How does the three dots parameter work in Lua?

October 21, 2024

A function in Lua can accept any number of arguments from zero to any amount. There might be a case where there is no exact number of arguments; string.format, from the standard library, is a good example of such a function. Where the first argument is format string with specifiers and after follow variables corresponding to the specifiers. The signature of this function is string.format(formatstring, ...). Such a function is called variadic, which means that the number of parameters is not determined. Variadic function is indicated by three dots ..., called vararg expression.

Consider:

-- no arguments
local function sayHello()
    return "Hello"
end

-- 2 arguments
local function sum2(a, b)
    return a + b
end

-- variadic function with undetermined number of arguments
local function sum(...)
    -- sum all arguments
end

vararg expression (...) contains a list of arguments, vararg expression always should be the last in the argument list.

local function sum(a, b, ...)
    print(a, b, ...)
end

sum(1, 2, 3, 4, 5) --> 1    2   3   4   5

Making a table from vararg expression using table constructor {}:

local function sum(a, b, ...)
    local t = {a, b, ...}
    print(t, #t)
end

sum(1, 2, 3, 4, 5) --> table:   0x11c2fc0   5

Warning

Here is the caveat: if in varag expression one of the values is nil, then {...} might not be a sequence. To avoid this, there is a function from the table module: table.pack.

lua function variadicFn(...) local args = table.pack(...) --[[ rest of code ]] end

Also, consider that table constructor {} works much faster than table.pack. I would recommend making a habit, always avoid nil values in the Lua tables.

Finally, let’s finish our sum.

local function sum(a, b, ...)
    local t = {a, b, ...}
    local s = 0
    for i = 1, #t do
        s = s + t[i]
    end
  return s
end

print(sum(1, 2, 3, 4, 5)) --> 15

Destruct vararg expression arguments to variables.

local function sum(a, b, ...)
  local c, d, e = ...
  return a + b + c + d + e
end

print(sum(1, 2, 3, 4, 5)) --> 15

In the example above, if any parameter is a non-number, then function will fail because the interpreter will try to sum non-numbers. This example is only for demonstration, in real code type checking and error handling should be done.

Word of warning¶

There might be issues that might be caused by using vararg expression. On lua-users.org there is full article about potential problems with vararg expression. In my daily job, I’d like to avoid such complex solutions, if possible. Code readability and maintainability are always one of the top priorities for me.

Tip

In JavaScript, rest parameters works mostly the same as vararg expression in Lua.

References¶

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