Welcome!
This tutorial aims to help people understand Luau’s type annotation feature, and how to write code that’s smarter and easier for other experienced developers to pick up and understand within Roblox’s ecosystem today!
What are Type Annotations?
Type annotations are a feature of Luau that allow you to define constraints to the variables in your code. They provide contextual clues to how your code is intended to be used, and blueprints for how to structure data. When these constraints are violated, warnings will be emitted by Luau’s analyzer suggesting that you fix them.
To make the most effective use of type annotations, it’s highly recommended that you add this comment to the top of your script:
--!strict
It ensures variables will always try to give you a properly inferred type instead of falling back to the default “any” type.
Basic Usage
When you mouse over a variable, a tooltip should appear that describes what the variable’s type is.
local value = 0 -- Should report as [number]
local part = Instance.new("Part") -- Should report as [Part]
You can explicitly define the type of a variable by writing them as such:
local value: number = 0
local part: Part = Instance.new("Part")
When annotations are explicitly defined, they will emit warnings if you attempt to assign values to them which don’t match their defined types:
value = 5 -- good!
value = "lol" -- bad!
part = workspace -- bad!
part = Instance.new("SpawnLocation") -- good!
Null Types and Refinement
Type names can be suffixed with a question mark (?) to mark them as potentially nil. For example, Instance:FindFirstChild returns a type of Instance? to indicate that it may not have found a child with the specified name.
-- `inst` has the inferred type: [Instance?]
local inst = workspace:FindFirstChild("Instance")
Attempting to index the fields of a nullable type will produce a warning. You must conditionally evaluate the variable’s existence. The nice thing is that Luau will automatically refine the type on the spot when this evaluation happens.
Consider the following example:
local maybe: number? = nil -- (Imagine this is assigned a value somewhere)
There are several ways to refine the nullability of this type:
Option 1: Inline refinement
-- `maybePlus1` is evaluated as [number?]
local maybePlus1 = (maybe and maybe + 1) -- `maybe` refines to [number] past `and`
Option 2: If statement refinement
if maybePlus1 then
-- `maybePlus1` refines to: [number] in this body.
print("maybePlus1 is defined!", maybePlus1)
else
-- `maybePlus1` is now refined to: [nil]
print("maybePlus1 is nil!")
end
Option 3: Return (or break/continue) refinement
if not maybePlus1 then
return
end
-- `maybePlus1` is now refined to: [number]
print("the maybePlus1 is real!!!", maybePlus1)
Option 4: Assertion refinement
assert(maybePlus1, "maybePlus1 is not defined!") -- Will error if maybePlus1 is nil!
-- maybePlus1 is now refined to: [number]
print("maybePlus2:", maybePlus1 + 1)
Built-in Types
Luau has 9 built-in type names defined:
nilstringnumberthreadbooleanbufferanyneverunknown
There are types for function and table as well, but they have a different structure which will be described later.
The last 3 types: any, never, and unknown, are special in that they don’t represent specific primitive types in Luau’s VM. They instead represent certain sets of all types in Luau:
any
any can store any type of value, effectively muting the type annotation system. You shouldn’t use this unless you have no choice.
local value: any = 0
value = "lol" -- valid!
value = nil -- valid!
value = 2 -- valid!
unknown
unknown must have it’s type evaluated through the type/typeof functions in order to be stored or used as any other variable type besides unknown. However, it can still be assigned to any value directly:
local value: unknown
value = "lol" -- valid!
value = nil -- valid!
value = 2 -- valid!
if type(value) == "number" then
-- `value` is [number] in here
elseif type(value) == "string" then
-- `value` is [string] in here
else
-- `value` is [unknown] in here
end
never
never cannot be refined into any type. It’s usually not defined directly, instead appearing when attempting to do a type refinement deemed impossible:
local value: unknown = true
if type(value) == "buffer" and type(value) == "boolean" then
-- `value is now [never] because it cannot
-- be a buffer and a boolean at the same time!
print("This message will never print!", value)
end
Concrete Types
Roblox’s Luau environment defines “concrete” types for each defined engine type in the API Reference.
