  	for row, entity in archetype.entities do
-			local pos = positions[row]
-			print(`Child ${entity} of ${p} is visible at {pos}`)
-		end
-	end
-end
-
--- Output:
---  Child $274 of $273 is visibile at 1,1,1

examples/luau/queries/basics.luau

@@ -1,5 +1,5 @@
 local jecs = require("@jecs")
-local world = jecs.world()
+local world = jecs.World.new()
 
 local Position = world:component()
 local Velocity = world:component()

examples/luau/queries/changetracking.luau

@@ -1,16 +1,19 @@
 local jecs = require("@jecs")
 local pair = jecs.pair
 
-local world = jecs.world()
+local world = jecs.World.new()
-local Name = world:component() :: jecs.Id<string>
+local Name = world:component()
 
-
+local function named(ctr, name)
-local function name(e: jecs.Entity): string
+	local e = ctr(world)
-	return assert(world:get(e, Name))
+	world:set(e, Name, name)
+	return e
+end
+local function name(e)
+	return world:get(e, Name)
 end
 
-local Position = world:component() :: jecs.Id<vector>
+local Position = named(world.component, "Position") :: jecs.Entity<vector>
-world:set(Position, Name, "Position")
 local Previous = jecs.Rest
 
 local added = world
@@ -26,14 +29,10 @@ local removed = world
 	:cached()
 
 
-local e1 = world:entity()
+local e1 = named(world.entity, "e1")
-world:set(e1, Name, "e1")
 world:set(e1, Position, vector.create(10, 20, 30))
-
+local e2 = named(world.entity, "e2")
-local e2 = world:entity()
-world:set(e2, Name, "e2")
 world:set(e2, Position, vector.create(10, 20, 30))
-
 for entity, p in added do
 	print(`Added {name(entity)}: \{{p.x}, {p.y}, {p.z}}`)
 	world:set(entity, pair(Previous, Position), p)
@@ -41,10 +40,10 @@ end
 
 world:set(e1, Position, vector.create(999, 999, 1998))
 
-for entity, new, old in changed do
+for _, archetype in changed:archetypes() do
 	if new ~= old then
-		print(`{name(entity)}'s Position changed from \{{old.x}, {old.y}, {old.z}\} to \{{new.x}, {new.y}, {new.z}\}`)
+		print(`{name(e)}'s Position changed from \{{old.x}, {old.y}, {old.z}\} to \{{new.x}, {new.y}, {new.z}\}`)
-		world:set(entity, pair(Previous, Position), new)
+		world:set(e, pair(Previous, Position), new)
 	end
 end
 

examples/luau/queries/spatial_grids.luau

@@ -3,47 +3,32 @@ local pair = jecs.pair
 local ChildOf = jecs.ChildOf
 local __ = jecs.Wildcard
 local Name = jecs.Name
-local world = jecs.world()
+local world = jecs.World.new()
 
-local Voxel = world:component() :: jecs.Id
+type Id<T = nil> = number & { __T: T }
-local Position = world:component() :: jecs.Id<vector>
+local Voxel = world:component() :: Id
-local Perception = world:component() :: jecs.Id<{
+local Position = world:component() :: Id<Vector3>
+local Perception = world:component() :: Id<{
 	range: number,
 	fov: number,
-	dir: vector,
+	dir: Vector3,
 }>
-type part = {
+local PrimaryPart = world:component() :: Id<Part>
-	Position: vector
-}
-local PrimaryPart = world:component() :: jecs.Id<part>
 
-local local_player = {
+local local_player = game:GetService("Players").LocalPlayer
-	Character = {
-		PrimaryPart = {
-			Position = vector.create(50, 0, 30)
-		}
-	}
-}
-local workspace = {
-	CurrentCamera = {
-		CFrame = {
-			LookVector = vector.create(0, 0, -1)
-		}
-	}
-}
 
-local function distance(a: vector, b: vector)
+local function distance(a: Vector3, b: Vector3)
-	return vector.magnitude((b - a))
+	return (b - a).Magnitude
 end
 
-local function is_in_fov(a: vector, b: vector, forward_dir: vector, fov_angle: number)
+local function is_in_fov(a: Vector3, b: Vector3, forward_dir: Vector3, fov_angle: number)
 	local to_target = b - a
 
