Editing Lua (section)

=== Functions ===
Lua's treatment of functions as [[first-class function|first-class]] values is shown in the following example, where the print function's behavior is modified:
<syntaxhighlight lang="lua">
do
  local oldprint = print
  -- Store current print function as oldprint
  function print(s)
    --[[ Redefine print function. The usual print function can still be used
      through oldprint. The new one has only one argument.]]
    oldprint(s == "foo" and "bar" or s)
  end
end
</syntaxhighlight>
Any future calls to <code>print</code> will now be routed through the new function, and because of Lua's [[Scope (programming)#Lexical scoping|lexical scoping]], the old print function will only be accessible by the new, modified print.

Lua also supports [[Closure (computer programming)|closures]], as demonstrated below:
<syntaxhighlight lang="lua">
function addto(x)
  -- Return a new function that adds x to the argument
  return function(y)
    --[[ When we refer to the variable x, which is outside the current
      scope and whose lifetime would be shorter than that of this anonymous
      function, Lua creates a closure.]]
    return x + y
  end
end
fourplus = addto(4)
print(fourplus(3)) -- Prints 7

--This can also be achieved by calling the function in the following way:
print(addto(4)(3))
--[[ This is because we are calling the returned function from 'addto(4)' with the argument '3' directly.
  This also helps to reduce data cost and up performance if being called iteratively.]]
</syntaxhighlight>
A new closure for the variable <code>x</code> is created every time <code>addto</code> is called, so that each new anonymous function returned will always access its own <code>x</code> parameter. The closure is managed by Lua's garbage collector, just like any other object.