Concrete types are statically defined and can extend from one another, but they cannot be defined by Luau code directly. They are declared by the Luau vendor (i.e. Roblox) at runtime.
Concrete types can be refined from any/unknown through the use of Luau’s typeof function:
local object: unknown
if typeof(object) == "Instance" then
-- `object` is now refined to [Instance]
print("object is an Instance:", object.Name)
else
-- `object` is still [unknown]
print("object type is not an Instance:", object)
end
All of Roblox’s top-level concrete types are listed in the datatypes documentation.
Refining Concrete Types via “Magic Functions”
Consider the following:
local part = Instance.new("Part")
print(typeof(part)) -- prints "Instance"
Why does this print Instance instead of Part? Well, it turns out the typeof function will only describe top-level concrete types!
For concrete types that are extensions of top-level concrete types, you’ll need to use certain magic functions in Luau to refine these extended types. (“Magic” in this case being a hack on the C++ end, but it works™)
An example of this is Roblox’s Instance:IsA function:
-- `object` starts as [Instance?]
local object = workspace:FindFirstChild("Terrain")
if object then
-- `object` is now refined to [Instance]
print(object.Name)
if object:IsA("BasePart") then
-- `object` is now refined to [BasePart]
print(object.Size)
if object:IsA("Terrain") then
-- `object` is now refined to [Terrain]
print(object:CountCells())
end
-- `object` is once again [BasePart]
print(object.Position)
end
-- `object` is once again [Instance]
print(object.Parent)
end
-- `object` is once again [Instance?]
print(object ~= nil)
Function Types
Arguments
When declaring functions, you can annotate their arguments with types to make them more explicitly defined:
local function addNumbers(a: number, b: number)
return a + b
end
addNumbers(5, 2) -- valid!
addNumbers(1) -- needs two arguments!
addNumbers("a", "b") -- argument types are wrong!
Return Types
You can explicitly annotate a function’s return type:
local function toVector2(vec3: Vector3): Vector2
return Vector2.new(vec3.X, vec3.Y)
end
local a: Vector2 = toVector2(Vector3.one) -- valid!
local b: number = toVector2(Vector3.one) -- wrong type for b!
local c = toVector2("lol") -- wrong argument type for vec3!
This is helpful when authoring a function knowing what it will return in advance. Luau will make sure to enforce that you’ve returned the correct type:
-- "not implemented yet!"
local function getNumber(object: Instance, name: string): number
-- A warning will appear reporting that the
-- function doesn't always return a `number`
end
Tuples
If you have a function that returns multiple values, you can wrap their return types in parentheses comma-separated as such:
local function getCFrameAndSize(part: BasePart?): (CFrame?, Vector3?)
if part then
return part.CFrame, part.Size
end
return -- explicit return is required
end
If your function doesn’t return anything, you can leave the contents of the parentheses empty:
local function reportIfPartFound(message: string): ()
if workspace:FindFirstChildOfClass("Part") then
print(message)
end
end
Variadics
You can typecheck variadic functions by annotating the ... argument of the function:
local function debugPrint(...: unknown)
if DEBUG then
print(...)
end
end
Functions that return a variable amount of some type can annotated as ...T
local function gimmeSomeNumbers(): ...number
if os.clock() % 2 > 1 then
return 1, 2, 3
else
return 4, 5, 6, 7
end
end
Function Type Annotating
A function variable is type annotated by wrapping the typenames of the function’s arguments and return types in parenthesis, separated by an arrow ->.
One basic example to start with is a function that takes no arguments and returns nothing. It would use the following type name: () -> ()
local noArgsOrReturn: () -> () = function()
print("This function takes no arguments and returns nothing")
end
-- valid!
noArgsOrReturn = function()
print("We can reassign the variable since we know its type!")
end
-- bad assignment: cannot convert a function with 1 arg into a function with no args.
noArgsOrReturn = function(arg: any)
print(arg, "????")
end
-- ok, but invalid in practice. see below.
noArgsOrReturn = function()
return "lol"
end
-- attempts to assign a variable to the "return" of this function
-- will warn that it's not supposed to return anything.