-	local forward_xz = vector.normalize(vector.create(forward_dir.x, 0, forward_dir.z))
+	local forward_xz = Vector3.new(forward_dir.X, 0, forward_dir.Z).Unit
-	local to_target_xz = vector.normalize(vector.create(to_target.x, 0, to_target.z))
+	local to_target_xz = Vector3.new(to_target.X, 0, to_target.Z).Unit
 
-	local angle_to_target = math.deg(math.atan2(to_target_xz.z, to_target_xz.x))
+	local angle_to_target = math.deg(math.atan2(to_target_xz.Z, to_target_xz.X))
-	local forward_angle = math.deg(math.atan2(forward_xz.z, forward_xz.z))
+	local forward_angle = math.deg(math.atan2(forward_xz.Z, forward_xz.X))
 
 	local angle_difference = math.abs(forward_angle - angle_to_target)
 
@@ -57,7 +42,7 @@ end
 local map = {}
 local grid = 50
 
-local function add_to_voxel(source: jecs.Entity, position: vector, prev_voxel_id: jecs.Entity?)
+local function add_to_voxel(source: number, position: Vector3, prev_voxel_id: number?)
 	local hash = position // grid
 	local voxel_id = map[hash]
 	if not voxel_id then
@@ -94,7 +79,7 @@ local function update_camera_direction(dt: number)
 end
 
 local function perceive_enemies(dt: number)
-	local it = world:query(Perception, Position, PrimaryPart):iter()
+	local it = world:query(Perception, Position, PrimaryPart)
 	-- There is only going to be one entity matching the query
 	local e, self_perception, self_position, self_primary_part = it()
 
@@ -108,28 +93,28 @@ local function perceive_enemies(dt: number)
 
 		if is_in_fov(self_position, target_position, self_perception.dir, self_perception.fov) then
 			local p = target_position
-			print(`Entity {world:get(e, Name)} can see target {world:get(enemy, Name)} at ({p.x}, {p.y}, {p.z})`)
+			print(`Entity {world:get(e, Name)} can see target {world:get(enemy, Name)} at ({p.X}, {p.Y}, {p.Z})`)
 		end
 	end
 end
 
 local player = world:entity()
 world:set(player, Perception, {
-	range = 200,
+	range = 100,
 	fov = 90,
-	dir = vector.create(1, 0, 0),
+	dir = Vector3.new(1, 0, 0),
 })
 world:set(player, Name, "LocalPlayer")
-local primary_part = local_player.Character.PrimaryPart
+local primary_part = (local_player.Character :: Model).PrimaryPart :: Part
 world:set(player, PrimaryPart, primary_part)
-world:set(player, Position, vector.zero)
+world:set(player, Position, Vector3.zero)
 
 local enemy = world:entity()
 world:set(enemy, Name, "Enemy $1")
-world:set(enemy, Position, vector.create(50, 0, 20))
+world:set(enemy, Position, Vector3.new(50, 0, 20))
 
 add_to_voxel(player, primary_part.Position)
-add_to_voxel(enemy, assert(world:get(enemy, Position)))
+add_to_voxel(enemy, world)
 
 local dt = 1 / 60
 reconcile_client_owned_assembly_to_voxel(dt)

jecs.d.ts

@@ -7,14 +7,10 @@ export type Entity<TData = unknown> = number & {
 	readonly __type_TData: TData;
 };
 
-type TagDiscriminator = {
-	readonly __nominal_Tag: unique symbol;
-};
-
 /**
  * An entity with no associated data when used as a component
  */
-export type Tag = Entity<TagDiscriminator>;
+export type Tag = Entity<undefined>;
 
