refactor: embrace scope cleanup, restructure code

pkgs/charm-sync/__benchmarks__/patch.bench.luau

@@ -0,0 +1,54 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+
+local patch = CharmSync.patch
+
+local USERS = 1000
+
+local function generate()
+	local users = {}
+
+	for user = 1, USERS do
+		users[user] = {
+			coins = math.random(1, 2),
+			gems = math.random(1, 2),
+		}
+	end
+
+	return { users = users }
+end
+
+local function update(state)
+	state = table.clone(state)
+	state.users = table.clone(state.users)
+
+	for user = 1, USERS do
+		local rng = math.random()
+
+		if rng < 1 / 3 then
+			continue
+		elseif rng < 2 / 3 then
+			state.users[user] = nil
+		else
+			state.users[user] = {
+				coins = math.random(1, 2),
+				gems = math.random(1, 2),
+			}
+		end
+	end
+
+	return state
+end
+
+return {
+	ParameterGenerator = function()
+		local state = generate()
+		return state, update(state)
+	end,
+	Functions = {
+		diff = function(_, a, b)
+			patch.diff(a, b)
+		end,
+	},
+}

pkgs/charm-sync/__tests__/client.spec.luau

@@ -0,0 +1,125 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+local suite = require(ServerScriptService.Tests.suite)
+
+local atom = Charm.atom
+local subscribe = Charm.subscribe
+local client = CharmSync.client
+local test = suite.test
+
+test("should sync state payload", function()
+	local a = atom(1)
+	local client = client({ atoms = { a = a } })
+
+	client:sync({
+		type = "init",
+		data = { a = 2 },
+	})
+
+	assert(a() == 2)
+end)
+
+test("should sync state patch", function()
+	local a = atom({ b = 1, c = 2 })
+	local previous = a()
+	local client = client({ atoms = { a = a } })
+
+	client:sync({
+		type = "init",
+		data = { a = a() },
+	})
+
+	assert(a() == previous)
+
+	client:sync({
+		type = "patch",
+		data = { a = { b = 3 } },
+	})
+
+	assert(a() ~= previous)
+	assert(a().b == 3)
+	assert(a().c == 2)
+end)
+
+test("should fix sparse array patches", function()
+	local a = atom({ 1 })
+	local client = client({ atoms = { a = a } })
+
+	client:sync({
+		type = "init",
+		data = { a = a() },
+	})
+
+	-- This patch would be the result of a diff between { 1 } and { 1, 2 } that
+	-- was JSON-serialized and deserialized. The client should be able to catch
+	-- this and convert the string key back to a number.
+	client:sync({
+		type = "patch",
+		data = { a = { ["2"] = 2 } },
+	})
+
+	assert(a()["2" :: any] == nil)
+	assert(a()[2] == 2)
+end)
+
+test("should apply multiple patches", function()
+	local a = atom(1)
+	local client = client({ atoms = { a = a } })
+	local calls = {}
+
+	subscribe(a, function(value)
+		table.insert(calls, value)
+	end)
+
+	client:sync({
+		type = "init",
+		data = { a = a() },
+	})
+
+	client:sync({
+		type = "patch",
+		data = { a = 2 },
+	}, {
+		type = "patch",
+		data = { a = 3 },
+	})
+
+	assert(a() == 3)
+	assert(calls[1] == 2)
+	assert(calls[2] == 3)
+end)
+
+test("should batch patches", function()
+	local a = atom(0)
+	local b = atom(0)
+	local client = client({ atoms = { a = a, b = b } })
+	local states = {}
+
+	subscribe(a, function()
+		states.b = b()
+	end)
+
+	subscribe(b, function()
+		states.a = a()
+	end)
+
+	client:sync({
+		type = "init",
+		data = { a = a(), b = b() },
+	})
+
+	client:sync({
+		type = "patch",
+		data = { a = 1, b = 2 },
+	})
+
+	assert(a() == 1)
+	assert(b() == 2)
+	assert(states.a == 1)
+	assert(states.b == 2)
+end)
+
+return {}

pkgs/charm-sync/__tests__/flatten.spec.luau

@@ -0,0 +1,28 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+local suite = require(ServerScriptService.Tests.suite)
+
+local atom = Charm.atom
+local flatten = CharmSync.flatten
+local test = suite.test
+
+test("should flatten atoms", function()
+	local input = {
+		a = atom(0),
+		b = { c = atom(0) },
+		d = { e = { f = atom(0) } },
+	}
+	local result = flatten(input)
+
+	assert(result.a == input.a)
+	assert(result["b/c"] == input.b.c)
+	assert(result["d/e/f"] == input.d.e.f)
+
+	assert(result.b == nil)
+	assert(result.d == nil)
+end)
+
+return {}

pkgs/charm-sync/__tests__/patch.spec.luau

@@ -0,0 +1,129 @@
+local HttpService = game:GetService("HttpService")
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+local suite = require("../suite")
+
+local patch = CharmSync.patch
+local test = suite.test
+
+local function serdes(value: { [any]: any })
+	return HttpService:JSONDecode(HttpService:JSONEncode(value))
+end
+
+test("returns a patch", function()
+	local prevState = { foo = "bar", baz = "qux" }
+	local nextState = { foo = "bar", baz = "quux" }
+	local patches = patch.diff(prevState, nextState)
+
+	assert(patches ~= nil)
+	assert(patches.baz == "quux")
+	assert(patches.foo == nil)
+end)
+
+test("applies a patch", function()
+	local prevState = { foo = "bar", baz = "qux" }
+	local patches = { baz = "quux", qaz = "qux" }
+	local applied = patch.apply(prevState, patches)
+
+	assert(applied ~= nil)
+	assert(applied.baz == "quux")
+	assert(applied.foo == "bar")
+	assert(applied.qaz == "qux")
+end)
+
+test("allows nested patches", function()
+	local prevState = { foo = { bar = "baz" } }
+	local nextState = { foo = { bar = "qux" } }
+	local patches = patch.diff(prevState, nextState)
+	local applied = patch.apply(prevState, patches)
+
+	assert(applied.foo.bar == "qux")
+end)
+
+test("preserves array density when applying", function()
+	local prevState = { "a", "b", "c", "d" }
+	local nextState = { ["2"] = "B", ["4"] = "D" }
+	local applied = patch.apply(prevState, nextState)
+
+	assert(applied[1] == "a")
+	assert(applied[2] == "B")
+	assert(applied[3] == "c")
+	assert(applied[4] == "D")
+
+	assert(applied["2"] == nil)
+	assert(applied["4"] == nil)
+end)
+
+test("preserves array density after serdes", function()
+	-- This particular change results in a patch that cannot be serialized
+	-- without losing information. Charm resolves this issue internally.
+	local prevState = { "a", "b", "c", "d", "e" }
+	local nextState = { "A", "b", "c", "D" }
+
+	local patches = serdes(patch.diff(prevState, nextState))
+	local applied = patch.apply(prevState, patches)
+
+	for index in next, prevState do
+		assert(applied[index] == nextState[index])
+	end
+
+	for index in next, applied do
+		assert(applied[index] == nextState[index])
+	end
+end)
+
+test("preserves large gaps between keys", function()
+	local prevState = table.create(10000, "foo")
+	local nextState = table.move(prevState, 2, 9998, 2, { [1] = "bar", [9999] = "bar" })
+
+	local start = os.clock()
+
+	local patches = serdes(patch.diff(prevState, nextState))
+	local applied = patch.apply(prevState, patches)
+
+	assert(os.clock() - start < 10e-3, `Performance is not acceptable ({(os.clock() - start) * 1e3}ms)`)
+
+	for index in next, prevState do
+		assert(applied[index] == nextState[index])
+	end
+end)
+
+test("cannot preserve sparse arrays", function()
+	local prevState = {}
+	local nextState = { [2] = "foo" }
+
+	local patches = serdes(patch.diff(prevState, nextState))
+	local applied = patch.apply(prevState, patches)
+
+	-- Not enough information is available to determine whether 'patches'
+	-- should keep the string keys added during remote event serialization.
+	assert(applied["2"] ~= nil)
+	assert(applied[2] == nil)
+end)
+
+test("optionally fixes sparse arrays", function()
+	for index in next, patch.diff({ 1, 1, 1, 1, 1 }, { 2, 1, 1, 2 }, true) do
+		assert(type(index) == "string")
+	end
+
+	for index in next, patch.diff({ 1, 1, 1, 1, 1 }, { 2, 1, 1, 2 }, false) do
+		assert(type(index) == "number")
+	end
+end)
+
+test("optionally validates state", function()
+	local success = pcall(function()
+		patch.diff({}, { [0.1] = true }, true)
+	end)
+
+	assert(not success)
+
+	success = pcall(function()
+		patch.diff({}, { [0.1] = true }, false)
+	end)
+
+	assert(success)
+end)
+
+return {}

pkgs/charm-sync/__tests__/server.spec.luau

@@ -0,0 +1,102 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+local suite = require(ServerScriptService.Tests.suite)
+
+type SyncPayload = CharmSync.SyncPayload
+
+local atom = Charm.atom
+local server = CharmSync.server
+local patch = CharmSync.patch
+local test = suite.test
+
+local target: Player = {} :: any
+
+test("sends state to new players", function()
+	local a = atom(1)
+	local server = server({ atoms = { a = a }, interval = -1 })
+	local player: Player, payload: SyncPayload
+
+	local disconnect = server:connect(function(...)
+		player, payload = ...
+	end)
+
+	server:hydrate(target)
+
+	assert(player == target)
+	assert(payload.type == "init")
+	assert(payload.data.a == 1)
+
+	disconnect()
+end)
+
+test("sends state patches", function()
+	local a = atom({ b = 1, c = 2 } :: { [string]: number? })
+	local server = server({ atoms = { a = a }, interval = -1 })
+	local player: Player, payload: SyncPayload
+
+	local disconnect = server:connect(function(...)
+		player, payload = ...
+	end)
+
+	a({ b = 1, c = 5 })
+	server:_sendPatch(target)
+
+	assert(player == target)
+	assert(payload.type == "patch")
+	assert(payload.data.a.b == nil)
+	assert(payload.data.a.c == 5)
+
+	a({ b = 1, d = 1 })
+	server:_sendPatch(target)
+
+	assert(payload.data.a.b == nil)
+	assert(patch.isNone(payload.data.a.c) == true)
+	assert(payload.data.a.d == 1)
+
+	disconnect()
+end)
+
+test("preserves history", function()
+	local a = atom(1)
+	local b = atom(1)
+	local payloads: { SyncPayload }
+
+	local server = server({
+		atoms = { a = a, b = b },
+		interval = -1,
+		preserveHistory = true,
+	})
+
+	local disconnect = server:connect(function(_, ...)
+		payloads = { ... }
+	end)
+
+	a(2)
+	b(2)
+	server:_sendPatch(target)
+
+	assert(#payloads == 1)
+	assert(payloads[1].type == "patch")
+	assert(payloads[1].data.a == 2)
+	assert(payloads[1].data.b == 2)
+
+	a(3)
+	a(4)
+	b(3)
+	server:_sendPatch(target)
+
+	assert(#payloads == 2)
+	assert(payloads[1].type == "patch")
+	assert(payloads[1].data.a == 3)
+	assert(payloads[1].data.b == nil)
+	assert(payloads[2].type == "patch")
+	assert(payloads[2].data.a == 4)
+	assert(payloads[2].data.b == 3)
+
+	disconnect()
+end)
+
+return {}

pkgs/charm-sync/__tests__/spec.luau

@@ -0,0 +1,19 @@
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local suite = require(ServerScriptService.Tests.suite)
+
+local describe = suite.describe
+
+local function spec(name: string)
+	describe(name, function()
+		return require(`./{name}.spec`) :: any
+	end)
+end
+
+spec("client")
+spec("server")
+spec("flatten")
+spec("patch")
+spec("validate")
+
+return {}

pkgs/charm-sync/__tests__/validate.spec.luau

@@ -0,0 +1,38 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+local suite = require(ServerScriptService.Tests.suite)
+
+local atom = Charm.atom
+local validate = CharmSync.validate
+local test = suite.test
+
+test("validates inputs", function()
+	local cases: { [string]: { value: any, expected: any } } = {
+		array = { value = { 1, 2, 3 }, expected = true },
+		array_sparse = { value = { 1, nil :: any, 3 }, expected = true },
+		array_sparse_invalid = { value = { [1] = 0, [3] = 0 }, expected = false },
+
+		map_decimal_key = { value = { [1.5] = 0 }, expected = false },
+		map_infinity_key = { value = { [math.huge] = true }, expected = false },
+		map_mixed_key = { value = { [1] = 0, a = 0 }, expected = false },
+
+		nested_atom = { value = atom(0), expected = false },
+	}
+
+	for key, case in next, cases do
+		local success, result = pcall(function()
+			validate(case.value, key)
+		end)
+
+		if case.expected and not success then
+			error(`Expected {key} to be valid, threw {result}`)
+		elseif not case.expected and success then
+			error(`Expected {key} to be invalid, but it was valid`)
+		end
+	end
+end)
+
+return {}

