Type checking

Luau supports a gradual type system through the use of type annotations and type inference.

Type inference modes

There are three modes currently available. They must be annotated on the top few lines among the comments.

  • --!nocheck,
  • --!nonstrict (default), and
  • --!strict

nocheck mode will simply not start the type inference engine whatsoever.

As for the other two, they are largely similar but with one important difference: in nonstrict mode, we infer any for most of the types if we couldn’t figure it out early enough. This means that given this snippet:

local foo = 1

We can infer foo to be of type number, whereas the foo in the snippet below is inferred any:

local foo
foo = 1

However, given the second snippet in strict mode, the type checker would be able to infer number for foo.

Unknown symbols

Consider how often you’re likely to assign a new value to a local variable. What if you accidentally misspelled it? Oops, it’s now assigned globally and your local variable is still using the old value.

local someLocal = 1

soeLocal = 2 -- the bug

print(someLocal)

Because of this, Luau type checker currently emits an error in strict mode; use local variables instead.

Structural type system

Luau’s type system is structural by default, which is to say that we inspect the shape of two tables to see if they are similar enough. This was the obvious choice because Lua 5.1 is inherently structural.

type A = {x: number, y: number, z: number?}
type B = {x: number, y: number, z: number}

local a1: A = {x = 1, y = 2}        -- ok
local b1: B = {x = 1, y = 2, z = 3} -- ok

local a2: A = b1 -- ok
local b2: B = a1 -- not ok

Primitive types

Lua VM supports 8 primitive types: nil, string, number, boolean, table, function, thread, and userdata. Of these, table and function are not represented by name, but have their dedicated syntax as covered in this syntax document, and userdata is represented by concrete types; other types can be specified by their name.

Additionally, we also have any which is a special built-in type. It effectively disables all type checking, and thus should be used as last resort.

local s = "foo"
local n = 1
local b = true
local t = coroutine.running()

local a: any = 1
print(a.x) -- Type checker believes this to be ok, but crashes at runtime.

There’s a special case where we intentionally avoid inferring nil. It’s a good thing because it’s never useful for a local variable to always be nil, thereby permitting you to assign things to it for Luau to infer that instead.

local a
local b = nil

Function types

Let’s start with something simple.

local function f(x) return x end

local a: number = f(1)     -- ok
local b: string = f("foo") -- ok
local c: string = f(true)  -- not ok

In strict mode, the inferred type of this function f is <A>(A) -> A (take a look at generics), whereas in nonstrict we infer (any) -> any. We know this is true because f can take anything and then return that. If we used x with another concrete type, then we would end up inferring that.

Similarly, we can infer the types of the parameters with ease. By passing a parameter into anything that also has a type, we are saying “this and that has the same type.”

local function greetingsHelper(name: string)
    return "Hello, " .. name
end

local function greetings(name)
    return greetingsHelper(name)
end

print(greetings("Alexander")          -- ok
print(greetings({name = "Alexander"}) -- not ok

Table types

From the type checker perspective, each table can be in one of three states. They are: unsealed table, sealed table, and generic table. This is intended to represent how the table’s type is allowed to change.

Unsealed tables

An unsealed table is a table whose properties could still be tacked on. This occurs when the table constructor literal had zero expressions. This is one way to accumulate knowledge of the shape of this table.

local t = {} -- {}
t.x = 1      -- {x: number}
t.y = 2      -- {x: number, y: number}

However, if this local were written as local t: {} = {}, it ends up sealing the table, so the two assignments henceforth will not be ok.

Furthermore, once we exit the scope where this unsealed table was created in, we seal it.

local function vec2(x, y)
    local t = {}
    t.x = x
    t.y = y
    return t
end

local v2 = vec2(1, 2)
v2.z = 3 -- not ok

Sealed tables

A sealed table is a table that is now locked down. This occurs when the table constructor literal had 1 or more expression, or when the table type is spelt out explicitly via a type annotation.

local t = {x = 1} -- {x: number}
t.y = 2           -- not ok

Generic tables

This typically occurs when the symbol does not have any annotated types or were not inferred anything concrete. In this case, when you index on a parameter, you’re requesting that there is a table with a matching interface.

local function f(t)
    return t.x + t.y
           --^   --^ {x: _, y: _}
end

f({x = 1, y = 2})        -- ok
f({x = 1, y = 2, z = 3}) -- ok
f({x = 1})               -- not ok

Table indexers

These are particularly useful for when your table is used similarly to an array.

local t = {"Hello", "world!"} -- {[number]: string}
print(table.concat(t, ", "))

Luau supports a concise declaration for array-like tables, {T} (for example, {string} is equivalent to {[number]: string}); the more explicit definition of an indexer is still useful when the key isn’t a number, or when the table has other fields like { [number]: string, n: number }.

