[Rd] is.environment, as.environment, and NULL

[Prof Brian Ripley]

On Tue, 28 Aug 2001, Kurt Hornik wrote:

> >>>>> Peter Dalgaard BSA writes:
>
> > John Chambers <jmc@research.bell-labs.com> writes:
> >> There isn't a completely consistent solution, given the use of NULL as
> >> an environment.  For the moment it seemed best to live with that one
> >> anomalous result in the tests.
>
> > The use of NULL for the base environment is something which I would
> > like to see go away anyway. It makes little sense semantically (if
> > anything, you would want the NULL environment to be an empty one), and
> > there are pitfalls where a NULL value returned by accident can cause
> > code to work in base but not in a package.
>
> > I did start to look into it at some point but ran out of stamina or
> > got distracted (probably by release timings as usual...). As I recall
> > it, NULL for base envir is only really used in a handful of places as
> > a sentinel for searches through the search path, as in:
>
> >     for (rho = R_GlobalEnv; rho != R_NilValue; rho = ENCLOS(rho)) ..
>
> > The base environment is special because the symbol lookups are
> > different there, but it could be represented by R_BaseEnv or some such
> > instead of R_NilValue. I doubt that much interpreted code needs to
> > explicitly test whether an environment is the base environment. One
> > thing to watch out for might be saved workspaces, though. On the other
> > hand, we do not save the search path, so maybe not...
>
> A question, and a remark.
>
> Question, would we want to keep as.environment(NULL) to give the base
> package, so that code like ls(envir = NULL) will still work?

Interesting.  Was it ever documented to?

> Remark, we once discussed making package environments (in particular for
> base) read-only.  One way of doing this is via setting attributes, but
> we cannot do this on NULL.  Not that I think this is the way to do it,
> though ...

NULL is just shorthand: there is a proper R object for base:

> .GlobalEnv
<environment: R_GlobalEnv>
> is.null(.GlobalEnv)
[1] FALSE

It was news to me that NULL for .GlobalEnv worked at R level.

Brian

-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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