--- permalink: /typecheck title: Type checking toc: true --- 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: ```lua local foo = 1 ``` We can infer `foo` to be of type `number`, whereas the `foo` in the snippet below is inferred `any`: ```lua 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. ```lua 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. ```lua 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](syntax), and `userdata` is represented by [concrete types](#Roblox-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. ```lua 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. ```lua local a local b = nil ``` ## Function types Let's start with something simple. ```lua 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` (take a look at [generics](#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." ```lua 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. ```lua 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. ```lua 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. ```lua 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. ```lua 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. ```lua 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`. ```lua type Array = {[number]: T} local strings: Array = {"Hello", "world!"} local numbers: Array = {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`? ```lua 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. ```lua 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 ``` ```lua 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))`. ```lua 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: ```lua 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: ```lua local maybeString: string? = nil if maybeString then local onlyString: string = maybeString -- ok end ``` And using `assert` will work with the above type guards: ```lua 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](https://developer.roblox.com/en-us/api-reference). 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: ```lua 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. ```lua -- 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 ``` ```lua -- 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.