Luau

A fast, small, safe, gradually typed embeddable scripting language derived from Lua


Why Luau?
Syntax
Sandboxing
Compatibility
Linting
Type checking
Performance


Syntax

Luau uses the baseline syntax of Lua 5.1. For detailed documentation, please refer to the Lua manual, this is an example:

local function tree_insert(tree, x)
    local lower, equal, greater = split(tree.root, x)
    if not equal then
        equal = {
            x = x,
            y = math.random(0, 2^31-1),
            left = nil,
            right = nil
        }
    end
    tree.root = merge3(lower, equal, greater)
end

Note that future versions of Lua extend the Lua 5.1 syntax with the following features; with the exception of the string literals, these are not supported by Luau:

For details please refer to compatibility section.

The rest of this document documents additional syntax used in Luau.

String literals

As noted above, Luau implements support for hexadecimal (\0x), Unicode (\u) and \z escapes for string literals. This syntax follows Lua 5.3 syntax:

Number literals

In addition to basic integer and floating-point decimal numbers, Luau supports:

Note that Luau only has a single number type, a 64-bit IEEE754 double precision number (which can represent integers up to 2^53 exactly), and larger integer literals are stored with precision loss.

Continue statement

In addition to break in all loops, Luau supports continue statement. Similar to break, continue must be the last statement in the block.

Note that unlike break, continue is not a keyword. This is required to preserve backwards compatibility with existing code; so this is a continue statement:

if x < 0 then
    continue
end

Whereas this is a function call:

if x < 0 then
    continue()
end

When used in repeat..until loops, continue can not skip the declaration of a local variable if that local variable is used in the loop condition; code like this is invalid and won’t compile:

repeat
    do continue end
    local a = 5
until a > 0

Compound assignments

Luau supports compound assignments with the following operators: +=, -=, *=, /=, %=, ^=, ..=. Just like regular assignments, compound assignments are statements, not expressions:

-- this works
a += 1

-- this doesn't work
print(a += 1)

Compound assignments only support a single value on the left and right hand side; additionally, the function calls on the left hand side are only evaluated once:

-- calls foo() twice
a[foo()] = a[foo()] + 1

-- calls foo() once
a[foo()] += 1

Compound assignments call the arithmetic metamethods (__add et al) and table indexing metamethods (__index and __newindex) as needed - for custom types no extra effort is necessary to support them.

Type annotations

To support gradual typing, Luau supports optional type annotations for variables and functions, as well as declaring type aliases.

Types can be declared for local variables, function arguments and function return types using : as a separator:

function foo(x: number, y: string): boolean
    local k: string = y:rep(x)
    return k == "a"
end

There are several simple builtin types: any (represents inability of the type checker to reason about the type), nil, boolean, number, string and thread.

Function types are specified using the arguments and return types, separated with ->:

local foo: (number, string) -> boolean

Table types are specified using the table literal syntax, using : to separate keys from values:

local array: { [number] : string }
local object: { x: number, y: string }

When the table consists of values keyed by numbers, it’s called an array-like table and has a special short-hand syntax, {T} (e.g. {string}).

Additionally, the type syntax supports type intersections (((number) -> string) & ((boolean) -> string)) and unions ((number | boolean) -> string). An intersection represents a type with values that conform to both sides at the same time, which is useful for overloaded functions; a union represents a type that can store values of either type - any is technically a union of all possible types.

It’s common in Lua for function arguments or other values to store either a value of a given type or nil; this is represented as a union (number | nil), but can be specified using ? as a shorthand syntax (number?).

In addition to declaring types for a given value, Luau supports declaring type aliases via type syntax:

type Point = { x: number, y: number }
type Array<T> = { [number]: T }
type Something = typeof(string.gmatch("", "\d"))

The right hand side of the type alias can be a type definition or a typeof expression; typeof expression doesn’t evaluate its argument at runtime.

By default type aliases are local to the file they are declared in. To be able to use type aliases in other modules using require, they need to be exported:

export type Point = { x: number, y: number }

For more information please refer to typechecking documentation.