pkgs/charm-sync/default.project.json

@@ -0,0 +1,6 @@
+{
+	"name": "CharmSync",
+	"tree": {
+		"$path": "src"
+	}
+}

pkgs/charm-sync/src/client.luau

@@ -0,0 +1,84 @@
+local Charm = require(script.Parent.Parent.Charm)
+local patch = require(script.Parent.patch)
+local types = require(script.Parent.types)
+
+type AtomMap = types.AtomMap
+type SyncPayload = types.SyncPayload
+
+type ClientOptions = {
+	--[=[
+		The atoms to synchronize with the server.
+	]=]
+	atoms: AtomMap,
+	--[=[
+		Whether to ignore patches sent before the client has been hydrated.
+		Default is `true`.
+	]=]
+	ignoreUnhydrated: boolean?,
+}
+
+type ClientSyncer = {
+	--[=[
+		Applies a patch or initializes the state of the atoms with the given
+		payload from the server.
+		
+		@param ...payloads The patches or hydration payloads to apply.
+	]=]
+	sync: (self: ClientSyncer, ...SyncPayload) -> (),
+}
+
+--[=[
+	Creates a `ClientSyncer` object that receives patches from the server and
+	applies them to the local state.
+	
+	@param options The atoms to synchronize with the server.
+	@return A `ClientSyncer` object.
+]=]
+local function client(options: ClientOptions): ClientSyncer
+	local atoms = options.atoms
+	local ignoreUnhydrated = options.ignoreUnhydrated ~= false
+
+	local self = {} :: ClientSyncer
+	local hydrated = false
+
+	local function hydrate(state: { [string | number]: any })
+		hydrated = true
+
+		for key, value in next, state do
+			local atom = atoms[key]
+			atom(value)
+		end
+	end
+
+	local function apply(data: { [string | number]: any })
+		local target = {}
+
+		for key, atom in next, atoms do
+			target[key] = atom()
+		end
+
+		target = patch.apply(target, data)
+
+		for key, atom in next, atoms do
+			atom(target[key])
+		end
+	end
+
+	function self:sync(...)
+		for index = 1, select("#", ...) do
+			local payload: SyncPayload = select(index, ...)
+
+			Charm.batched(function()
+				if payload.type == "init" then
+					hydrate(payload.data)
+				elseif not ignoreUnhydrated or hydrated then
+					apply(payload.data)
+				end
+			end)
+		end
+	end
+
+	return self
+end
+
+return client

pkgs/charm-sync/src/flatten.luau

@@ -0,0 +1,29 @@
+local types = require(script.Parent.types)
+
+type AtomMap = types.AtomMap
+
+type NestedAtomMap = {
+	[string]: NestedAtomMap | () -> (),
+}
+
+local function flatten(atoms: NestedAtomMap): AtomMap
+	local result: AtomMap = {}
+
+	local function visit(node: NestedAtomMap, path: string)
+		for key, value in node do
+			local location = if path == "" then key else path .. "/" .. key
+
+			if type(value) == "table" then
+				visit(value, location)
+			else
+				result[location] = value
+			end
+		end
+	end
+
+	visit(atoms, "")
+
+	return result
+end
+
+return flatten

pkgs/charm-sync/src/init.luau

@@ -0,0 +1,21 @@
+local client = require(script.client)
+local flatten = require(script.flatten)
+local patch = require(script.patch)
+local server = require(script.server)
+local types = require(script.types)
+local validate = require(script.validate)
+
+export type SyncPayload = types.SyncPayload
+
+--[=[
+	Synchronizes state between the client and server. The server sends patches
+	to the client, which applies them to its local state.
+]=]
+return {
+	client = client,
+	server = server,
+	flatten = flatten,
+	validate = validate,
+	patch = patch,
+	isNone = patch.isNone,
+}

pkgs/charm-sync/src/interval.luau

@@ -0,0 +1,30 @@
+local RunService = game:GetService("RunService")
+
+--[=[
+	Schedules a function to be called at a fixed interval of `seconds`.
+	
+	@param callback The function to call.
+	@param seconds The interval in seconds.
+	@return A function that cancels the interval.
+]=]
+local function interval(callback: () -> (), seconds: number): () -> ()
+	if seconds < 0 then
+		return function() end
+	end
+
+	local clock = 0
+
+	local connection = RunService.Heartbeat:Connect(function(delta)
+		clock += delta
+		if clock >= seconds then
+			clock = 0
+			callback()
+		end
+	end)
+
+	return function()
+		connection:Disconnect()
+	end
+end
+
+return interval

pkgs/charm-sync/src/patch.luau

@@ -0,0 +1,117 @@
+local validate = require(script.Parent.validate)
+
+local __DEV__ = _G.__DEV__
+
+--[=[
+	A special symbol that denotes the absence of a value. Used to represent
+	deleted values in patches.
+]=]
+local None = { __none = "__none" }
+
+local function isNone(value: any): boolean
+	return type(value) == "table" and value.__none == "__none"
+end
+
+--[=[
+	JSON serialization can drop the last values of a sparse array. For example,
+	`{ [1] = 1, [3] = 3 }` becomes `[1]` in some cases. Ambiguity in the
+	serialization of sparse arrays can be resolved by converting them to
+	dictionaries with string keys.
+
+	The `apply` function will convert these dictionaries back into arrays as
+	long as the target array is not sparse. This function ensures that the diff
+	function produces a patch that can be correctly applied.
+]=]
+local function stringifySparseArray(object: { any }): { [string | number]: any }
+	local maxn = table.maxn(object)
+
+	if maxn == 0 or maxn == #object then
+		-- The object is either not an array or a dense array
+		return object
+	end
+
+	local dictionary = {}
+
+	for index, value in next, object do
+		dictionary[tostring(index)] = value
+	end
+
+	return dictionary
+end
+
+--[=[
+	Compares two states and returns a patch that can be applied to the previous
+	state to produce the next state.
+
+	@param prevState The previous state.
+	@param nextState The next state.
+	@param serialize Whether to convert sparse arrays into dictionaries. Defaults to `true`.
+	@returns A patch that can be applied to the previous state.
+]=]
+local function diff(prevState: { [any]: any }, nextState: { [any]: any }, serialize: boolean?)
+	serialize = serialize ~= false
+
+	local patches = table.clone(nextState)
+
+	for key, previous in next, prevState do
+		local current = nextState[key]
+
+		if previous == current then
+			patches[key] = nil
+		elseif current == nil then
+			patches[key] = None
+		elseif type(previous) == "table" and type(current) == "table" then
+			patches[key] = diff(previous, current, serialize)
+		end
+	end
+
+	-- Coerce sparse array patches into dictionaries
+	if serialize and (prevState[1] ~= nil or nextState[1] ~= nil) then
+		patches = stringifySparseArray(patches)
+	end
+
+	if serialize and __DEV__ then
+		for key, value in next, patches do
+			validate(value, key)
+		end
+	end
+
+	return patches
+end
+
+--[=[
+	Applies a patch to a state and returns the next state.
+
+	@param state The current state.
+	@param patches The patches to apply.
+	@returns The next state.
+]=]
+local function apply(state: any, patches: any): any
+	if type(patches) == "table" and patches.__none == "__none" then
+		return nil
+	elseif type(state) ~= "table" or type(patches) ~= "table" then
+		return patches
+	end
+
+	local nextState = table.clone(state)
+	local stateIsArray = state[1] ~= nil
+
+	for key, patch in next, patches do
+		-- Diff-checking an array produces a sparse array, which will not be
+		-- preserved when converted to JSON. To prevent this, we turn string
+		-- keys back into numeric keys.
+		if stateIsArray and type(key) == "string" then
+			key = tonumber(key) or key
+		end
+
+		nextState[key] = apply(nextState[key], patch)
+	end
+
+	return nextState
+end
+
+return {
+	isNone = isNone,
+	diff = diff,
+	apply = apply,
+}

pkgs/charm-sync/src/server.luau

@@ -0,0 +1,268 @@
+local Players = game:GetService("Players")
+
+local Charm = require(script.Parent.Parent.Charm)
+local interval = require(script.Parent.interval)
+local patch = require(script.Parent.patch)
+local types = require(script.Parent.types)
+
+type AtomMap = types.AtomMap
+type SyncPayload = types.SyncPayload
+
+type ServerOptions = {
+	--[=[
+		The atoms to synchronize with the client.
+	]=]
+	atoms: AtomMap,
+	--[=[
+		The interval at which to send patches to the client, in seconds.
+		Defaults to `0` (patches are sent up to once per frame). Set to a
+		negative value to disable automatic syncing.
+	]=]
+	interval: number?,
+	--[=[
+		Whether the history of state changes since the client's last update
+		should be preserved. This is useful for values that change multiple times
+		per frame, where each individual change is important. Defaults to `false`.
+	]=]
+	preserveHistory: boolean?,
+	--[=[
+		When `true`, Charm will apply validation and serialize unsafe arrays
+		to address remote event argument limitations. Defaults to `true`.
+
+		This option should be disabled if your network library uses a custom
+		serialization method (i.e. Zap, ByteNet) to prevent interference.
+	]=]
+	autoSerialize: boolean?,
+}
+
+type ServerSyncer = {
+	--[=[
+		Sets up a subscription to each atom that schedules a patch to be sent to
+		the client whenever the state changes. When a change occurs, the `callback`
+		is called with the player and the payload to send.
+
+		Note that the `payload` object should not be mutated. If you need to
+		modify the payload, apply the changes to a copy of the object.
+
+		@param callback The function to call when the state changes.
+		@return A cleanup function that unsubscribes all listeners.
+	]=]
+	connect: (self: ServerSyncer, callback: (player: Player, ...SyncPayload) -> ()) -> () -> (),
+	--[=[
+		Hydrates the client's state with the server's state. This should be
+		called when a player joins the game and requires the server's state.
+
+		@param player The player to hydrate.
+	]=]
+	hydrate: (self: ServerSyncer, player: Player) -> (),
+	--[=[
+		@deprecated For internal use only.
+	]=]
+	_sendPatch: (self: ServerSyncer, player: Player) -> (),
+}
+
+--[=[
+	Creates a `ServerSyncer` object that sends patches to the client and
+	hydrates the client's state.
+	
+	@param options The atoms to synchronize with the client.
+	@return A `ServerSyncer` object.
+]=]
+local function server(options: ServerOptions): ServerSyncer
+	local atoms = options.atoms
+	local autoSerialize = options.autoSerialize ~= false
+	local preserveHistory = options.preserveHistory
+	local syncRate = options.interval or 0
+
+	local self = {} :: ServerSyncer
+	local sync: (player: Player, payload: SyncPayload) -> ()
+
+	local function createSnapshot()
+		local snapshot = {}
+
+		for key, atom in next, atoms do
+			snapshot[key] = atom()
+		end
+
+		return snapshot
+	end
+
+	-- Send the initial state to a player when they join the server.
+	function self:hydrate(player)
+		assert(sync, "connect() must be called before hydrate()")
+
+		sync(player, {
+			type = "init",
+			data = createSnapshot(),
+		})
+	end
+
+	-- If the history of state changes should be preserved, keep a list of
+	-- previous snapshots and diff-check each one from the last.
+	if preserveHistory then
+		local snapshots = { createSnapshot() }
+		local changed = false
+
+		function self:connect(callback)
+			local subscriptions = {}
+
+			sync = callback
+
+			local function pushSnapshot(key: string | number, current: any, previous: any)
+				local lastSnapshot = snapshots[#snapshots]
+				local previousSnapshot = snapshots[#snapshots - 1]
+
+				-- Optimize snapshots by updating the most recent snapshot if the
+				-- previous and current values are the same, since this allows us
+				-- to group multiple changes into a single snapshot.
+				if previousSnapshot and previousSnapshot[key] == previous and lastSnapshot[key] == previous then
+					lastSnapshot[key] = current
+				else
+					local nextSnapshot = table.clone(lastSnapshot)
+					nextSnapshot[key] = current
+					table.insert(snapshots, nextSnapshot)
+				end
+			end
+
+			-- Populate the snapshot with the initial state of each atom.
+			-- Subscribe to each atom and update the state when it changes.
+			for key, atom in next, atoms do
+				subscriptions[key] = Charm.subscribe(atom, function(current, previous)
+					pushSnapshot(key, current, previous)
+					changed = true
+				end)
+			end
+
+			local disconnect = interval(function()
+				if not changed then
+					return
+				end
+
+				local payloads: { SyncPayload } = {}
+				local lastSnapshot
+
+				for index, snapshot in next, snapshots do
+					lastSnapshot = snapshot
+
+					if index == 1 then
+						continue
+					end
+
+					table.insert(payloads, {
+						type = "patch",
+						data = patch.diff(snapshots[index - 1], snapshot, autoSerialize),
+					})
+				end
+
+				snapshots = { lastSnapshot }
+				changed = false
+
+				for _, player in next, Players:GetPlayers() do
+					callback(player, unpack(payloads))
+				end
+			end, syncRate)
+
+			return function()
+				disconnect()
+
+				for _, unsubscribe in next, subscriptions do
+					unsubscribe()
+				end
+			end
+		end
+
+		function self:_sendPatch(player: Player)
+			assert(sync, "connect() must be called before _sendPatch()")
+
+			if not changed then
+				return
+			end
+
+			local payloads: { SyncPayload } = {}
+
+			for index, snapshot in next, snapshots do
+				if index == 1 then
+					continue
+				end
+
+				table.insert(payloads, {
+					type = "patch",
+					data = patch.diff(snapshots[index - 1], snapshot, autoSerialize),
+				})
+			end
+
+			snapshots = { snapshots[#snapshots] }
+			changed = false
+
+			sync(player, unpack(payloads))
+		end
+
+		return self
+	end
+
+	local previousSnapshot = createSnapshot()
+	local currentSnapshot = table.clone(previousSnapshot)
+	local changed = false
+
+	function self:connect(callback)
+		local subscriptions = {}
+
+		sync = callback
+
+		-- Subscribe to each atom and update the state when one changes.
+		for key, atom in next, atoms do
+			subscriptions[key] = Charm.subscribe(atom, function(state)
+				currentSnapshot[key] = state
+				changed = true
+			end)
+		end
+
+		local disconnect = interval(function()
+			if not changed then
+				return
+			end
+
+			local payload: SyncPayload = {
+				type = "patch",
+				data = patch.diff(previousSnapshot, currentSnapshot, autoSerialize),
+			}
+
+			previousSnapshot = table.clone(currentSnapshot)
+			changed = false
+
+			for _, player in next, Players:GetPlayers() do
+				callback(player, payload)
+			end
+		end, syncRate)
+
+		return function()
+			disconnect()
+
+			for _, unsubscribe in next, subscriptions do
+				unsubscribe()
+			end
+		end
+	end
+
+	function self:_sendPatch(player)
+		assert(sync, "connect() must be called before _sendPatch()")
+
+		if not changed then
+			return
+		end
+
+		local payload: SyncPayload = {
+			type = "patch",
+			data = patch.diff(previousSnapshot, currentSnapshot, autoSerialize),
+		}
+
+		previousSnapshot = table.clone(currentSnapshot)
+		changed = false
+
+		sync(player, payload)
+	end
+
+	return self
+end
+
+return server

