This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
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16.5 – The Single-Method Approach

A particular case of the previous approach for OO programming occurs when an object has a single method. In such cases, we do not need to create an interface table; instead, we can return this single method as the object representation. If this sounds a little weird, it is worth remembering Section 7.1, where we saw how to construct iterator functions that keep state as closures. An iterator that keeps state is nothing more than a single-method object.

Another interesting case of single-method objects occurs when this single-method is actually a dispatch method that performs different tasks based on a distinguished argument. A possible implementation for such object is as follows:

    function newObject (value)
      return function (action, v)
        if action == "get" then return value
        elseif action == "set" then value = v
        else error("invalid action")
Its use is straightforward:
    d = newObject(0)
    print(d("get"))    --> 0
    d("set", 10)
    print(d("get"))    --> 10

This unconventional implementation for objects is quite effective. The syntax d("set",10), although peculiar, is only two characters longer than the more conventional d:set(10). Each object uses one single closure, which is cheaper than one table. There is no inheritance, but we have full privacy: The only way to access an object state is through its sole method.

Tcl/Tk uses a similar approach for its widgets. The name of a widget in Tk denotes a function (a widget command) that can perform all kinds of operations over the widget.