TopolearnFunctions are first-class values in Lua and the main tool for structuring code.
function add(a, b) return a + b end is sugar for assigning a function value to a variable; prefer local function add(a, b) ... end so it doesn't become a global. Functions are values: store them in tables, pass them as arguments, return them.
Arguments are positional. Missing arguments are nil and extra ones are dropped, so a = a or 1 is the usual way to give a default.
A function can return multiple values: return q, r. The caller receives as many as it asks for. The adjustment rule matters: a call returns all its values only when it is the last expression in a list. In the middle, or wrapped in parentheses, it is cut to one value.
Variadic functions use .... Inside, select("#", ...) counts the arguments (including nils) and {...} or table.pack(...) collects them; table.unpack(t) does the reverse, spreading a table into arguments.
When a function takes a single table or string literal, you can drop the parentheses: f{x = 1} and print "hi". The table form is a common way to fake named arguments.
Lua performs proper tail calls: return f(x) reuses the stack frame, so tail-recursive loops never overflow.
Example
local function divmod(a, b)
return a // b, a % b
end
local q, r = divmod(17, 5)
print(q, r) --> 3 2
print(divmod(17, 5), "end") --> 3 end (cut to one value)
print((divmod(17, 5))) --> 3
local function sum(...)
local total = 0
for _, v in ipairs({...}) do total = total + v end
return total, select("#", ...)
end
print(sum(1, 2, 3)) --> 6 3
print(sum(table.unpack({4, 5}))) --> 9 2
local function greet(opts)
local name = opts.name or "world"
return "hello " .. name
end
print(greet{name = "Ana"}) --> hello AnaRemember
- Use
local functionso functions don't become globals - A call expands to all its results only as the last item in a list
...,select("#", ...)andtable.unpackhandle varargsf{...}with a table fakes named arguments
Watch out for
- Expecting
f(), xto keep all of f's return values; only the last call expands.