pkgs/charm-sync/src/types.luau

@@ -0,0 +1,21 @@
+local Charm = require(script.Parent.Parent.Charm)
+
+type Atom<T> = Charm.Atom<T>
+
+--[=[
+	A payload that can be sent from the server to the client to synchronize
+	state between the two.
+]=]
+export type SyncPayload = {
+	type: "init",
+	data: { [string | number]: any },
+} | {
+	type: "patch",
+	data: { [string | number]: any },
+}
+
+export type AtomMap = {
+	[string | number]: Atom<any>,
+}
+
+return nil

pkgs/charm-sync/src/validate.luau

@@ -0,0 +1,68 @@
+local SAFE_KEYS = { string = true, number = true }
+local UNSAFE_VALUES = { ["function"] = true, thread = true }
+
+local function isTableUnsafe(object: { [any]: any })
+	local keyType = nil
+	local objectSize = 0
+
+	-- All keys must have the same type
+	for key in next, object do
+		local currentType = type(key)
+
+		if not keyType and SAFE_KEYS[currentType] then
+			keyType = currentType
+		elseif keyType ~= currentType then
+			return true
+		end
+
+		objectSize += 1
+	end
+
+	-- If there are more keys than the length of the array, it's an array with
+	-- non-sequential keys.
+	if objectSize > #object and keyType == "number" then
+		return true
+	end
+
+	return false
+end
+
+--[=[
+	Validates a value to ensure it can be synced over a remote event.
+
+	@param value The value to validate.
+	@param key The key of the value in the table.
+	@error Throws an error if the value cannot be synced.
+]=]
+local function validate(value: any, key: any)
+	local typeOfKey = type(key)
+	local typeOfValue = type(value)
+
+	if not SAFE_KEYS[typeOfKey] then
+		error(`Invalid key type '{typeOfKey}' at key '{key}'`)
+	elseif UNSAFE_VALUES[typeOfValue] then
+		error(`Invalid value type '{typeOfValue}' at key '{key}'`)
+	elseif typeOfValue == "table" then
+		if getmetatable(value) ~= nil then
+			error(`Cannot sync tables with metatables! Got {value} at key '{key}'`)
+		elseif isTableUnsafe(value) then
+			error(
+				`Cannot sync tables unsupported by remote events! The value has the key '{key}'.\n\n`
+					.. "This can be for the following reasons:\n"
+					.. "1. The object is an array with non-sequential keys\n"
+					.. "2. The object is a dictionary with mixed key types (e.g. string and number)\n\n"
+					.. "Read more: https://create.roblox.com/docs/scripting/events/remote#argument-limitations"
+			)
+		end
+	end
+
+	if typeOfKey == "number" then
+		if key == math.huge or key == -math.huge then
+			error("Cannot sync infinity as key")
+		elseif key ~= math.floor(key) then
+			error("Cannot sync non-integer number as key")
+		end
+	end
+end
+
+return validate

pkgs/charm-sync/wally.toml

@@ -0,0 +1,9 @@
+[package]
+name = "littensy/charm-sync"
+version = "0.3.0"
+registry = "https://github.com/UpliftGames/wally-index"
+realm = "shared"
+exclude = ["Charm.luau"]
+
+[dependencies]
+Charm = "littensy/charm@^0.10"

pkgs/charm/__benchmarks__/effect.bench.luau

@@ -0,0 +1,39 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+
+local computed = Charm.computed
+local effect = Charm.effect
+local atom = Charm.atom
+
+return {
+	Functions = {
+		atom = function()
+			local state = atom(0)
+
+			effect(function()
+				state()
+			end)
+
+			for _ = 1, 500 do
+				state(math.random(1, 2))
+			end
+		end,
+
+		computed = function()
+			local state = atom(0)
+
+			local double = computed(function()
+				return 2 * state()
+			end)
+
+			effect(function()
+				double()
+			end)
+
+			for _ = 1, 500 do
+				state(math.random(1, 2))
+			end
+		end,
+	},
+}

pkgs/charm/__benchmarks__/propagate.bench.luau

@@ -0,0 +1,63 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+
+local atom = Charm.atom
+local computed = Charm.computed
+local effect = Charm.effect
+local listen = Charm.listen
+
+local function propagate(w: number, h: number)
+	local src = atom(1)
+	for _ = 1, w do
+		local last = src
+		for _ = 1, h do
+			local prev = last
+			last = computed(function()
+				return prev() + 1
+			end)
+		end
+		effect(function()
+			last()
+		end)
+	end
+	src(src() + 1)
+end
+
+local function propagateListen(w: number, h: number)
+	local src = atom(1)
+	for _ = 1, w do
+		local last = src
+		for _ = 1, h do
+			local prev = last
+			last = computed(function()
+				return prev() + 1
+			end)
+		end
+		listen(last, function() end)
+	end
+	src(src() + 1)
+end
+
+return {
+	Functions = {
+		["effect 1*100"] = function()
+			propagate(1, 100)
+		end,
+		["effect 100*1"] = function()
+			propagate(100, 1)
+		end,
+		["effect 10*10"] = function()
+			propagate(10, 10)
+		end,
+		["listen 1*100"] = function()
+			propagateListen(1, 100)
+		end,
+		["listen 100*1"] = function()
+			propagateListen(100, 1)
+		end,
+		["listen 10*10"] = function()
+			propagateListen(10, 10)
+		end,
+	},
+}

pkgs/charm/__tests__/signals/computed.spec.luau

@@ -0,0 +1,83 @@
+local signals = require("../../src/signals")
+local suite = require("@tests/suite")
+
+local computed = signals.computed
+local signal = signals.signal
+local test = suite.test
+
+test("should correctly propagate changes through computed signals", function()
+	local src, setSrc = signal(0)
+	local c1 = computed(function()
+		return src() % 2
+	end)
+	local c2 = computed(function()
+		return c1()
+	end)
+	local c3 = computed(function()
+		return c2()
+	end)
+
+	c3()
+	setSrc(1) -- c1 -> dirty, c2 -> toCheckDirty, c3 -> toCheckDirty
+	c2() -- c1 -> none, c2 -> none
+	setSrc(3) -- c1 -> dirty, c2 -> toCheckDirty
+
+	assert(c3() == 1, "bad final value")
+end)
+
+test("should propagate updated source value through chained computations", function()
+	local src, setSrc = signal(0)
+	local a = computed(function()
+		return src()
+	end)
+	local b = computed(function()
+		return a() % 2
+	end)
+	local c = computed(function()
+		return src()
+	end)
+	local d = computed(function()
+		return b() + c()
+	end)
+
+	assert(d() == 0, "bad initial value")
+	setSrc(2)
+	assert(d() == 2, "bad updated value")
+end)
+
+test("should handle flags are indirectly updated during checkDirty", function()
+	local a, setA = signal(false)
+	local b = computed(function()
+		return a()
+	end)
+	local c = computed(function()
+		b()
+		return 0
+	end)
+	local d = computed(function()
+		c()
+		return b()
+	end)
+
+	assert(d() == false, "bad initial value")
+	setA(true)
+	assert(d() == true, "bad updated value")
+end)
+
+test("should not update if the signal value is reverted", function()
+	local times = 0
+
+	local src, setSrc = signal(0)
+	local c1 = computed(function()
+		times += 1
+		return src()
+	end)
+	c1()
+	assert(times == 1, "computed did not run")
+	setSrc(1)
+	setSrc(0)
+	c1()
+	assert(times == 1, "computed ran when it should not")
+end)
+
+return {}

pkgs/charm/__tests__/signals/effect.spec.luau

@@ -0,0 +1,325 @@
+local signals = require("../../src/signals")
+local suite = require("@tests/suite")
+
+local computed = signals.computed
+local effect = signals.effect
+local effectScope = signals.effectScope
+local endBatch = signals.endBatch
+local pauseTracking = signals.pauseTracking
+local resumeTracking = signals.resumeTracking
+local onEffectCleanup = signals.onEffectCleanup
+local signal = signals.signal
+local startBatch = signals.startBatch
+local test = suite.test
+
+test("should clear subscriptions when untracked by all subscribers", function()
+	local bRunTimes = 0
+
+	local a, setA = signal(1)
+	local b = computed(function()
+		bRunTimes += 1
+		return a() * 2
+	end)
+	local stopEffect = effect(function()
+		b()
+	end)
+
+	assert(bRunTimes == 1, "should run initially")
+	setA(2)
+	assert(bRunTimes == 2, "should run after change")
+	stopEffect()
+	setA(3)
+	assert(bRunTimes == 2, "should not run after stop")
+end)
+
+test("should not run untracked inner effect", function()
+	local a, setA = signal(3)
+	local b = computed(function()
+		return a() > 0
+	end)
+
+	effect(function()
+		if b() then
+			effect(function()
+				if a() == 0 then
+					error("bad")
+				end
+			end)
+		end
+	end)
+
+	setA(2)
+	setA(1)
+	setA(0)
+end)
+
+test("should run outer effect first", function()
+	local a, setA = signal(1)
+	local b, setB = signal(1)
+
+	effect(function()
+		if a() > 0 then
+			effect(function()
+				b()
+				if a() == 0 then
+					error("bad")
+				end
+			end)
+		end
+	end)
+
+	startBatch()
+	setB(0)
+	setA(0)
+	endBatch()
+end)
+
+test("should not trigger inner effect when resolve maybe dirty", function()
+	local a, setA = signal(0)
+	local b = computed(function()
+		return a() % 2
+	end)
+
+	local innerTriggerTimes = 0
+
+	effect(function()
+		effect(function()
+			b()
+			innerTriggerTimes += 1
+			if innerTriggerTimes >= 2 then
+				error("bad")
+			end
+		end)
+	end)
+
+	setA(2)
+end)
+
+test("should trigger inner effects in sequence", function()
+	local a, setA = signal(0)
+	local b, setB = signal(0)
+	local c = computed(function()
+		return a() - b()
+	end)
+	local order: { string } = {}
+
+	effect(function()
+		c()
+
+		effect(function()
+			table.insert(order, "first inner")
+			a()
+		end)
+
+		effect(function()
+			table.insert(order, "last inner")
+			a()
+			b()
+		end)
+	end)
+
+	table.clear(order)
+
+	startBatch()
+	setB(1)
+	setA(1)
+	endBatch()
+
+	assert(table.concat(order, ", ") == "first inner, last inner", `wrong order: {table.concat(order, ", ")}`)
+end)
+
+test("should trigger inner effects in sequence in effect scope", function()
+	local a, setA = signal(0)
+	local b, setB = signal(0)
+	local order: { string } = {}
+
+	effectScope(function()
+		effect(function()
+			table.insert(order, "first inner")
+			a()
+		end)
+
+		effect(function()
+			table.insert(order, "last inner")
+			a()
+			b()
+		end)
+	end)
+
+	table.clear(order)
+
+	startBatch()
+	setB(1)
+	setA(1)
+	endBatch()
+
+	assert(table.concat(order, ", ") == "first inner, last inner", `wrong order: {table.concat(order, ", ")}`)
+end)
+
+test("should custom effect support batch", function()
+	local function batchEffect(fn: () -> ())
+		return effect(function()
+			startBatch()
+			local success, result: unknown = pcall(fn)
+			endBatch()
+			if not success then
+				error(result)
+			end
+		end)
+	end
+
+	local logs: { string } = {}
+	local a = signal(0)
+	local b, setB = signal(0)
+
+	local aa = computed(function()
+		table.insert(logs, "aa-0")
+		if a() == 0 then
+			setB(1)
+		end
+		table.insert(logs, "aa-1")
+		return nil
+	end)
+
+	local bb = computed(function()
+		table.insert(logs, "bb")
+		return b()
+	end)
+
+	batchEffect(function()
+		bb()
+	end)
+	batchEffect(function()
+		aa()
+	end)
+
+	assert(table.concat(logs, ", ") == "bb, aa-0, aa-1, bb", `wrong order: {table.concat(logs, ", ")}`)
+end)
+
+test("should duplicate subscribers do not affect the notify order", function()
+	local src1, setSrc1 = signal(0)
+	local src2, setSrc2 = signal(0)
+	local order: { string } = {}
+
+	effect(function()
+		table.insert(order, "a")
+		pauseTracking()
+		local isOne = src2() == 1
+		resumeTracking()
+		if isOne then
+			src1()
+		end
+		src2()
+		src1()
+	end)
+	effect(function()
+		table.insert(order, "b")
+		src1()
+	end)
+	setSrc2(1) -- src1.subs: a -> b -> a
+
+	table.clear(order)
+	setSrc1(src1() + 1)
+
+	assert(table.concat(order) == "ab", `wrong order: {table.concat(order)}`)
+end)
+
+test("should handle side effect with inner effects", function()
+	local a, setA = signal(0)
+	local b, setB = signal(0)
+	local order: { string } = {}
+
+	effect(function()
+		effect(function()
+			a()
+			table.insert(order, "a")
+		end)
+		effect(function()
+			b()
+			table.insert(order, "b")
+		end)
+		assert(table.concat(order) == "ab", `wrong order: {table.concat(order)}`)
+
+		table.clear(order)
+		setB(1)
+		setA(1)
+		assert(table.concat(order) == "ba", `wrong order: {table.concat(order)}`)
+	end)
+end)
+
+test("should handle flags are indirectly updated during checkDirty", function()
+	local a, setA = signal(false)
+	local b = computed(function()
+		return a()
+	end)
+	local c = computed(function()
+		b()
+		return 0
+	end)
+	local d = computed(function()
+		c()
+		return b()
+	end)
+
+	local triggers = 0
+
+	effect(function()
+		d()
+		triggers += 1
+	end)
+	assert(triggers == 1, "did not run initially")
+	setA(true)
+	assert(triggers == 2, "did not update indirectly")
+end)
+
+test("should run cleanups in sequence", function()
+	local order = {}
+
+	local a, setA = signal(false)
+	local stopEffect = effect(function()
+		a()
+		onEffectCleanup(function()
+			table.insert(order, "a")
+		end)
+		onEffectCleanup(function()
+			table.insert(order, "b")
+		end)
+	end)
+
+	assert(table.concat(order) == "", "should not cleanup immediately")
+
+	setA(true)
+	assert(table.concat(order) == "ab", `bad update: {table.concat(order)}`)
+
+	table.clear(order)
+	stopEffect()
+	assert(table.concat(order) == "ab", `bad dispose: {table.concat(order)}`)
+
+	table.clear(order)
+	setA(false)
+	assert(table.concat(order) == "", "should not cleanup after update")
+end)
+
+test("should run inner effect cleanup", function()
+	local cleanups = 0
+
+	local a, setA = signal(false)
+	local stopEffect = effect(function()
+		a()
+		effect(function()
+			onEffectCleanup(function()
+				cleanups += 1
+			end)
+		end)
+	end)
+
+	assert(cleanups == 0, "should not cleanup immediately")
+	setA(true)
+	assert(cleanups == 1, "should cleanup after outer update")
+	stopEffect()
+	assert(cleanups == 2, "should cleanup after outer dispose")
+	setA(false)
+	assert(cleanups == 2, "should not cleanup after update")
+end)
+
+return {}