Generics

The type inference engine was built from the ground up to recognize generics. A generic is simply a type parameter in which another type could be slotted in. It’s extremely useful because it allows the type inference engine to remember what the type actually is, unlike any.

type Array<T> = {[number]: T}

local strings: Array<string> = {"Hello", "world!"}
local numbers: Array<number> = {1, 2, 3, 4, 5, 6}

Union types

A union type represents one of the types in this set. If you try to pass a union onto another thing that expects a more specific type, it will fail.

For example, what if this string | number was passed into something that expects number, but the passed in value was actually a string?

local stringOrNumber: string | number = "foo"

local onlyString: string = stringOrNumber -- not ok
local onlyNumber: number = stringOrNumber -- not ok

Note: it’s impossible to be able to call a function if there are two or more function types in this union.

Intersection types

An intersection type represents all of the types in this set. It’s useful for two main things: to join multiple tables together, or to specify overloadable functions.

type XCoord = {x: number}
type YCoord = {y: number}
type ZCoord = {z: number}

type Vector2 = XCoord & YCoord
type Vector3 = XCoord & YCoord & ZCoord

local vec2: Vector2 = {x = 1, y = 2}        -- ok
local vec3: Vector3 = {x = 1, y = 2, z = 3} -- ok
type SimpleOverloadedFunction = ((string) -> number) & ((number) -> string)

local f: SimpleOverloadedFunction

local r1: number = f("foo") -- ok
local r2: number = f(12345) -- not ok
local r3: string = f("foo") -- not ok
local r4: string = f(12345) -- ok

Note: it’s impossible to create an intersection type of some primitive types, e.g. string & number, or string & boolean, or other variations thereof.

Note: Luau still does not support user-defined overloaded functions. Some of Roblox and Lua 5.1 functions have different function signature, so inherently requires overloaded functions.

Typing idiomatic OOP

One common pattern we see throughout Roblox is this OOP idiom. A downside with this pattern is that it does not automatically create a type binding for an instance of that class, so one has to write type Account = typeof(Account.new("", 0)).

local Account = {}
Account.__index = Account

function Account.new(name, balance)
    local self = {}
    self.name = name
    self.balance = balance

    return setmetatable(self, Account)
end

function Account:deposit(credit)
    self.balance += credit
end

function Account:withdraw(debit)
    self.balance -= debit
end

local account: Account = Account.new("Alexander", 500)
             --^^^^^^^ not ok, 'Account' does not exist

Type refinements

When we check the type of a value, what we’re doing is we’re refining the type, hence “type refinement.” Currently, the support for this is somewhat basic.

Using type comparison:

local stringOrNumber: string | number = "foo"

if type(x) == "string" then
    local onlyString: string = stringOrNumber -- ok
    local onlyNumber: number = stringOrNumber -- not ok
end

local onlyString: string = stringOrNumber -- not ok
local onlyNumber: number = stringOrNumber -- not ok

Using truthy test:

local maybeString: string? = nil

if maybeString then
    local onlyString: string = maybeString -- ok
end

And using assert will work with the above type guards:

local stringOrNumber: string | number = "foo"

assert(type(x) == "string")

local onlyString: string = stringOrNumber -- ok
local onlyNumber: number = stringOrNumber -- not ok

Roblox types

Roblox supports a rich set of classes and data types, documented here. All of them are readily available for the type checker to use by their name (e.g. Part or RaycastResult).

When one type inherits from another type, the type checker models this relationship and allows to cast a subclass to the parent class implicitly, so you can pass a Part to a function that expects an Instance.

All enums are also available to use by their name as part of the Enum type library, e.g. local m: Enum.Material = part.Material.

Finally, we can automatically deduce what calls like Instance.new and game:GetService are supposed to return:

local part = Instance.new("Part")
local basePart: BasePart = part

Note that many of these types provide some properties and methods in both lowerCase and UpperCase; the lowerCase variants are deprecated, and the type system will ask you to use the UpperCase variants instead.

Module interactions

Let’s say that we have two modules, Foo and Bar. Luau will try to resolve the paths if it can find any require in any scripts. In this case, when you say script.Parent.Bar, Luau will resolve it as: relative to this script, go to my parent and get that script named Bar.

-- Module Foo
local Bar = require(script.Parent.Bar)

local baz1: Bar.Baz = 1     -- not ok
local baz2: Bar.Baz = "foo" -- ok

print(Bar.Quux)         -- ok
print(Bar.FakeProperty) -- not ok

Bar.NewProperty = true -- not ok
-- Module Bar
export type Baz = string

local module = {}

module.Quux = "Hello, world!"

return module

There are some caveats here though. For instance, the require path must be resolvable statically, otherwise Luau cannot accurately type check it.