Re: Lua Basics: function environments and function parameters?

[Rici Lake <lua@...>]

On 25-Aug-05, at 3:33 PM, William Trenker wrote:

> I've been experimenting with introspection in Lua (5.0.2).  If I
> understand correctly, a function's parameters are not included in that
> function's environment table.  For example:

That's correct. They are local variables.

> What I find curious is what happens when I redfine the function in the
> same chunk:

You've set p in the global environment.

The environment table is just a table; when a function closure is 
created, it is created with the same environment table as its creator 
(*not* a copy of the table):

> ------------------------ chunk -----------------------
> function f(p)  -- same as previous chunk
>     local env = getfenv(1)
-- env is now the global environment
>     setmetatable(env, {__index=_G})
>     env.p = p
-- so this sets p in the global environment
>     setfenv(1, env)
>     print(string.format("<%s>: p = %s", tostring(f),p))
>     print(string.format("<%s>: getfenv(1).p = %s",tostring(f),
> tostring(getfenv(1).p)))
> end

What you probably wanted to do was:

function f(p)
  local env = {}  -- a new table
  setmetatable(env, {__index = _G})
    -- now the global environment is the "backup" for the new table
  env.p = p
  setfenv(1, env)
  -- now the function has a different environment table
  -- ...
end

The following is identical but maybe not so easy to read:

function f(p)
  setfenv(1, setmetatable({p = p}, {__index = _G}))
  -- ...
end