pkgs/charm/__tests__/signals/effectScope.spec.luau

@@ -0,0 +1,75 @@
+local signals = require("../../src/signals")
+local suite = require("@tests/suite")
+
+local effect = signals.effect
+local effectScope = signals.effectScope
+local signal = signals.signal
+local onEffectCleanup = signals.onEffectCleanup
+local onScopeDispose = signals.onScopeDispose
+local test = suite.test
+
+test("should not trigger after stop", function()
+	local count, setCount = signal(1)
+
+	local triggers = 0
+
+	local stopScope = effectScope(function()
+		effect(function()
+			triggers += 1
+			count()
+		end)
+		assert(triggers == 1, "did not run initially")
+
+		setCount(2)
+		assert(triggers == 2, "did not run after update")
+	end)
+
+	setCount(3)
+	assert(triggers == 3, "did not run after update outside scope")
+	stopScope()
+	setCount(4)
+	assert(triggers == 3, "ran after stop")
+end)
+
+test("should run indirect scope cleanup", function()
+	local cleanups = 0
+
+	local cleanup
+	local stopScope = effectScope(function()
+		cleanup = effectScope(function()
+			onScopeDispose(function()
+				cleanups += 1
+			end)
+		end)
+	end)
+
+	assert(cleanups == 0, "should not cleanup immediately")
+	stopScope()
+	assert(cleanups == 1, "should cleanup from parent dispose")
+	cleanup()
+	assert(cleanups == 1, "should skip redundant dispose")
+end)
+
+test("should run indirect effect cleanup", function()
+	local cleanups = 0
+
+	local a, setA = signal(false)
+	local stopScope = effectScope(function()
+		effect(function()
+			a()
+			onEffectCleanup(function()
+				cleanups += 1
+			end)
+		end)
+	end)
+
+	assert(cleanups == 0, "should not cleanup immediately")
+	setA(true)
+	assert(cleanups == 1, "should cleanup once")
+	stopScope()
+	assert(cleanups == 2, "should cleanup after outer dispose")
+	setA(false)
+	assert(cleanups == 2, "should not cleanup after update")
+end)
+
+return {}

pkgs/charm/__tests__/signals/issue_48.spec.luau

@@ -0,0 +1,114 @@
+local signals = require("../../src/signals")
+local suite = require("@tests/suite")
+
+local computed = signals.computed
+local effect = signals.effect
+local pauseTracking = signals.pauseTracking
+local resumeTracking = signals.resumeTracking
+local signal = signals.signal
+local test = suite.test
+
+type ReactionOptions<T = unknown> = {
+	fireImmediately: boolean?,
+	equals: ((a: T, b: T?) -> boolean)?,
+	onError: (error: unknown) -> ()?,
+	scheduler: (fn: () -> ()) -> ()?,
+	once: boolean?,
+}
+
+local function untracked<T>(callback: () -> T): T
+	pauseTracking()
+	local success, result = pcall(callback)
+	resumeTracking()
+	if not success then
+		error(result)
+	end
+	return result
+end
+
+local function reaction<T>(
+	dataFn: () -> T,
+	effectFn: (newValue: T, oldValue: T?) -> (),
+	options: ReactionOptions<T>?
+): () -> ()
+	local options: ReactionOptions<T> = options or {}
+	local scheduler: (() -> ()) -> () = options.scheduler or function(fn)
+		return fn()
+	end
+	local equals: (T, T?) -> boolean = options.equals or function(a, b)
+		return a == b
+	end
+	local onError = options.onError
+	local once = options.once or false
+	local fireImmediately = options.fireImmediately or false
+
+	local prevValue: T?
+	local version = 0
+
+	local tracked = computed(function()
+		local success, result = pcall(dataFn)
+		if not success then
+			untracked(function()
+				if onError then
+					onError(result)
+				end
+				return nil
+			end)
+			return prevValue
+		end
+		return result
+	end)
+
+	local dispose
+	dispose = effect(function()
+		local current = tracked()
+		if not fireImmediately and version == 0 then
+			prevValue = current
+		end
+		version += 1
+		if equals(current :: T, prevValue) then
+			return
+		end
+		local oldValue = prevValue
+		prevValue = current
+		untracked(function()
+			scheduler(function()
+				local success = pcall(effectFn, current :: T, oldValue)
+				if not success and onError then
+					onError(error)
+				end
+				if once then
+					if (fireImmediately and version > 1) or (not fireImmediately and version > 0) then
+						dispose()
+					end
+				end
+			end)
+			return nil
+		end)
+	end)
+
+	return dispose
+end
+
+test("#48", function()
+	local source, setSource = signal(0)
+	local disposeInner: () -> ()
+
+	reaction(function()
+		return source()
+	end, function(val)
+		if val == 1 then
+			disposeInner = reaction(function()
+				return source()
+			end, function() end)
+		elseif val == 2 then
+			disposeInner()
+		end
+	end)
+
+	setSource(1)
+	setSource(2)
+	setSource(3)
+end)
+
+return {}

pkgs/charm/__tests__/signals/spec.luau

@@ -0,0 +1,18 @@
+local suite = require("@tests/suite")
+
+local describe = suite.describe
+
+local function spec(name: string)
+	describe(name, function()
+		return require(`./{name}.spec`) :: any
+	end)
+end
+
+spec("computed")
+spec("effect")
+spec("effectScope")
+spec("issue_48")
+spec("topology")
+spec("untrack")
+
+return {}

