Struct function 

pub struct function;

The type of Lua and C functions.

Functions are first-class values: they can be stored in variables and tables, passed as arguments, and returned from other functions.

A variadic function collects its extra arguments in a vararg table, whose field n holds their count. Since Lua 5.5 that table can be given a name after the three dots, becoming a read-only local:

local function f(...args)
  print(args.n, args[1])
end
f("a", "b")    --> 2   a

local greet = function(name)
  return "hello, " .. name
end
print(greet("world"))