local value = noArgsOrReturn()
-- attempting to call it with arguments will also report
-- that it's not supposed to take arguments.
noArgsOrReturn("lol")
Annotating Arguments and Returns
Function arguments and return types are defined in a similar way to how they are directly declared. They also may be optionally given argument names to help contextualize their use:
local coolFunction: (part: BasePart) -> (CFrame, Vector3) = function (part)
-- `part` is inferred as [BasePart]
return part.CFrame, part.Size
end
Nullable Functions
If you want to define a function as nullable, you can put a question mark next to the return type:
local maybeFunc: (Instance, string) -> ()?
if maybeFunc then
-- `maybeFunc` is now [(Instance, string) -> ()]
maybeFunc()
end
HOWEVER, there are a few places where you need to be careful with how you define them.
For example: (Instance, string) -> Part? is not a nullable function type, it’s a function that returns a nullable Part. You can fix this by putting parenthesis in the right place:
((Instance, string) -> Part)?
Tupled Arguments and Returns
Functions that take variadic arguments are written as ...T, and are not allowed to have a named argument:
local goodFunc: (format: string, ...any) -> ()? -- valid!
local badFunc: (format: string, args: ...any) -> ()? -- syntax error!
Functions as Arguments
You can define the type of a function argument as a function type annotation:
local function awaitChild(object: Instance, name: string, andThen: (child: Instance) -> ())
task.spawn(function()
local child = object:WaitForChild(name)
andThen(child)
end)
end
Arrays
Arrays in Luau’s type checker are defined by wrapping a type name in curly braces {}. For example, the return type of Instance:GetChildren() is defined as { Instance }
local objects = workspace:GetChildren() -- objects has type: [{Instance}]
for i, child in objects do
-- child is [Instance]
print(child.Name)
end
You can define what kind of values an array expects by annotating their type:
local t: {number} = {}
table.insert(t, 5) -- good!
table.insert(t, "lol") -- invalid!
Dictionaries
Dictionary types are a bit more in-depth. There are two ways you can define dictionary fields, both of which can be used at the same time.
Indexer Type
Array types in Luau are actually just a shorthand for a number dictionary indexer. In practice this means {T} is a shorthand for { [number]: T } which is how dictionary indexers are defined:
local t = {} :: {
[number]: number
}
table.insert(t, 5) -- good!
table.insert(t, "lol") -- invalid!
t[1] = 2 -- good!
t.lol = 3 -- invalid!
t[Vector3.zero] = 4 -- invalid!
(Note: The typecast operator :: will be explained more later, it’s being used here to make the table declaration look a little cleaner)
You can define the indexer to be any type! For example, here’s a way to map Player objects to their positions:
--!strict
local Players = game:GetService("Players")
local positionMap = {} :: {
[Player]: Vector3?
}
local function onPlayerRemoving(player: Player)
-- This assignment is valid because we're removing
-- the player from the dictionary index.
positionMap[player] = nil
end
local function updatePositions()
for i, player in Players:GetPlayers() do
-- Character is a reference, so its type is [Model?]
local character = player.Character
if character then
local cf = character:GetPivot()
positionMap[player] = cf.Position
end
end
end
Players.PlayerRemoving:Connect(onPlayerRemoving)
Explicit Fields & Type Declarations
Type declarations are a core feature of Luau that I’ve refrained from talking about so far because their best use case is with dictionaries. You can use them to create concisely shaped structures for your tables!
type SimpleType = {
Number: number,
String: string,
Function: (...any) -> (...any),
}
local value: SimpleType = {
Number = 0,
String = "lol",
Function = print,
}
Dictionaries are types in the same way that functions and other primitives are types.
If you want to declare a table that starts nil but will exist later, you can do that!