pkgs/charm/__tests__/signals/topology.spec.luau

@@ -0,0 +1,492 @@
+local signals = require("../../src/signals")
+local suite = require("@tests/suite")
+
+local computed = signals.computed
+local effect = signals.effect
+local signal = signals.signal
+local describe = suite.describe
+local test = suite.test
+
+--[[
+	Tests adopted with thanks from preact-signals implementation at
+	https://github.com/preactjs/signals/blob/main/packages/core/test/signal.test.tsx
+
+	The MIT License (MIT)
+
+	Copyright (c) 2022-present Preact Team
+
+	Permission is hereby granted, free of charge, to any person obtaining a copy
+	of this software and associated documentation files (the "Software"), to deal
+	in the Software without restriction, including without limitation the rights
+	to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+	copies of the Software, and to permit persons to whom the Software is
+	furnished to do so, subject to the following conditions:
+
+	The above copyright notice and this permission notice shall be included in all
+	copies or substantial portions of the Software.
+
+	THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+	IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+	FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+	AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+	LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+	OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+	SOFTWARE
+]]
+
+describe("graph updates", function()
+	test("should drop A->B->A updates", function()
+		--     A
+		--   / |
+		--  B  | <- Looks like a flag doesn't it? :D
+		--   \ |
+		--     C
+		--     |
+		--     D
+		local a, setA = signal(2)
+
+		local b = computed(function()
+			return a() - 1
+		end)
+		local c = computed(function()
+			return a() + b()
+		end)
+
+		local computeCalls = 0
+		local compute = function()
+			computeCalls += 1
+			return "d: " .. c()
+		end
+		local d = computed(compute)
+
+		-- Trigger read
+		assert(d() == "d: 3", "d should be 3")
+		assert(computeCalls == 1, "compute should be called once")
+		computeCalls = 0
+
+		setA(4)
+		d()
+		assert(computeCalls == 1, "compute should drop update")
+	end)
+
+	test("should only update every signal once (diamond graph)", function()
+		-- In this scenario "D" should only update once when "A" receives
+		-- an update. This is sometimes referred to as the "diamond" scenario.
+		--     A
+		--   /   \
+		--  B     C
+		--   \   /
+		--     D
+
+		local a, setA = signal("a")
+		local b = computed(function()
+			return a()
+		end)
+		local c = computed(function()
+			return a()
+		end)
+
+		local spyCalls = 0
+		local spy = function()
+			spyCalls += 1
+			return b() .. " " .. c()
+		end
+		local d = computed(spy)
+
+		assert(d() == "a a", "d should be 'a a'")
+		assert(spyCalls == 1, "compute should be called once")
+
+		setA("aa")
+		assert(d() == "aa aa", "d should be 'aa aa'")
+		assert(spyCalls == 2, "compute should be called only twice")
+	end)
+
+	test("should only update every signal once (diamond graph + tail)", function()
+		-- "E" will be likely updated twice if our mark+sweep logic is buggy.
+		--     A
+		--   /   \
+		--  B     C
+		--   \   /
+		--     D
+		--     |
+		--     E
+
+		local a, setA = signal("a")
+		local b = computed(function()
+			return a()
+		end)
+		local c = computed(function()
+			return a()
+		end)
+
+		local d = computed(function()
+			return b() .. " " .. c()
+		end)
+
+		local spyCalls = 0
+		local spy = function()
+			spyCalls += 1
+			return d()
+		end
+		local e = computed(spy)
+
+		assert(e() == "a a", "e should be 'a a'")
+		assert(spyCalls == 1, "compute should be called once")
+
+		setA("aa")
+		assert(e() == "aa aa", "e should be 'aa aa'")
+		assert(spyCalls == 2, "compute should be called only twice")
+	end)
+
+	test("should bail out if result is the same", function()
+		-- Bail out if value of "B" never changes
+		-- A->B->C
+		local a, setA = signal("a")
+		local b = computed(function()
+			a()
+			return "foo"
+		end)
+
+		local spyCalls = 0
+		local spy = function()
+			spyCalls += 1
+			return b()
+		end
+		local c = computed(spy)
+
+		assert(c() == "foo", "c should be 'foo'")
+		assert(spyCalls == 1, "compute should be called once")
+
+		setA("aa")
+		assert(c() == "foo", "c should be 'foo'")
+		assert(spyCalls == 1, "compute should bail out")
+	end)
+
+	test("should only update every signal once (jagged diamond graph + tails)", function()
+		-- "F" and "G" will be likely updated twice if our mark+sweep logic is buggy.
+		--     A
+		--   /   \
+		--  B     C
+		--  |     |
+		--  |     D
+		--   \   /
+		--     E
+		--   /   \
+		--  F     G
+		local a, setA = signal("a")
+
+		local b = computed(function()
+			return a()
+		end)
+		local c = computed(function()
+			return a()
+		end)
+
+		local d = computed(function()
+			return c()
+		end)
+
+		local eSpyCalls = 0
+		local eSpyTime = 0
+		local eSpy = function()
+			eSpyCalls += 1
+			eSpyTime = os.clock()
+			return b() .. " " .. d()
+		end
+		local e = computed(eSpy)
+
+		local fSpyCalls = 0
+		local fSpyTime = 0
+		local fSpy = function()
+			fSpyCalls += 1
+			fSpyTime = os.clock()
+			return e()
+		end
+		local f = computed(fSpy)
+		local gSpyCalls = 0
+		local gSpyTime = 0
+		local gSpy = function()
+			gSpyCalls += 1
+			gSpyTime = os.clock()
+			return e()
+		end
+		local g = computed(gSpy)
+
+		assert(f() == "a a", "f should be 'a a'")
+		assert(fSpyCalls == 1, "f should be called once")
+
+		assert(g() == "a a", "g should be 'a a'")
+		assert(gSpyCalls == 1, "g should be called once")
+
+		eSpyCalls, eSpyTime = 0, 0
+		fSpyCalls, fSpyTime = 0, 0
+		gSpyCalls, gSpyTime = 0, 0
+
+		setA("b")
+
+		assert(e() == "b b", "e should be 'b b'")
+		assert(eSpyCalls == 1, "e should run once after update")
+
+		assert(f() == "b b", "f should be 'b b'")
+		assert(fSpyCalls == 1, "f should run once after update")
+
+		assert(g() == "b b", "g should be 'b b'")
+		assert(gSpyCalls == 1, "g should run once after update")
+
+		eSpyCalls, eSpyTime = 0, 0
+		fSpyCalls, fSpyTime = 0, 0
+		gSpyCalls, gSpyTime = 0, 0
+
+		setA("c")
+
+		assert(e() == "c c", "e should be 'c c'")
+		assert(eSpyCalls == 1, "e should run once after update")
+
+		assert(f() == "c c", "f should be 'c c'")
+		assert(fSpyCalls == 1, "f should run once after update")
+
+		assert(g() == "c c", "g should be 'c c'")
+		assert(gSpyCalls == 1, "g should run once after update")
+
+		-- top to bottom
+		assert(eSpyTime < fSpyTime, "e should be called before f")
+		-- left to right
+		assert(eSpyTime < gSpyTime, "e should be called before g")
+	end)
+
+	test("should only subscribe to signals listened to", function()
+		--    *A
+		--   /   \
+		-- *B     C <- we don't listen to C
+		local a, setA = signal("a")
+
+		local b = computed(function()
+			return a()
+		end)
+		local spyCalls = 0
+		local spy = function()
+			spyCalls += 1
+			return a()
+		end
+		computed(spy)
+
+		assert(b() == "a", "b should be 'a'")
+		assert(spyCalls == 0, "compute should not run")
+
+		setA("aa")
+		assert(b() == "aa", "b should be 'aa'")
+		assert(spyCalls == 0, "compute should not run")
+	end)
+
+	test("should only subscribe to signals listened to II", function()
+		-- Here both "B" and "C" are active in the beginning, but
+		-- "B" becomes inactive later. At that point it should
+		-- not receive any updates anymore.
+		--    *A
+		--   /   \
+		-- *B     D <- we don't listen to C
+		--  |
+		-- *C
+		local a, setA = signal("a")
+		local spyBCalls = 0
+		local spyB = function()
+			spyBCalls += 1
+			return a()
+		end
+		local b = computed(spyB)
+
+		local spyCCalls = 0
+		local spyC = function()
+			spyCCalls += 1
+			return b()
+		end
+		local c = computed(spyC)
+
+		local d = computed(function()
+			return a()
+		end)
+
+		local result = ""
+		local unsub = effect(function()
+			result = c()
+		end)
+
+		assert(result == "a", "result should be 'a'")
+		assert(d() == "a", "d should be 'a'")
+
+		spyBCalls = 0
+		spyCCalls = 0
+		unsub()
+
+		setA("aa")
+
+		assert(spyBCalls == 0, "b should not run")
+		assert(spyCCalls == 0, "c should not run")
+		assert(d() == "aa", "d should be 'aa'")
+	end)
+
+	test("should ensure subs update even if one dep unmarks it", function()
+		-- In this scenario "C" always returns the same value. When "A"
+		-- changes, "B" will update, then "C" at which point its update
+		-- to "D" will be unmarked. But "D" must still update because
+		-- "B" marked it. If "D" isn't updated, then we have a bug.
+		--     A
+		--   /   \
+		--  B     *C <- returns same value every time
+		--   \   /
+		--     D
+		local a, setA = signal("a")
+		local b = computed(function()
+			return a()
+		end)
+		local c = computed(function()
+			a()
+			return "c"
+		end)
+		local spyResult: string?
+		local spy = function()
+			spyResult = b() .. " " .. c()
+			return spyResult
+		end
+		local d = computed(spy)
+
+		assert(d() == "a c", "d should be 'a c'")
+		spyResult = nil
+
+		setA("aa")
+		d()
+		assert(spyResult == "aa c", "d should have returned 'aa c'")
+	end)
+
+	test("should ensure subs update even if two deps unmark it", function()
+		-- In this scenario both "C" and "D" always return the same
+		-- value. But "E" must still update because "A" marked it.
+		-- If "E" isn't updated, then we have a bug.
+		--     A
+		--   / | \
+		--  B *C *D
+		--   \ | /
+		--     E
+		local a, setA = signal("a")
+		local b = computed(function()
+			return a()
+		end)
+		local c = computed(function()
+			a()
+			return "c"
+		end)
+		local d = computed(function()
+			a()
+			return "d"
+		end)
+		local spyResult: string?
+		local spy = function()
+			spyResult = b() .. " " .. c() .. " " .. d()
+			return spyResult
+		end
+		local e = computed(spy)
+
+		assert(e() == "a c d", "e should be 'a c d'")
+		spyResult = nil
+
+		setA("aa")
+		e()
+		assert(spyResult == "aa c d", "e should have returned 'aa c d'")
+	end)
+
+	test("should support lazy branches", function()
+		local a, setA = signal(0)
+		local b = computed(function()
+			return a()
+		end)
+		local c = computed(function()
+			return if a() > 0 then a() else b()
+		end)
+
+		assert(c() == 0, "c should be 0")
+		setA(1)
+		assert(c() == 1, "c should be 1")
+
+		setA(0)
+		assert(c() == 0, "c should be 0 again")
+	end)
+
+	test("should not update a sub if all deps unmark it", function()
+		-- In this scenario "B" and "C" always return the same value. When "A"
+		-- changes, "D" should not update.
+		--     A
+		--   /   \
+		-- *B     *C
+		--   \   /
+		--     D
+		local a, setA = signal("a")
+		local b = computed(function()
+			a()
+			return "b"
+		end)
+		local c = computed(function()
+			a()
+			return "c"
+		end)
+		local spyCalls = 0
+		local spy = function()
+			spyCalls += 1
+			return b() .. " " .. c()
+		end
+		local d = computed(spy)
+
+		assert(d() == "b c", "d should be 'b c'")
+		spyCalls = 0
+
+		setA("aa")
+		assert(spyCalls == 0, "d should not run")
+	end)
+end)
+
+describe("error handling", function()
+	test("should keep graph consistent on errors during activation", function()
+		local a, setA = signal(0)
+		local b = computed(function()
+			error("fail")
+		end)
+		local c = computed(function()
+			return a()
+		end)
+
+		local success, result = pcall(function()
+			b()
+		end)
+		assert(not success, "should throw")
+		assert(string.match(result, "fail"), "should throw fail")
+
+		setA(1)
+		assert(c() == 1, "c should recover and return 1")
+	end)
+
+	test("should keep graph consistent on errors in computeds", function()
+		local a, setA = signal(0)
+		local b = computed(function()
+			if a() == 1 then
+				error("fail")
+			end
+			return a()
+		end)
+		local c = computed(function()
+			return b()
+		end)
+
+		assert(c() == 0, "c should be 0")
+
+		setA(1)
+		local success, result = pcall(function()
+			b()
+		end)
+		assert(not success, "should throw")
+		assert(string.match(result, "fail"), "should throw fail")
+
+		setA(2)
+		assert(c() == 2, "c should recover and return 2")
+	end)
+end)
+
+return {}

pkgs/charm/__tests__/signals/untrack.spec.luau

@@ -0,0 +1,24 @@
+local signals = require("../../src/signals")
+local suite = require("@tests/suite")
+
+local computed = signals.computed
+local pauseTracking = signals.pauseTracking
+local resumeTracking = signals.resumeTracking
+local signal = signals.signal
+local test = suite.test
+
+test("should pause tracking", function()
+	local src, setSrc = signal(0)
+	local c = computed(function()
+		pauseTracking()
+		local value = src()
+		resumeTracking()
+		return value
+	end)
+	assert(c() == 0, "should not track src")
+
+	setSrc(1)
+	assert(c() == 0, "should not track src")
+end)
+
+return {}

pkgs/charm/__tests__/spec.luau

@@ -0,0 +1,20 @@
+local suite = require("@tests/suite")
+
+local describe = suite.describe
+
+local function collect(name: string)
+	describe(name, function()
+		return require(`./{name}/spec`) :: any
+	end)
+end
+
+local function spec(name: string)
+	describe(name, function()
+		return require(`./{name}.spec`) :: any
+	end)
+end
+
+collect("signals")
+spec("subscribe")
+
+return {}

pkgs/charm/__tests__/subscribe.spec.luau

@@ -0,0 +1,28 @@
+local atom = require("../src/atom")
+local subscribe = require("../src/subscribe")
+local suite = require("@tests/suite")
+
+local test = suite.test
+
+test("should subscribe to atom", function()
+	local source = atom(0)
+	local current, previous
+
+	local unsub = subscribe(source, function(a, b)
+		current = a
+		previous = b
+	end)
+
+	assert(not current and not previous, "should not run initially")
+	source(1)
+	assert(current == 1, "should run with new value")
+	assert(previous == 0, "should run with old value")
+	source(2)
+	assert(current == 2, "should run with new value 2")
+	assert(previous == 1, "should run with old value 2")
+	unsub()
+	source(3)
+	assert(current == 2, "should not run after unsubscribing")
+end)
+
+return {}

pkgs/charm/default.project.json

@@ -0,0 +1,6 @@
+{
+	"name": "Charm",
+	"tree": {
+		"$path": "src"
+	}
+}

pkgs/charm/src/atom.luau

@@ -0,0 +1,54 @@
+local signals = require("./signals")
+local types = require("./types")
+
+type Atom<T> = types.Atom<T>
+type AtomOptions<T> = types.AtomOptions<T>
+
+local signal = signals.signal
+
+local function atomGetterSetterEquals<T>(
+	source: () -> T,
+	setSource: (((T) -> T) | T) -> T,
+	equals: (T, T) -> boolean
+): Atom<T>
+	return function(...: ((T) -> T) | T): T
+		if select("#", ...) == 0 then
+			return source()
+		end
+
+		local nextState = (...)
+
+		if type(nextState) == "function" then
+			return setSource(function(currentState): T
+				local nextState = nextState(currentState)
+				return if equals(currentState, nextState) then currentState else nextState
+			end)
+		else
+			return setSource(function(currentState): T
+				return if equals(currentState, nextState) then currentState else nextState
+			end)
+		end
+	end
+end
+
+local function atomGetterSetter<T>(source: () -> T, setSource: (((T) -> T) | T) -> T): Atom<T>
+	return function(...: ((T) -> T) | T): T
+		if select("#", ...) == 0 then
+			return source()
+		else
+			return setSource((...))
+		end
+	end
+end
+
+local function atom<T>(state: T, equals: ((previous: T, current: T) -> boolean)?): Atom<T>
+	local source, setSource = signal(state)
+
+	if equals then
+		return atomGetterSetterEquals(source, setSource, equals)
+	else
+		return atomGetterSetter(source, setSource)
+	end
+end
+
+return atom

pkgs/charm/src/batched.luau

@@ -0,0 +1,16 @@
+local signals = require("./signals")
+
+local startBatch = signals.startBatch
+local endBatch = signals.endBatch
+
+local function batched(callback: () -> ())
+	startBatch()
+	local success, result: any = pcall(callback)
+	endBatch()
+
+	if not success then
+		error(result)
+	end
+end
+
+return batched

pkgs/charm/src/effect.luau

@@ -0,0 +1,27 @@
+local signals = require("./signals")
+
+local _effect = signals.effect
+local onEffectCleanup = signals.onEffectCleanup
+
+local function effect(callback: (dispose: () -> ()) -> ...() -> ()?): () -> ()
+	local disposeImmediately = false
+
+	local function dispose()
+		disposeImmediately = true
+	end
+
+	dispose = _effect(function()
+		local cleanup = callback(dispose)
+		if cleanup then
+			onEffectCleanup(cleanup)
+		end
+	end)
+
+	if disposeImmediately then
+		dispose()
+	end
+
+	return dispose
+end
+
+return effect

pkgs/charm/src/effectScope.luau

@@ -0,0 +1,15 @@
+local signals = require("./signals")
+
+local _effectScope = signals.effectScope
+local onScopeDispose = signals.onScopeDispose
+
+local function effectScope(callback: () -> ...() -> ()?): () -> ()
+	return _effectScope(function()
+		local cleanup = callback()
+		if cleanup then
+			onScopeDispose(cleanup)
+		end
+	end)
+end
+
+return effectScope