 /**
  * A pair of entities:
@@ -30,12 +26,12 @@ export type Pair<P = unknown, O = unknown> = number & {
  * An `Id` can be either a single Entity or a Pair of Entities.
  * By providing `TData`, you can specifically require an Id that yields that type.
  */
-export type Id<TData = unknown> = Entity<TData> | Pair<TData, unknown> | Pair<TagDiscriminator, TData>;
+export type Id<TData = unknown> = Entity<TData> | Pair<TData, unknown> | Pair<undefined, TData>;
 
 export type InferComponent<E> = E extends Entity<infer D>
 	? D
 	: E extends Pair<infer P, infer O>
-	? P extends TagDiscriminator
+	? P extends undefined
 		? O
 		: P
 	: never;
@@ -47,30 +43,22 @@ type InferComponents<A extends Id[]> = { [K in keyof A]: InferComponent<A[K]> };
 type ArchetypeId = number;
 export type Column<T> = T[];
 
-export type Archetype<T extends Id[]> = {
+export type Archetype<T extends unknown[]> = {
 	id: number;
-	types: Entity[];
+	types: number[];
 	type: string;
-	entities: Entity[];
+	entities: number[];
 	columns: Column<unknown>[];
-	columns_map: { [K in T[number]]: Column<InferComponent<K>> };
+	columns_map: Record<Id, Column<T[number]>>
 };
 
-type IterFn<T extends Id[]> = IterableFunction<LuaTuple<[Entity, ...InferComponents<T>]>>;
+type Iter<T extends unknown[]> = IterableFunction<LuaTuple<[Entity, ...T]>>;
-type Iter<T extends Id[]> = IterFn<T> & {
-	/**
-	 * This isn't callable
-	 * @hidden
-	 * @deprecated
-	 */
-	(): never
-};
 
-export type CachedQuery<T extends Id[]> = {
+export type CachedQuery<T extends unknown[]> = {
 	/**
 	 * Returns an iterator that produces a tuple of [Entity, ...queriedComponents].
 	 */
-	iter(): IterFn<T>;
+	iter(): Iter<T>;
 
 	/**
 	 * Returns the matched archetypes of the query
@@ -79,11 +67,11 @@ export type CachedQuery<T extends Id[]> = {
 	archetypes(): Archetype<T>[];
 } & Iter<T>;
 
-export type Query<T extends Id[]> = {
+export type Query<T extends unknown[]> = {
 	/**
 	 * Returns an iterator that produces a tuple of [Entity, ...queriedComponents].
 	 */
-	iter(): IterFn<T>;
+	iter(): Iter<T>;
 
 	/**
 	 * Creates and returns a cached version of this query for efficient reuse.
@@ -131,7 +119,7 @@ export class World {
 	 * @returns An entity (Tag) with no data.
 	 */
 	entity(): Tag;
-	entity<T extends Entity>(id: T): T;
+	entity<T extends Entity>(id: T): InferComponent<T> extends undefined ? Tag : T;
 
 	/**
 	 * Creates a new entity in the first 256 IDs, typically used for static
@@ -160,7 +148,7 @@ export class World {
 	 * @param entity The target entity.
 	 * @param component The component (or tag) to add.
 	 */
-	add<C>(entity: Entity, component: TagDiscriminator extends InferComponent<C> ? C : Id<TagDiscriminator>): void;
+	add<C>(entity: Entity, component: undefined extends InferComponent<C> ? C : Id<undefined>): void;
 
 	/**
 	 * Installs a hook on the given component.
@@ -184,11 +172,6 @@ export class World {
 	 */
 	cleanup(): void;
 
-	/**
-	 * Removes all instances of specified component
-	 */
-	// purge<T>(component: Id<T>): void
-
 	/**
 	 * Clears all components and relationships from the given entity, but
 	 * does not delete the entity from the world.
@@ -263,11 +246,11 @@ export class World {
 	 * @param components The list of components to query.
 	 * @returns A Query object to iterate over results.
 	 */
-	query<T extends Id[]>(...components: T): Query<T>;
+	query<T extends Id[]>(...components: T): Query<InferComponents<T>>;
 