--!strict
local pendingData = nil :: {
Player: Player,
UserId: number,
Coins: number,
}? -- Question mark is important!
local Players = game:GetService("Players")
local player = assert(Players.LocalPlayer)
local userId = player.UserId
local coins = math.random(1, 1000)
pendingData = {
Player = assert(player),
UserId = userId,
Coins = coins,
}
Here are a few real use-cases:
-- This is an entry in the array returned by `HumanoidDescription:GetAccessories`
type AccessoryInfo = {
AccessoryType: Enum.AccessoryType,
AssetId: number,
IsLayered: boolean,
Order: number?,
Puffiness: number?,
}
-- With this, we can iterate over the contents of the table with a type annotation!
local accessories: { AccessoryInfo } = hDesc:GetAccessories(true)
for i, info in accessories do
-- Inferred type of `info` is [AccessoryInfo]
print("Got accessory with type", info.AccessoryType, "and AssetId", info.AssetId)
if info.IsLayered then
print("\tAccessory is layered! Order is:", assert(info.Order))
end
end
Metatable Types and Object Oriented-ish Modules!
Metatables can facilitate artificial object-oriented classes, and they are supported in Luau’s type annotations! The syntax for it is a little strange, but the benefits of it outweigh the quirkiness by a long shot.
Here’s a simple Person ModuleScript example to start with:
--!strict
local Person = {}
Person.__index = Person
export type Class = typeof(setmetatable({} :: {
FirstName: string,
LastName: string,
}, Person))
function Person.new(firstName: string, lastName: string): Class
return setmetatable({
FirstName = firstName,
LastName = lastName,
}, Person)
end
function Person.GetFullName(self: Class): string
return `{self.FirstName} {self.LastName}`
end
return Person
There’s a LOT to unpack here, so here’s all the technical details if you’re interested to know more:
-
Person.__index = Persondefines a fallback table to search against when usingPersonas a metatable.- When we create an “instance” of the Person type using
Person.new, this will allow us to callperson:GetFullName(), because it will look for that function in thePersontable!
- When we create an “instance” of the Person type using
-
export typemeans the declared type can be imported into other scripts that require the module.- It would be defined something along the lines of…
local Person = require(script.Parent.Person)type Person = Person.Class
- It would be defined something along the lines of…
-
type Class = typeof(setmetatable({} :: {is the beginning of a metatable type declaration!- The
typeoffunction behaves differently in Luau’s type declaration syntax. Instead of being executed like a Luau function, Luau will instead evaluate the expression in its parenthesis as a Luau type.- This is useful for aliasing types with special behavior that can’t be defined with Luau syntax alone (i.e. concrete types, Roblox instances tied to the
DataModel, and the magic type returned bysetmetatable)
- This is useful for aliasing types with special behavior that can’t be defined with Luau syntax alone (i.e. concrete types, Roblox instances tied to the
- We typecast an empty table as a dictionary containing the fields that are required to be defined when creating an “instance” of the
Classtype throughPerson.new- This is allowed because of some internal Luau jank.
setmetatable’s first argument type is a magic “generic table” type that is allowed to be typecasted into any table type, regardless of its contents. As such, there’s no reason to put any fields into the empty table since it’s better to define the fields with their respective types explicitly.
- This is allowed because of some internal Luau jank.
- The evaluated return type of
setmetatableis a magic type that tags the provided table type with the provided metatable type, andtypeofscopes this all back into a type.
- The
-
function Person.GetFullName(self: Class): string- In order to use Luau’s
:member function syntax sugar,selfhas to be explicitly annotated with theClasstype we’ve created.- This resolves ambiguity that Luau has regarding what the type of
selfis, at the cost of needing to define the function with a.instead of a:.
- This resolves ambiguity that Luau has regarding what the type of
- There is an alternate way described in Luau’s official typechecking documentation (see the
AccountImpltype) but it requires you to manually define a type for the metatable which I personally don’t think is as ergonomic as this strategy.
- In order to use Luau’s
With all of that out of the way, now you can require this Person module from another script, import its Class type, and use it in both functions and dictionary types as we see fit!