pkgs/charm/src/init.luau

@@ -0,0 +1,7 @@
+local lib = require(script.lib)
+
+export type Atom<T = any> = lib.Atom<T>
+export type AtomOptions<T = any> = lib.AtomOptions<T>
+export type Selector<T = any> = lib.Selector<T>
+
+return lib

pkgs/charm/src/lib.luau

@@ -0,0 +1,35 @@
+local atom = require("./atom")
+local batched = require("./batched")
+local effect = require("./effect")
+local listen = require("./listen")
+local mapped = require("./mapped")
+local observe = require("./observe")
+local signals = require("./signals")
+local subscribe = require("./subscribe")
+local types = require("./types")
+local untracked = require("./untracked")
+
+export type Atom<T = any> = types.Atom<T>
+export type AtomOptions<T = any> = types.AtomOptions<T>
+export type Selector<T = any> = types.Selector<T>
+
+local computed = signals.computed
+local effectScope = signals.effectScope
+local onEffectCleanup = signals.onEffectCleanup
+local onScopeDispose = signals.onScopeDispose
+
+return {
+	atom = atom,
+	batched = batched,
+	computed = computed,
+	effect = effect,
+	effectScope = effectScope,
+	listen = listen,
+	mapped = mapped,
+	observe = observe,
+	subscribe = subscribe,
+	untracked = untracked,
+	onEffectCleanup = onEffectCleanup,
+	onScopeDispose = onScopeDispose,
+	signals = signals,
+}

pkgs/charm/src/listen.luau

@@ -0,0 +1,36 @@
+local signals = require("./signals")
+local spawn = require("./spawn")
+
+local effect = signals.effect
+local pauseTracking = signals.pauseTracking
+local resumeTracking = signals.resumeTracking
+
+local function listen<T>(selector: () -> T, listener: (state: T, previous: T?, dispose: () -> ()) -> ()): () -> ()
+	local disposeImmediately = false
+	local current: T?
+
+	local function dispose()
+		disposeImmediately = true
+	end
+
+	dispose = effect(function()
+		local value = selector()
+
+		if value ~= current then
+			local previous = current
+			current = value
+
+			pauseTracking()
+			spawn(listener, value, previous, dispose)
+			resumeTracking()
+		end
+	end)
+
+	if disposeImmediately then
+		dispose()
+	end
+
+	return dispose
+end
+
+return listen

pkgs/charm/src/mapped.luau

@@ -0,0 +1,131 @@
+local signals = require("./signals")
+
+local computed = signals.computed
+local pauseTracking = signals.pauseTracking
+local resumeTracking = signals.resumeTracking
+
+type MappedKey<VI, KO, VO> = {
+	key: KO,
+	value: VO,
+}
+
+local function mapped<KI, VI, KO, VO>(selector: () -> { [KI]: VI }, transform: (item: VI, key: KI) -> (VO, KO?)): () -> { [KO]: VO }
+	local mappedKeys: { [KI]: MappedKey<VI, KO, VO> } = {}
+	local previousItems: { [KI]: VI }?
+
+	local function getTransform(value: VI, key: KI): (VO, KO?)
+		pauseTracking()
+		local success, valueOut, keyOut = pcall(transform, value, key)
+		resumeTracking()
+
+		if not success then
+			error(`mapped() transform function failed: {valueOut}`)
+		end
+
+		return valueOut, keyOut
+	end
+
+	return computed(function(previousMappedItems)
+		local currentItems = selector()
+
+		-- If this is the initial run, create a map of the items and store the
+		-- updated keys and values.
+		if not previousItems or not previousMappedItems then
+			local mappedItems: { [KO]: VO } = {}
+			previousItems = currentItems
+
+			for key, value in currentItems do
+				local valueOut, keyOut = getTransform(value, key)
+				local keyOut = if keyOut ~= nil then keyOut else key :: never
+
+				if valueOut ~= nil then
+					mappedKeys[key] = { key = keyOut, value = valueOut }
+					mappedItems[keyOut] = valueOut
+				end
+			end
+
+			return mappedItems
+		end
+
+		if previousItems == currentItems then
+			return previousMappedItems
+		end
+
+		local mappedItems = table.clone(previousMappedItems)
+
+		-- Update and remove mapped keys if the source values changed
+		for key, oldValue in previousItems do
+			local currentValue = currentItems[key]
+			local mappedKey = mappedKeys[key]
+
+			-- Remove the key if the source value changed from non-nil to nil
+			if currentValue == nil then
+				mappedKeys[key] = nil
+				if mappedKey then
+					mappedItems[mappedKey.key] = nil
+				end
+				continue
+			end
+
+			-- Do not call transform if the value has not changed
+			if currentValue == oldValue then
+				continue
+			end
+
+			local valueOut, keyOut = getTransform(currentValue, key)
+			local keyOut = if keyOut ~= nil then keyOut else key :: never
+
+			-- If there is no previous mappedKey, then this item was filtered
+			-- out to optimize memory. We need to check if the value is nil to
+			-- determine if we should add it back to the list.
+			if not mappedKey then
+				if valueOut ~= nil then
+					mappedKeys[key] = { key = keyOut, value = valueOut }
+					mappedItems[keyOut] = valueOut
+				end
+				continue
+			end
+
+			-- If the mapped value is nil, then delete mappedKey to save memory.
+			-- This is treated the same as setting mappedKey.value to nil.
+			if valueOut == nil then
+				mappedKeys[key] = nil
+				mappedItems[mappedKey.key] = nil
+				continue
+			end
+
+			-- If the key changed since the last transform call, remove
+			-- the old key and update mappedKey.
+			if keyOut ~= mappedKey.key then
+				mappedItems[mappedKey.key] = nil
+				mappedKey.key = keyOut
+			end
+
+			mappedKey.value = valueOut
+			mappedItems[keyOut] = valueOut
+		end
+
+		-- Add new keys if the value was not present in the previous state
+		for key, currentValue in currentItems do
+			local oldValue = previousItems[key]
+
+			if oldValue ~= nil then
+				continue
+			end
+
+			local valueOut, keyOut = getTransform(currentValue, key)
+			local keyOut = if keyOut ~= nil then keyOut else key :: never
+
+			if valueOut ~= nil then
+				mappedKeys[key] = { key = keyOut, value = valueOut }
+				mappedItems[keyOut] = valueOut
+			end
+		end
+
+		previousItems = currentItems
+
+		return mappedItems
+	end)
+end
+
+return mapped

pkgs/charm/src/observe.luau

@@ -0,0 +1,60 @@
+local signals = require("./signals")
+
+local computed = signals.computed
+local effect = signals.effect
+local getCurrentSub = signals.getCurrentSub
+local onEffectCleanup = signals.onEffectCleanup
+local onScopeDispose = signals.onScopeDispose
+
+local function noop() end
+
+local function observe<K, V>(selector: () -> { [K]: V }, observer: (item: V, key: K) -> ...() -> ()?): () -> ()
+	local tracked = computed(function()
+		return selector()
+	end)
+
+	local cleanups: { [K]: () -> () } = {}
+	local currentItems: { [K]: V } = {}
+
+	local function update(currentItems: { [K]: V }, previousItems: { [K]: V })
+		for key in previousItems do
+			local cleanup = cleanups[key]
+			if currentItems[key] == nil and cleanup then
+				cleanups[key] = nil
+				cleanup()
+			end
+		end
+
+		for key, value in currentItems do
+			if previousItems[key] == nil then
+				cleanups[key] = observer(value, key) or noop
+			end
+		end
+	end
+
+	local function teardown()
+		for _, cleanup in cleanups do
+			cleanup()
+		end
+		table.clear(cleanups)
+	end
+
+	local disposeEffect = effect(function()
+		local previousItems = currentItems
+		currentItems = tracked()
+		update(currentItems, previousItems)
+	end)
+
+	if getCurrentSub() then
+		onEffectCleanup(teardown)
+	else
+		onScopeDispose(teardown)
+	end
+
+	return function()
+		disposeEffect()
+		teardown()
+	end
+end
+
+return observe

