[Rd] How to understand packages, namespaces, environments

[Prof Brian Ripley]

On Fri, 6 May 2005, Alexander Stoddard wrote:

> I would be very glad of pointers to information on how the concepts of 
> packages, namespaces and environments are interrelated in R.
>
> I am trying to get a handle on this both so I can delve further into 
> understanding other people's code and so I can organize my own in a more 
> coherent manner.
>
> From my reading about environments it seems they function as what I would 
> intuitively call namespaces. However, the documentation for the 'library' 
> function implies that "namespace" has a specific meaning in R that I have so 
> far failed to grasp. What is that meaning (or where should I look to read up 
> on it) ?

Look in `Writing R Extensions' and on developer.r-project.org.  I don't 
know about `intuitively' but they are similar to other systems (e.g. 
Perl's) namespaces.

More user-level documentation of namespaces (e.g. in R-lang) is on the 
WOULD-BE-NICE list, but there is enough in `Writing R Extensions' for many 
package writers and we are talking about more details of what happens 
under the skin.

> I can think of the following more specific questions. Perhaps they may most 
> usefully serve to reveal my misconceptions, so corrections would be very 
> helpful.

It seems you have revealed that you have not studied the manuals nor the 
help pages.  Please do so before posting (see the posting guide).

> Does saying the following load package 'foo' into its own environment ?
>> library(foo)
>
> Do environments have names?

Not in general.  You will see <environment: some-hex-address> printed 
quite often.  I am not at all sure you have grasped what an environment is 
in R: see the R-lang manual.  (Look it up in the index, as at present the 
term is used before it is defined.)

> Of what does the list returned by search() actually consist? Is it a list of 
> environments, a list that may include environments, or something else?

search() does not return a list: it returns a character vector.  It seems 
you have not read its help page, which tells you that quite explicitly.

> What is actually designated by a character vector of form "package:foo"?

That is actually a character string, not a character vector.  It is a name 
that can be used to refer to attach'ed objects, as the search() help page 
says.

> In what ways can I use the character vector "package:foo" when interacting 
> with R?

I am aware of attach(), detach() and as.environment() (which is used by 
quite a few functions, e.g. get()).  (Asking what you can do it an 
extensible system such as R is not very helpful.)  Generally it is a name 
used when a package or another attach'ed object might be meant: it is not 
used if a package is all that can be specified.

> Many thanks,
> Alex Stoddard
>
> P.S. It took me a long time of flailing about to discover the 'search' 
> function. Perhaps it could be included in the "See Also" section of the help 
> for 'ls'.

This *is* in `An Introduction to R', something we ask all R users to read.

I (and I am biased) suggest the right way to get to grips with things like 
this is to read a good book on the S/R language.

-- 
Brian D. Ripley,                  ripley at 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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595