Here’s a super lazy example of a School using the Person type we created:
--!strict
local Person = require(script.Parent.Person)
type Person = Person.Class
type School = {
Name: string,
Principal: Person,
Teachers: { Person },
Students: { Person },
}
local function addTeacher(school: School, student: Person)
table.insert(school.Teachers, student)
end
local function addStudent(school: School, student: Person)
table.insert(school.Students, student)
end
local coolSchool: School = {
Name = "Hella Cool School",
Principal = Person.new("John", "Doe"),
Teachers = {},
Students = {},
}
local janeDoe = Person.new("Jane", "Doe")
addStudent(coolSchool, janeDoe)
local coolTeacher = Person.new("Cool", "Teacher")
addTeacher(coolSchool, coolTeacher)
----------------------------------------------------------------------------
local function printSchoolInfo(school: School)
print("School Name:", school.Name)
print("Principal:", school.Principal:GetFullName())
print("Teachers:")
for i, teacher in school.Teachers do
print(`\t{teacher:GetFullName()}`)
end
print("Students:")
for i, student in school.Students do
print(`\t{student:GetFullName()}`)
end
end
printSchoolInfo(coolSchool)
----------------------------------------------------------------------------
Typecasting
So what was the deal with that :: operator earlier? This is a feature of Luau called typecasting, which allows you to override what the automatically inferred type of a value is.
Typecasts are allowed if the provided type can be converted into the target type. There are a few rules and points to be made of this:
- All types can be casted into
any/unknown/never. Likewise,any/unknown/nevercan be casted into any type.- Do this with caution understanding what you’re trying to do, because this effectively bypasses Luau’s type contracting and puts the responsibility onto you and whoever may maintain your code in the future to ensure it complies.
- Untyped empty tables can be casted into any indexer table.
- As soon as Luau gets a contextual clue to what the type is, the type will become inferred and cannot be inferred as another type.
- Concrete types can be casted up into their base classes (i.e.
BasePart→PVInstance→Instance), but not the other way around.
Use Cases
Inline Typing
Consider the following untyped definition:
local data = {
Kills = {},
Deaths = {},
}
If we want to convert this to type annotations, we could declare the type explicitly like this:
type PlayersData = {
Kills: {
[Player]: number
},
Deaths: {
[Player]: number
},
}
local data: PlayersData = {
Kills = {},
Deaths = {},
}
This is fine, but we might only use this PlayersData type annotation once for the data variable, so we could just inline the type instead of defining a type alias for it:
local data: {
Kills: {
[Player]: number
},
Deaths: {
[Player]: number
},
} = {
Kills = {},
Deaths = {},
}
As you can see though… this looks a little bit rough. It’s not very hygenic to have a multi-line type inline like this.
This is where typecasting empty tables comes in handy:
local data = {
Kills = {} :: {
[Player]: number
},
Deaths = {} :: {
[Player]: number
},
}
Now we effectively have the same type behavior, but in a more compact and readable structure on-site!
Unsafe Dynamic Access
At the moment, Luau warns when trying to perform dynamic table indexing on concrete types. If you absolutely know what you’re doing is fine, you can use a cast to any to get around this.
For example:
local bodyColors = Instance.new("BodyColors")
for i, bodyPart in Enum.BodyPart:GetEnumItems() do
(bodyColors :: any)[`{bodyPart.Name}Color3`] = Color3.new(1, 0, 0)
end
That’s all (for now) folks!
This isn’t every aspect of Luau, but it’s a lot of the core stuff I desperately felt was in need of a full proper best-practices tutorial. I’ll definitely update this more in the future if people would like to see more areas covered (such as generics and type unioning).
Feel free to check out Roblox’s official typechecking documentation as well for coverage of additional things I may not have covered here yet: Type checking - Luau
If it’s any help, I also have a few open source projects and modules that are fully --!strict
Feel free to check them out:
- Character Realism
- BinarySearchTree
- Terrain Generator
- Parallel Worker
- sm64-roblox
- Quaternion
- Moonlite
- Ragdoll
- Region
- Octree
- Bitbuf
If you have any questions or addendums, feel free to reply. I’ll keep an eye on this post so long as there’s interest.
Have fun!
. P.S I’m all for static typing, but I’m not super sure why there is such a push to add complex static typing to a dynamic language - do you want the autocomplete that badly? I don’t want to have to learn type theory to understand someone else’s code.
. Still think types are very important everywhere else