pkgs/charm/src/signals.luau

@@ -0,0 +1,436 @@
+--[=[
+	alien-signals 2.0.1 implementation adopted from
+	https://github.com/Nicell/alien-signals-luau
+	https://github.com/stackblitz/alien-signals
+
+	MIT License
+
+	Copyright (c) 2024-present Johnson Chu, Nicholas Winans
+
+	Permission is hereby granted, free of charge, to any person obtaining a copy
+	of this software and associated documentation files (the "Software"), to deal
+	in the Software without restriction, including without limitation the rights
+	to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+	copies of the Software, and to permit persons to whom the Software is
+	furnished to do so, subject to the following conditions:
+
+	The above copyright notice and this permission notice shall be included in all
+	copies or substantial portions of the Software.
+
+	THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+	IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+	FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+	AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+	LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+	OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+	SOFTWARE.
+]=]
+
+local system = require("./system")
+
+export type ReactiveFlags = system.ReactiveFlags
+export type ReactiveNode = system.ReactiveNode
+export type Link = system.Link
+
+local ReactiveFlags = system.ReactiveFlags
+local createReactiveSystem = system.createReactiveSystem
+
+local NONE = 0b0000000
+local MUTABLE = 0b0000001
+local WATCHING = 0b0000010
+local DIRTY = 0b0010000
+local PENDING = 0b0100000
+local QUEUED = 0b1000000
+
+type EffectScope = ReactiveNode & {
+	cleanup: (() -> ())?,
+}
+
+type Effect = ReactiveNode & {
+	fn: () -> (),
+	cleanup: (() -> ())?,
+}
+
+type Computed<T = any> = ReactiveNode & {
+	value: T?,
+	getter: (previousValue: T?) -> T,
+}
+
+type Signal<T = any> = ReactiveNode & {
+	previousValue: T,
+	value: T,
+}
+
+local pauseStack: { ReactiveNode? } = {}
+local pauseStackCount = 0
+local queuedEffects: { Effect | EffectScope } = {}
+
+local link: (dep: ReactiveNode, sub: ReactiveNode) -> ()
+local unlink: (link: Link, sub: ReactiveNode?) -> Link?
+local propagate: (link: Link) -> ()
+local checkDirty: (link: Link, sub: ReactiveNode) -> boolean
+local endTracking: (sub: ReactiveNode) -> ()
+local startTracking: (sub: ReactiveNode) -> ()
+local shallowPropagate: (link: Link) -> ()
+
+local batchDepth = 0
+
+local notifyIndex = 0
+local queuedEffectsLength = 0
+local activeSub: ReactiveNode?
+local activeScope: EffectScope?
+
+local function getCurrentSub(): ReactiveNode?
+	return activeSub
+end
+
+local function setCurrentSub(sub: ReactiveNode?)
+	local prevSub = activeSub
+	activeSub = sub
+	return prevSub
+end
+
+local function getCurrentScope(): EffectScope?
+	return activeScope
+end
+
+local function setCurrentScope(scope: EffectScope?)
+	local prevScope = activeScope
+	activeScope = scope
+	return prevScope
+end
+
+local function addCleanup(e: EffectScope, cleanup: () -> ())
+	local prevCleanup = e.cleanup
+	if prevCleanup then
+		e.cleanup = function()
+			prevCleanup()
+			cleanup()
+		end
+	else
+		e.cleanup = cleanup
+	end
+end
+
+local function onEffectCleanup(cleanup: () -> ())
+	if activeSub then
+		addCleanup(activeSub :: Effect, cleanup)
+	end
+end
+
+local function onScopeDispose(cleanup: () -> ())
+	if activeScope then
+		addCleanup(activeScope, cleanup)
+	end
+end
+
+local function cleanupEffect(e: EffectScope)
+	if e.cleanup then
+		local prev = setCurrentSub(nil)
+		local success, result: unknown = pcall(e.cleanup)
+		setCurrentSub(prev)
+		e.cleanup = nil
+		if not success then
+			error(result)
+		end
+	end
+end
+
+local function run(e: EffectScope, flags: ReactiveFlags): ()
+	if bit32.band(flags, DIRTY) ~= 0 or (bit32.band(flags, PENDING) ~= 0 and checkDirty(e.deps :: Link, e)) then
+		cleanupEffect(e)
+		local prev = setCurrentSub(e)
+		startTracking(e)
+		local success, result: unknown = pcall((e :: Effect).fn)
+		setCurrentSub(prev)
+		endTracking(e)
+		if not success then
+			error(result)
+		end
+		return
+	elseif bit32.band(flags, PENDING) ~= 0 then
+		e.flags = bit32.band(flags, bit32.bnot(PENDING))
+	end
+	local link = e.deps
+	while link do
+		local dep = link.dep
+		local depFlags = dep.flags
+		if bit32.band(depFlags, QUEUED) ~= 0 then
+			depFlags = bit32.band(depFlags, bit32.bnot(QUEUED))
+			dep.flags = depFlags
+			run(dep :: EffectScope, depFlags)
+		end
+		link = link.nextDep
+	end
+end
+
+local function flush(): ()
+	while notifyIndex < queuedEffectsLength do
+		notifyIndex += 1
+		local effect = queuedEffects[notifyIndex] :: EffectScope
+		queuedEffects[notifyIndex] = nil
+		effect.flags = bit32.band(effect.flags, bit32.bnot(QUEUED))
+		run(effect, effect.flags)
+	end
+	notifyIndex = 0
+	queuedEffectsLength = 0
+end
+
+local function getBatchDepth(): number
+	return batchDepth
+end
+
+local function startBatch()
+	batchDepth += 1
+end
+
+local function endBatch()
+	batchDepth -= 1
+	if batchDepth == 0 then
+		flush()
+	end
+end
+
+local function pauseTracking()
+	pauseStackCount += 1
+	pauseStack[pauseStackCount] = activeSub
+	activeSub = nil
+end
+
+local function resumeTracking()
+	activeSub = pauseStack[pauseStackCount]
+	pauseStack[pauseStackCount] = nil
+	pauseStackCount -= 1
+end
+
+local function updateComputed(c: Computed): boolean
+	local prevSub = setCurrentSub(c)
+	startTracking(c)
+	local oldValue = c.value
+	local success, result = pcall(c.getter, oldValue)
+	setCurrentSub(prevSub)
+	endTracking(c)
+	if not success then
+		error(result)
+	end
+	if oldValue ~= result then
+		c.value = result
+		return true
+	else
+		return false
+	end
+end
+
+local function updateSignal(s: Signal, value: any): boolean
+	s.flags = MUTABLE
+	if s.previousValue ~= value then
+		s.previousValue = value
+		return true
+	else
+		return false
+	end
+end
+
+local function signalGetterSetter<T>(this: Signal<T>): (() -> T, (value: ((T) -> T) | T) -> T)
+	local function getter(): T
+		local value = this.value
+		if bit32.band(this.flags, DIRTY) ~= 0 then
+			if updateSignal(this, value) then
+				local subs = this.subs
+				if subs then
+					shallowPropagate(subs)
+				end
+			end
+		end
+		if activeSub then
+			link(this, activeSub)
+		end
+		return value
+	end
+
+	local function setter(newValue: ((T) -> T) | T): T
+		local value = this.value
+		local newValue = newValue :: T
+		if type(newValue) == "function" then
+			local prev = setCurrentSub(nil)
+			local success, result = pcall(newValue, value)
+			setCurrentSub(prev)
+			if not success then
+				error(result)
+			end
+			newValue = result
+		end
+		if value ~= newValue then
+			this.value = newValue
+			this.flags = bit32.bor(MUTABLE, DIRTY)
+			local subs = this.subs
+			if subs then
+				propagate(subs)
+				if batchDepth == 0 then
+					flush()
+				end
+			end
+		end
+		return newValue
+	end
+
+	return getter, setter
+end
+
+local function computedGetter<T>(this: Computed<T>): () -> T
+	return function()
+		local flags = this.flags
+		if
+			bit32.band(flags, DIRTY) ~= 0
+			or (bit32.band(flags, PENDING) ~= 0 and checkDirty(this.deps :: Link, this))
+		then
+			if updateComputed(this) then
+				local subs = this.subs
+				if subs then
+					shallowPropagate(subs)
+				end
+			end
+		elseif bit32.band(flags, PENDING) ~= 0 then
+			this.flags = bit32.band(flags, bit32.bnot(PENDING))
+		end
+		if activeSub then
+			link(this, activeSub)
+		elseif activeScope then
+			link(this, activeScope)
+		end
+		return this.value :: T
+	end
+end
+
+local function effectStop(this: EffectScope): () -> ()
+	return function()
+		local dep = this.deps
+		while dep do
+			dep = unlink(dep, this)
+		end
+		local sub = this.subs
+		while sub do
+			unlink(sub)
+			sub = this.subs
+		end
+		this.flags = NONE
+		cleanupEffect(this)
+	end
+end
+
+local function signal<T>(initialValue: T): (() -> T, (value: ((T) -> T) | T) -> T)
+	return signalGetterSetter({
+		previousValue = initialValue,
+		value = initialValue,
+		flags = MUTABLE,
+	})
+end
+
+local function computed<T>(getter: (previousValue: T?) -> T): () -> T
+	return computedGetter({
+		flags = bit32.bor(MUTABLE, DIRTY),
+		getter = getter,
+	})
+end
+
+local function effect(fn: () -> ()): () -> ()
+	local e: Effect = {
+		fn = fn,
+		flags = WATCHING,
+	}
+	if activeSub then
+		link(e, activeSub)
+	elseif activeScope then
+		link(e, activeScope)
+	end
+	local prev = setCurrentSub(e)
+	local success, result: unknown = pcall(fn)
+	setCurrentSub(prev)
+	if not success then
+		error(result)
+	end
+	return effectStop(e)
+end
+
+local function effectScope(fn: () -> ()): () -> ()
+	local e: EffectScope = {
+		flags = NONE,
+	}
+	if activeScope then
+		link(e, activeScope)
+	end
+	local prev = setCurrentScope(e)
+	local success, result: unknown = pcall(fn)
+	setCurrentScope(prev)
+	if not success then
+		error(result)
+	end
+	return effectStop(e)
+end
+
+local function notify(e: ReactiveNode)
+	local flags = e.flags
+	if bit32.band(flags, QUEUED) == 0 then
+		e.flags = bit32.bor(flags, QUEUED)
+		local subs = e.subs
+		if subs then
+			notify(subs.sub :: Effect | EffectScope)
+		else
+			queuedEffectsLength += 1
+			queuedEffects[queuedEffectsLength] = e :: Effect
+		end
+	end
+end
+
+local function update(signal: ReactiveNode): boolean
+	if (signal :: Computed).getter then
+		return updateComputed(signal :: Computed)
+	else
+		return updateSignal(signal :: Signal, (signal :: Signal).value)
+	end
+end
+
+local function unwatched(signal: ReactiveNode)
+	local toRemove = signal.deps
+	if toRemove then
+		repeat
+			toRemove = unlink(toRemove, signal)
+		until not toRemove
+		signal.flags = bit32.bor(signal.flags, DIRTY)
+	end
+	cleanupEffect(signal :: EffectScope)
+end
+
+local reactiveSystem = createReactiveSystem({
+	update = update,
+	notify = notify,
+	unwatched = unwatched,
+})
+
+link = reactiveSystem.link
+unlink = reactiveSystem.unlink
+propagate = reactiveSystem.propagate
+checkDirty = reactiveSystem.checkDirty
+endTracking = reactiveSystem.endTracking
+startTracking = reactiveSystem.startTracking
+shallowPropagate = reactiveSystem.shallowPropagate
+
+return {
+	ReactiveFlags = ReactiveFlags,
+	createReactiveSystem = createReactiveSystem,
+	getCurrentSub = getCurrentSub,
+	setCurrentSub = setCurrentSub,
+	getCurrentScope = getCurrentScope,
+	setCurrentScope = setCurrentScope,
+	getBatchDepth = getBatchDepth,
+	startBatch = startBatch,
+	endBatch = endBatch,
+	pauseTracking = pauseTracking,
+	resumeTracking = resumeTracking,
+	onEffectCleanup = onEffectCleanup,
+	onScopeDispose = onScopeDispose,
+	signal = signal,
+	computed = computed,
+	effect = effect,
+	effectScope = effectScope,
+}

pkgs/charm/src/spawn.luau

@@ -0,0 +1,36 @@
+local freeThread: thread?
+
+local function run<T...>(thread: thread, fn: (T...) -> (), ...: T...): boolean
+	fn(...)
+	if freeThread then
+		return true
+	else
+		freeThread = thread
+		return false
+	end
+end
+
+local function runner()
+	local quit = false
+	repeat
+		quit = run(coroutine.yield())
+	until quit
+end
+
+local function spawn<T...>(callback: (T...) -> (), ...: T...)
+	local thread
+
+	if freeThread then
+		thread = freeThread
+		freeThread = nil
+	elseif task then
+		thread = task.spawn(runner)
+	else
+		thread = coroutine.create(runner)
+		coroutine.resume(thread)
+	end
+
+	coroutine.resume(thread, thread, callback, ...)
+end
+
+return spawn

pkgs/charm/src/subscribe.luau

@@ -0,0 +1,29 @@
+local signals = require("./signals")
+local spawn = require("./spawn")
+
+local effect = signals.effect
+local pauseTracking = signals.pauseTracking
+local resumeTracking = signals.resumeTracking
+
+local function subscribe<T>(selector: () -> T, listener: (T, T) -> ()): () -> ()
+	local current: T
+	local mounted = true
+
+	return effect(function()
+		local value = selector()
+
+		if mounted then
+			current = value
+			mounted = false
+		elseif value ~= current then
+			local previous = current
+			current = value
+
+			pauseTracking()
+			spawn(listener, value, previous)
+			resumeTracking()
+		end
+	end)
+end
+
+return subscribe