-	added<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>, value: T) => void): () => void;
+	added<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>, value: T) => void): () => void
-	changed<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>, value: T) => void): () => void;
+	changed<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>, value: T) => void): () => void
-	removed<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>) => void): () => void;
+	removed<T>(component: Entity<T>, listener: (e: Entity, id: Id<T>) => void): () => void
 }
 
 export function world(): World;
@@ -314,11 +297,11 @@ export function ECS_PAIR_FIRST(pair: Pair): number;
 export function ECS_PAIR_SECOND(pair: Pair): number;
 
 type StatefulHook = Entity<<T>(e: Entity<T>, id: Id<T>, data: T) => void> & {
-	readonly __nominal_StatefulHook: unique symbol;
+	readonly __nominal_StatefulHook: unique symbol,
-};
+}
 type StatelessHook = Entity<<T>(e: Entity<T>, id: Id<T>) => void> & {
-	readonly __nominal_StatelessHook: unique symbol;
+	readonly __nominal_StatelessHook: unique symbol,
-};
+}
 
 export declare const OnAdd: StatefulHook;
 export declare const OnRemove: StatelessHook;
@@ -336,27 +319,12 @@ export declare const Exclusive: Tag;
 export declare const Rest: Entity;
 
 export type ComponentRecord = {
-	records: Map<Id, number>;
+	records: Map<Id, number>,
-	counts: Map<Id, number>;
+	counts: Map<Id, number>,
-	size: number;
+	size: number,
-};
+}
 
-export function component_record(world: World, id: Id): ComponentRecord;
+export function component_record(world: World, id: Id): ComponentRecord
 
-type TagToUndefined<T> = T extends TagDiscriminator ? undefined : T
+export function bulk_insert<const C extends Id[]>(world: World, entity: Entity, ids: C, values: InferComponents<C>): void
-
+export function bulk_remove(world: World, entity: Entity, ids: Id[]): void
-export function bulk_insert<const C extends Id[]>(
-	world: World,
-	entity: Entity,
-	ids: C,
-	values: { [K in keyof C]: TagToUndefined<InferComponent<C[K]>> },
-): void;
-export function bulk_remove(world: World, entity: Entity, ids: Id[]): void;
-
-export type EntityRecord<T extends Id[]> = {
-	archetype: Archetype<T>,
-	row: number,
-	dense: number,
-};
-
-export function record<T extends Id[] = []>(world: World, entity: Entity): EntityRecord<T>;

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "@rbxts/jecs",
-	"version": "0.9.0-rc.12",
+	"version": "0.9.0-rc.7",
 	"description": "Stupidly fast Entity Component System",
 	"main": "jecs.luau",
 	"repository": {

wally.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ukendio/jecs"
-version = "0.9.0-rc.12"
+version = "0.9.0-rc.7"
 registry = "https://github.com/UpliftGames/wally-index"
 realm = "shared"
 license = "MIT"

werewitch.luau

@@ -0,0 +1,66 @@
+local jecs = require("@jecs")
+
+local world = jecs.world()
+local pair = jecs.pair
+
+local IsA = world:entity()
+
+local traits = {
+	IsA = IsA
+}
+
+world:set(IsA, jecs.OnAdd, function(component, id)
+	local second = jecs.pair_second(world, id)
+	assert(second ~= component, "circular")
+
+	local is_tag = jecs.is_tag(world, second)
+	world:added(component, function(entity, _, value)
+		if is_tag then
+			world:add(entity, second)
+		else
+			world:set(entity, second, value)
+		end
+	end)
+
+	world:removed(component, function(entity)
+		world:remove(entity, second)
+	end)
+
+	if not is_tag then
+		world:changed(component, function(entity, _, value)
+			world:set(entity, second, value)
+		end)
+	end
+
+	local r = jecs.record(world, second) :: jecs.Record
+	local archetype = r.archetype
+	if not archetype then
+		return
+	end
+	local types = archetype.types
+
+	for _, id in types do
+		if jecs.is_tag(world, id) then
+			world:add(component, id)
+		else
+			local metadata = world:get(second, id)
+			if not world:has(component, id) then
+				world:set(component, id, metadata)
+			end
+		end
+	end
+
+	-- jecs.bulk_insert(world, component, ids, values)
+end)
+
+local Witch = world:entity()
+local Werewolf = world:entity()
+
+local WereWitch = world:entity()
+world:add(WereWitch, pair(IsA, Witch))
+world:add(WereWitch, pair(IsA, Werewolf))
+
+local e = world:entity()
+world:add(e, WereWitch)
+print(world:has(e, pair(IsA, Witch))) -- false
+print(world:has(e, Witch)) -- true