Re: setfenv on userdata

[Rici Lake <lua@...>]

On 23-Aug-05, at 6:51 PM, David Burgess wrote:

> Hi Rici,
>
> I am obviously missing something. The way I read it, each object has
> an environment (LUA_ENVIRONINDEX) that by default points at
> the global environment (LUA_GLOBALSINDEX). Am I wrong?

There is no global environment in 5.1. By default it gets set to the 
current environment, which is the environment of the currently running 
function.

> Any table can then be set as the environment that can
> replace the environment for an object.  Yes?

Yes, indeed. But you cannot set "no table" as the environment for an 
object.

Consider the following case: I bind myFancyWidget to a Lua userdata and 
export it into the Lua environment.

The Lua script may want to override some method in a particular 
instance of MyFancyWidget (to make it fancier, maybe :). Now, it could 
create an entire new object to do that, but it would be a lot simpler 
to do this:

function overrideCtlY(widget)
  local oldDoKeyPress = myFancyWidget.doKeyPress
  function widget:doKeyPress(key)
    if key == "ctl-y" then
      -- handle control y the way I want to
    else
      return oldDoKeyPress(widget, key)
    end
  end
  return widget
end

local widget = overrideCtlY(MyFancyWidget.new())

....

If I want to allow the Lua script to do that, then I need to have a 
place to store the overloaded doKeyPress member function. I can't store 
it in the standard metatable; that would apply to all instances. 
Logically, I should store it in the widget's environment table, since 
that is local to the widget instance.

In the common case, of course, no methods are overridden. So I don't 
want an environment table at all; I want the method lookup to go 
directly to the metatable. If I can't set the environment table to nil, 
then I have to set it to some sentinel and test for that on every 
lookup. So I was looking for something that:

a) semantically corresponded to (my) expected use of environment tables.

b) involved fewer API calls on common operations.