pkgs/charm/src/system.luau

@@ -0,0 +1,357 @@
+--[=[
+	alien-signals 2.0.1 implementation adopted from
+	https://github.com/Nicell/alien-signals-luau
+	https://github.com/stackblitz/alien-signals
+
+	MIT License
+
+	Copyright (c) 2024-present Johnson Chu, Nicholas Winans
+
+	Permission is hereby granted, free of charge, to any person obtaining a copy
+	of this software and associated documentation files (the "Software"), to deal
+	in the Software without restriction, including without limitation the rights
+	to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+	copies of the Software, and to permit persons to whom the Software is
+	furnished to do so, subject to the following conditions:
+
+	The above copyright notice and this permission notice shall be included in all
+	copies or substantial portions of the Software.
+
+	THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+	IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+	FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+	AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+	LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+	OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+	SOFTWARE.
+]=]
+
+export type ReactiveNode = {
+	deps: Link?,
+	depsTail: Link?,
+	subs: Link?,
+	subsTail: Link?,
+	flags: ReactiveFlags,
+}
+
+export type Link = {
+	dep: ReactiveNode,
+	sub: ReactiveNode,
+	prevSub: Link?,
+	nextSub: Link?,
+	prevDep: Link?,
+	nextDep: Link?,
+}
+
+type Stack<T> = {
+	value: T,
+	prev: Stack<T>?,
+}
+
+export type ReactiveFlags = number
+
+local NONE = 0b0000000
+local MUTABLE = 0b0000001
+local WATCHING = 0b0000010
+local RECURSED_CHECK = 0b0000100
+local RECURSED = 0b0001000
+local DIRTY = 0b0010000
+local PENDING = 0b0100000
+
+local ReactiveFlags = {
+	None = NONE,
+	Mutable = MUTABLE,
+	Watching = WATCHING,
+	RecursedCheck = RECURSED_CHECK,
+	Recursed = RECURSED,
+	Dirty = DIRTY,
+	Pending = PENDING,
+}
+
+local function createReactiveSystem(props: {
+	update: (sub: ReactiveNode) -> boolean,
+	notify: (sub: ReactiveNode) -> (),
+	unwatched: (sub: ReactiveNode) -> (),
+})
+	local update = props.update
+	local notify = props.notify
+	local unwatched = props.unwatched
+
+	local function isValidLink(checkLink: Link, sub: ReactiveNode): boolean
+		local depsTail = sub.depsTail
+		if depsTail then
+			local link = sub.deps :: Link
+			repeat
+				if link == checkLink then
+					return true
+				end
+				if link == depsTail then
+					break
+				end
+				link = link.nextDep :: Link
+			until not link
+		end
+		return false
+	end
+
+	local function link(dep: ReactiveNode, sub: ReactiveNode): ()
+		local prevDep = sub.depsTail
+		if prevDep and prevDep.dep == dep then
+			return
+		end
+		local nextDep: Link?
+		local recursedCheck = bit32.band(sub.flags, RECURSED_CHECK) ~= 0
+		if recursedCheck then
+			nextDep = if prevDep then prevDep.nextDep else sub.deps
+			if nextDep and nextDep.dep == dep then
+				sub.depsTail = nextDep
+				return
+			end
+		end
+		local prevSub = dep.subsTail
+		if prevSub and prevSub.sub == sub and (not recursedCheck or isValidLink(prevSub, sub)) then
+			return
+		end
+		local newLink: Link = {
+			dep = dep,
+			sub = sub,
+			prevDep = prevDep,
+			nextDep = nextDep,
+			prevSub = prevSub,
+		}
+		sub.depsTail = newLink
+		dep.subsTail = newLink
+		if nextDep then
+			nextDep.prevDep = newLink
+		end
+		if prevDep then
+			prevDep.nextDep = newLink
+		else
+			sub.deps = newLink
+		end
+		if prevSub then
+			prevSub.nextSub = newLink
+		else
+			dep.subs = newLink
+		end
+	end
+
+	local function unlink(link: Link, sub: ReactiveNode?): Link?
+		local sub = sub or link.sub
+		local dep = link.dep
+		local prevDep = link.prevDep
+		local nextDep = link.nextDep
+		local nextSub = link.nextSub
+		local prevSub = link.prevSub
+		if nextDep then
+			nextDep.prevDep = prevDep
+		else
+			sub.depsTail = prevDep
+		end
+		if prevDep then
+			prevDep.nextDep = nextDep
+		else
+			sub.deps = nextDep
+		end
+		if nextSub then
+			nextSub.prevSub = prevSub
+		else
+			dep.subsTail = prevSub
+		end
+		if prevSub then
+			prevSub.nextSub = nextSub
+		else
+			dep.subs = nextSub
+			if not nextSub then
+				unwatched(dep)
+			end
+		end
+		return nextDep
+	end
+
+	local function propagate(link: Link): ()
+		local next = link.nextSub
+		local stack: Stack<Link?>?
+
+		repeat
+			local sub = link.sub
+
+			local flags = sub.flags
+
+			if bit32.band(flags, bit32.bor(MUTABLE, WATCHING)) ~= 0 then
+				if bit32.band(flags, bit32.bor(RECURSED_CHECK, RECURSED, DIRTY, PENDING)) == 0 then
+					sub.flags = bit32.bor(flags, PENDING)
+				elseif bit32.band(flags, bit32.bor(RECURSED_CHECK, RECURSED)) == 0 then
+					flags = NONE
+				elseif bit32.band(flags, RECURSED_CHECK) == 0 then
+					sub.flags = bit32.bor(bit32.band(flags, bit32.bnot(RECURSED)), PENDING)
+				elseif bit32.band(flags, bit32.bor(DIRTY, PENDING)) == 0 and isValidLink(link, sub) then
+					sub.flags = bit32.bor(flags, bit32.bor(RECURSED, PENDING))
+					flags = bit32.band(flags, MUTABLE)
+				else
+					-- selene: allow(if_same_then_else)
+					flags = NONE
+				end
+
+				if bit32.band(flags, WATCHING) ~= 0 then
+					notify(sub)
+				end
+
+				if bit32.band(flags, MUTABLE) ~= 0 then
+					local subSubs = sub.subs
+					if subSubs then
+						link = subSubs
+						if subSubs.nextSub then
+							stack = { value = next, prev = stack }
+							next = link.nextSub
+						end
+						continue
+					end
+				end
+			end
+
+			link = next :: Link
+
+			if link then
+				next = link.nextSub
+				continue
+			end
+
+			local continueTop = false
+
+			while stack do
+				link = stack.value :: Link
+				stack = stack.prev
+				if link then
+					next = link.nextSub
+					continueTop = true
+					break
+				end
+			end
+
+			if continueTop then
+				continue
+			end
+
+			break
+		until false
+	end
+
+	local function startTracking(sub: ReactiveNode): ()
+		sub.depsTail = nil
+		sub.flags = bit32.bor(bit32.band(sub.flags, bit32.bnot(bit32.bor(RECURSED, DIRTY, PENDING))), RECURSED_CHECK)
+	end
+
+	local function endTracking(sub: ReactiveNode): ()
+		local depsTail = sub.depsTail
+		local toRemove = if depsTail then depsTail.nextDep else sub.deps
+		while toRemove do
+			toRemove = unlink(toRemove, sub)
+		end
+		sub.flags = bit32.band(sub.flags, bit32.bnot(RECURSED_CHECK))
+	end
+
+	local function shallowPropagate(link: Link): ()
+		repeat
+			local sub = link.sub
+			local subFlags = sub.flags
+			if bit32.band(subFlags, bit32.bor(PENDING, DIRTY)) == PENDING then
+				sub.flags = bit32.bor(subFlags, DIRTY)
+				if bit32.band(subFlags, WATCHING) ~= 0 then
+					notify(sub)
+				end
+			end
+			link = link.nextSub :: Link
+		until not link
+	end
+
+	local function checkDirty(link: Link, sub: ReactiveNode): boolean
+		local stack: Stack<Link>?
+		local checkDepth = 0
+
+		repeat
+			local dep = link.dep
+			local depFlags = dep.flags
+
+			local dirty = false
+
+			if bit32.band(sub.flags, DIRTY) ~= 0 then
+				dirty = true
+			elseif bit32.band(depFlags, bit32.bor(MUTABLE, DIRTY)) == bit32.bor(MUTABLE, DIRTY) then
+				if update(dep) then
+					local subs = dep.subs :: Link
+					if subs.nextSub then
+						shallowPropagate(subs)
+					end
+					dirty = true
+				end
+			elseif bit32.band(depFlags, bit32.bor(MUTABLE, PENDING)) == bit32.bor(MUTABLE, PENDING) then
+				if link.nextSub or link.prevSub then
+					stack = { value = link, prev = stack }
+				end
+				link = dep.deps :: Link
+				sub = dep
+				checkDepth += 1
+				continue
+			end
+
+			if not dirty and link.nextDep then
+				link = link.nextDep
+				continue
+			end
+
+			local continueTop = false
+
+			while checkDepth ~= 0 do
+				checkDepth -= 1
+				local firstSub = sub.subs :: Link
+				local hasMultipleSubs = firstSub.nextSub ~= nil
+				if hasMultipleSubs then
+					link = (stack :: Stack<Link>).value
+					stack = (stack :: Stack<Link>).prev
+				else
+					link = firstSub
+				end
+				if dirty then
+					if update(sub) then
+						if hasMultipleSubs then
+							shallowPropagate(firstSub)
+						end
+						sub = link.sub
+						continue
+					end
+				else
+					sub.flags = bit32.band(sub.flags, bit32.bnot(PENDING))
+				end
+				sub = link.sub
+				if link.nextDep then
+					link = link.nextDep
+					continueTop = true
+					break
+				end
+				dirty = false
+			end
+
+			if continueTop then
+				continue
+			end
+
+			return dirty
+		until false
+	end
+
+	return {
+		link = link,
+		unlink = unlink,
+		propagate = propagate,
+		checkDirty = checkDirty,
+		endTracking = endTracking,
+		startTracking = startTracking,
+		shallowPropagate = shallowPropagate,
+	}
+end
+
+return {
+	ReactiveFlags = ReactiveFlags,
+	createReactiveSystem = createReactiveSystem,
+}

pkgs/charm/src/types.luau

@@ -0,0 +1,9 @@
+export type Selector<T = any> = () -> T
+
+export type Atom<T = any> = ((((T) -> T) | T) -> T) & (() -> T)
+
+export type AtomOptions<T = any> = {
+	equals: ((prev: T, next: T) -> boolean)?,
+}
+
+return {}

pkgs/charm/src/untracked.luau

@@ -0,0 +1,18 @@
+local signals = require("./signals")
+
+local pauseTracking = signals.pauseTracking
+local resumeTracking = signals.resumeTracking
+
+local function untracked<T, Args...>(callback: ((Args...) -> T) | (Args...) -> (), ...: Args...): T
+	pauseTracking()
+	local success, result = pcall(callback, ...)
+	resumeTracking()
+
+	if not success then
+		error(result)
+	end
+
+	return result
+end
+
+return untracked

pkgs/charm/wally.toml

@@ -0,0 +1,7 @@
+[package]
+name = "littensy/charm"
+version = "0.10.0"
+registry = "https://github.com/UpliftGames/wally-index"
+realm = "shared"
+
+[dependencies]

pkgs/react-charm/__tests__/helpers/renderHook.luau

@@ -0,0 +1,42 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+
+_G.__ROACT_17_MOCK_SCHEDULER__ = true
+
+local React = require(ReplicatedStorage.Packages.React)
+local ReactRoblox = require(ReplicatedStorage.Packages.ReactRoblox)
+
+type RenderedHook<Props, Result> = {
+	result: Result,
+	rerender: (Props?) -> (),
+	unmount: () -> (),
+}
+
+local function renderHook<Props, Result>(callback: (Props) -> Result, props: Props?): RenderedHook<Props, Result>
+	local root = ReactRoblox.createLegacyRoot(Instance.new("Folder"))
+	local hook = {}
+
+	local function Component(props)
+		hook.result = callback(props)
+	end
+
+	function hook.rerender(newProps)
+		props = newProps or props
+		ReactRoblox.act(function()
+			root:render(React.createElement(Component, props))
+		end)
+	end
+
+	function hook.unmount()
+		ReactRoblox.act(function()
+			root:unmount()
+		end)
+	end
+
+	ReactRoblox.act(function()
+		hook.rerender(props)
+	end)
+
+	return hook
+end
+
+return renderHook

pkgs/react-charm/__tests__/spec.luau

@@ -0,0 +1,15 @@
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local suite = require(ServerScriptService.Tests.suite)
+
+local describe = suite.describe
+
+local function spec(name: string)
+	describe(name, function()
+		return require(`./{name}.spec`) :: any
+	end)
+end
+
+spec("useAtom")
+
+return {}

pkgs/react-charm/__tests__/useAtom.spec.luau

@@ -0,0 +1,50 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+local ReactCharm = require(ReplicatedStorage.Packages.ReactCharm)
+local renderHook = require(script.Parent.helpers.renderHook)
+local suite = require(ServerScriptService.Tests.suite)
+
+local atom = Charm.atom
+local useAtom = ReactCharm.useAtom
+local test = suite.test
+
+test("tracks atom state", function()
+	local state = atom(0)
+	local hook = renderHook(function()
+		return useAtom(state)
+	end)
+	assert(hook.result == 0)
+	state(1)
+	assert(hook.result == 1)
+end)
+
+test("accepts a selector", function()
+	local state = atom(0)
+	local hook = renderHook(function()
+		return useAtom(function()
+			return 2 * state()
+		end)
+	end)
+	assert(hook.result == 0)
+	state(1)
+	assert(hook.result == 2)
+end)
+
+test("tracks dependencies", function()
+	local state = atom(0)
+	local hook = renderHook(function()
+		return useAtom(state, { state })
+	end)
+	assert(hook.result == 0)
+	state(1)
+	assert(hook.result == 1)
+	state = atom(0)
+	hook.rerender()
+	assert(hook.result == 0)
+	state(1)
+	assert(hook.result == 1)
+end)
+
+return {}

pkgs/react-charm/default.project.json

@@ -0,0 +1,6 @@
+{
+	"name": "ReactCharm",
+	"tree": {
+		"$path": "src"
+	}
+}

pkgs/react-charm/src/init.luau

@@ -0,0 +1,32 @@
+local Charm = require(script.Parent.Charm)
+local React = require(script.Parent.React)
+
+local listen = Charm.listen
+local useEffect = React.useEffect
+local useState = React.useState
+
+--[=[
+	A hook that subscribes to changes in the given atom or selector. The
+	component is re-rendered whenever the state changes.
+	
+	If the `dependencies` array is provided, the subscription to the atom or
+	selector is re-created whenever the dependencies change. Otherwise, the
+	subscription is created once when the component is mounted.
+	
+	@param callback The atom or selector to subscribe to.
+	@param dependencies An array of values that the subscription depends on.
+	@return The current state.
+]=]
+local function useAtom<State>(callback: () -> State, dependencies: { any }?): State
+	local state, setState = useState(callback)
+
+	useEffect(function()
+		return listen(callback, setState)
+	end, dependencies or {})
+
+	return state
+end
+
+return {
+	useAtom = useAtom,
+}

pkgs/react-charm/wally.toml

@@ -0,0 +1,10 @@
+[package]
+name = "littensy/react-charm"
+version = "0.3.0"
+registry = "https://github.com/UpliftGames/wally-index"
+realm = "shared"
+exclude = ["Charm.luau", "React.luau"]
+
+[dependencies]
+Charm = "littensy/charm@^0.10"
+React = "jsdotlua/react@^17"

pkgs/vide-charm/__tests__/spec.luau

@@ -0,0 +1,15 @@
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local suite = require(ServerScriptService.Tests.suite)
+
+local describe = suite.describe
+
+local function spec(name: string)
+	describe(name, function()
+		return require(`./{name}.spec`) :: any
+	end)
+end
+
+spec("useAtom")
+
+return {}

pkgs/vide-charm/__tests__/useAtom.spec.luau

@@ -0,0 +1,40 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+local Vide = require(ReplicatedStorage.Packages.Vide)
+local VideCharm = require(ReplicatedStorage.Packages.VideCharm)
+local suite = require(ServerScriptService.Tests.suite)
+
+local atom = Charm.atom
+local root = Vide.root
+local useAtom = VideCharm.useAtom
+local test = suite.test
+
+test("tracks atom state", function()
+	local state = atom(0)
+	local source
+	local cleanup = root(function()
+		source = useAtom(state)
+	end)
+	assert(source() == 0)
+	state(1)
+	assert(source() == 1)
+	cleanup()
+end)
+
+test("accepts a selector", function()
+	local state = atom(0)
+	local source
+	local cleanup = root(function()
+		source = useAtom(function()
+			return 2 * state()
+		end)
+	end)
+	assert(source() == 0)
+	state(1)
+	assert(source() == 2)
+	cleanup()
+end)
+
+return {}

pkgs/vide-charm/default.project.json

@@ -0,0 +1,6 @@
+{
+	"name": "VideCharm",
+	"tree": {
+		"$path": "src"
+	}
+}

pkgs/vide-charm/src/init.luau

@@ -0,0 +1,22 @@
+local Charm = require(script.Parent.Charm)
+local Vide = require(script.Parent.Vide)
+
+local subscribe = Charm.subscribe
+local source = Vide.source
+local cleanup = Vide.cleanup
+
+--[=[
+	Subscribes to the state of an atom and returns a Vide source.
+	
+	@param callback The atom or selector to subscribe to.
+	@return The reactive source.
+]=]
+local function useAtom<State>(callback: () -> State): () -> State
+	local state = source(callback())
+	cleanup(subscribe(callback, state))
+	return state
+end
+
+return {
+	useAtom = useAtom,
+}

pkgs/vide-charm/wally.toml

@@ -0,0 +1,10 @@
+[package]
+name = "littensy/vide-charm"
+version = "0.3.0"
+registry = "https://github.com/UpliftGames/wally-index"
+realm = "shared"
+exclude = ["Charm.luau", "Vide.luau"]
+
+[dependencies]
+Charm = "littensy/charm@^0.10"
+Vide = "centau/vide@^0.3"