Re: setfenv (was Re: tuples)

["Rici Lake" <lua@...>]

Mark Hamburg said:
> on 9/15/05 10:42 AM, Rici Lake at lua@ricilake.net wrote:
>
>> Lua functions are
>> actually mutable objects -- you can call setfenv on them
>
> Which is an annoyance in a variety of ways.
>
> 1. It cries out for a way to duplicate a function so that you can set the
> environment for the duplicate without changing the original.

Agreed.

> An alternative to setfenv which would be less powerful but would avoid
> these
> issues would be to provide the environment as an argument to the chunk
> compiler.

That's certainly worth thinking about. Another possibility, which Wim and
I discussed at some length a couple of years ago (and which recently
resurfaced, I believe) is the "in <expr> do ... end" statement, which
makes <expr> the table for unbound variable lookup within the syntactic
scope of the do...end clause.

This allows you to create a wrapper which is quite flexible:

function(env)
  in env do
    return function()
      -- insert code here
    end
  end
end

Now every time this factory is executed, it creates a new instance of the
embedded code with a different environment.

You could also create objects with mutable environments:

function(env)
  in env do
    return {
      setenv = function(e) env = e end,
      call = function()
        -- insert code here
    }
  end
end

This seems to capture a wide variety of use cases while still permitting
that:

> ... it might be nice to fully embrace the mutability of
                                       ^^ immutability, no?
> functions and provide a way to duplicate them.
>
> Mark
>