R devel - Feb 2005 - Environment with no parent?

Is it possible to create an environment that has no parent (or an
empty parent)?  I would have thought

 e <- new.env(parent=NULL)

would work, but it acts as though the parent is the base namespace:
> get("close", envir = e)
function (con, ...) 
UseMethod("close")
<environment: namespace:base>

I can use inherits = FALSE in this case:
> get("close", envir = e, inherits = F)
Error in get(x, envir, mode, inherits) : variable "close" was not
found

but what I want to do is to create my own hierarchy of environments
that allow inheritance from their parents, but which stop when they
get to e, and don't continue on into base.  For example
> assign('x', 1, envir = e)
> 
> f <- new.env(parent = e)
> assign('y', 2, envir = f)
> 
>#  The first two of these work as desired, but I'd like a "not
found" error from the last:
>
> get('y', envir=f)
[1] 2
> get('x', envir=f)
[1] 1
> get('close', envir=f)
function (con, ...) 
UseMethod("close")
<environment: namespace:base>

Looking in envir.c, I see this:

    /* env is now R_NilValue, the base environment */

which doesn't give me much hope, but maybe there's a trick....

If not, would it be reasonable to install a magic "EmptyEnv" to use as
a parent in this sort of situation?

Duncan Murdoch

Duncan Murdoch <murdoch@stats.uwo.ca> writes:
> Looking in envir.c, I see this:
> 
>     /* env is now R_NilValue, the base environment */
> 
> which doesn't give me much hope, but maybe there's a trick....
> 
> If not, would it be reasonable to install a magic "EmptyEnv" to
use as
> a parent in this sort of situation?
> 
I'm fairly sure the answer is "nope".

It's been annoying me for years, for language aesthetic reasons
mostly, but also with some consideration of cases like yours, and I've
been on the brink of implementing a version where the base environment
was a true environment. Apart from the usual issue of "round tuits",
I was held back by the fact that one has to consider at least two things:

(a) efficiency. Is it expensive no longer to have the base functions
bound directly to their symbol? (My gut feeling is that with suitable
hashing and cacheing, the penalty is minimal.)

(b) you can *only* use get and simple variable retrieval in a non-base
environment with a NULL parent (eval(x <- 1, envir=foo) would give
'couldn't find function "<-"' or so). This could cause
some confusion.


-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907