Functions

Named Functions

function add(a: number, b: number): number
    return a + b
end

Local Functions

local function multiply(x: number, y: number): number
    return x * y
end

Method Syntax

function obj:method(x: number): string
    return tostring(x)
end

Anonymous Functions

local square = function(x: number): number
    return x * x
end

table.sort(items, function(a, b) return a < b end)

Inside a table constructor a function field may carry its name directly - see Named Functions in tables:

local ops = { function add(a: number, b: number): number return a + b end }

Default Parameters

function greet(name: string, greeting: string = "Hello"): string
    return greeting .. ", " .. name
end

greet("Alice")              -- "Hello, Alice"
greet("Bob", "Hi")          -- "Hi, Bob"

Default parameters compile to if param == nil then param = default end checks at the start of the function body.

Variadic Functions

Standard Varargs

function printf(fmt: string, ...)
    print(string.format(fmt, ...))
end

Named Varargs

Give a name and type to the variadic parameter:

function sum(...values: number): number
    local total = 0
    for _, v in ipairs({...}) do
        total = total + v
    end
    return total
end

Overloading

Multiple functions with the same name but different parameter counts:

function format(x: number): string
    return tostring(x)
end

function format(x: number, decimals: number): string
    return string.format("%." .. decimals .. "f", x)
end

format(3.14)        -- "3.14"
format(3.14, 1)     -- "3.1"

The compiler generates a single dispatch function that selects the right implementation based on argument count.

Multi-Return

function divide(a: number, b: number): (number, number)
    return math.floor(a / b), a % b
end

local quotient, remainder = divide(10, 3)

Return Statement

function getValue(): number
    return 42
end

-- No return value
function doWork(): void
    print("working")
end

-- Never returns: always raises, exits, or loops forever
function panic(message: string): never
    error("panic: " .. message)
end

A never function may not return a value, and the compiler rejects it if any path can reach its end. Code after a call to one is reported as unreachable. See The never Type.