luau/docs/_pages/syntax.md

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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 more features; Luau does support string literal extensions but does not support other 5.x additions; for details please refer to compatibility section.

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

String literals

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

• \xAB inserts a character with the code 0xAB into the string
• \u{ABC} inserts a UTF8 byte sequence that encodes U+0ABC character into the string (note that braces are mandatory)
• \z at the end of the line inside a string literal ignores all following whitespace including newlines, which can be helpful for breaking long literals into multiple lines.

Number literals

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

• Hexadecimal integer literals, 0xABC or 0XABC
• Binary integer literals, 0b01010101 or 0B01010101
• Decimal separators in all integer literals, using _ for readability: 1_048_576, 0xFFFF_FFFF, 0b_0101_0101

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

In addition, the type of any expression can be overridden using a type cast :::

local k = (y :: string):rep(x)

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

To return no values or more than one, you need to wrap the return type position with parentheses, and then list your types there.

local no_returns: (number, string) -> ()
local returns_boolean_and_string: (number, string) -> (boolean, string)

function foo(x: number, y: number): (number, string)
    return x + y, tostring(x) .. tostring(y)
end

Note that function types are specified without the argument names in the examples above, but it's also possible to specify the names (that are not semantically significant but can show up in documentation and autocomplete):

local callback: (errorCode: number, errorText: string) -> ()

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 }

An exported type can be used in another module by prefixing its name with the require alias that you used to import the module.

local M = require(Other.Module)

local a: M.Point = {x=5, y=6}

For more information please refer to typechecking documentation.

If-then-else expressions

In addition to supporting standard if statements, Luau adds support for if expressions. Syntactically, if-then-else expressions look very similar to if statements. However instead of conditionally executing blocks of code, if expressions conditionally evaluate expressions and return the value produced as a result. Also, unlike if statements, if expressions do not terminate with the end keyword.

Here is a simple example of an if-then-else expression:

local maxValue = if a > b then a else b

if-then-else expressions may occur in any place a regular expression is used. The if-then-else expression must match if <expr> then <expr> else <expr>; it can also contain an arbitrary number of elseif clauses, like if <expr> then <expr> elseif <expr> then <expr> else <expr>. Note that in either case, else is mandatory.

Here's is an example demonstrating elseif:

local sign = if x < 0 then -1 elseif x > 0 then 1 else 0

Note: In Luau, the if-then-else expression is preferred vs the standard Lua idiom of writing a and b or c (which roughly simulates a ternary operator). However, the Lua idiom may return an unexpected result if b evaluates to false. The if-then-else expression will behave as expected in all situations.

Generalized iteration

Luau uses the standard Lua syntax for iterating through containers, for vars in values, but extends the semantics with support for generalized iteration. In Lua, to iterate over a table you need to use an iterator like next or a function that returns one like pairs or ipairs. In Luau, you can simply iterate over a table:

for k, v in {1, 4, 9} do
    assert(k * k == v)
end

This works for tables but can also be extended for tables or userdata by implementing __iter metamethod that is called before the iteration begins, and should return an iterator function like next (or a custom one):

local obj = { items = {1, 4, 9} }
setmetatable(obj, { __iter = function(o) return next, o.items end })

for k, v in obj do
    assert(k * k == v)
end

The default iteration order for tables is specified to be consecutive for elements 1..#t and unordered after that, visiting every element; similarly to iteration using pairs, modifying the table entries for keys other than the current one results in unspecified behavior.