[Rd] unloadNamespace (Was: How to unload a dll loaded via library.dynam()?)

[Berwin A Turlach]

G'day all,

On Fri, 20 Feb 2009 04:01:07 +0000 (GMT)
Prof Brian Ripley <ripley at stats.ox.ac.uk> wrote:

> library.dynam.unload() does work if the OS is cooperative.  And if
> you have your package set up correctly and unload the namespace (and
> not just detach the package if there is a namespace) then the shared 
> object/DLL will be unloaded. [...]

I guess I have a similar code-install-test development cycle as Alex;
and I seem to work on a cooperative OS (Kubuntu 8.04).

My set up is that I install packages on which I work into a separate
library.  To test changes to such packages, I start R in a directory
which contains a .Rprofile file which, via .libPaths(), adds the above
library to the library path.  In this R session I then test the changes.

I also used to quit and restart R whenever I re-installed a package with
namespace to test the changes made.  Somehow I got the impression that
this was the way to proceed when namespaces were introduced; and I did
not realise until recently that better ways (unloading the namespace)
exist.

However, I noticed the following behaviour under R 2.8.1 and "R version
2.9.0 Under development (unstable) (2009-02-19 r47958)" which I found
surprising:

1) In the running R session, issue the command "unloadNamespace(XXX)"
2) Do changes to the code of the package; e.g. add a "print("hello
   world")" statement to one of the R functions.
3) Install the new package
4) In the running R session, issue the command "library(XXX)" and call
   the R function that was changed.  
   
Result: "Hello world" is not printed, somehow the old R function is
still used.  If I issue the commands "unloadNamespace(XXX)" and
"library(XXX)" once more then a call to the R function that was changed
will print "Hello world"; i.e. the new code is used.

If the above sequence is changed to 2), 3) and then 1), then 4) behaves
"as expected" and the new R code is used immediately.

As far as I can tell, if via the .onUnload() hook the shared object is
unloaded via library.dynam.unload(), changes in the C code take effect
no matter whether I perform the above steps in the sequence 1-2-3-4 or
2-3-1-4.

My preference is to use the sequence 1-2-3-4 since it seems to be the
"more logical and cleaner" sequence; and I have vague memories that I
managed to crash R in the past after using 2-3 and then trying to quit
R.

I am wondering why does it make a difference with respect to R code in
which order these steps are done but not with respect to compiled
code.  Well, I guess I understand why the order does not matter for
compiled code, but I do not understand why the order matters for R
code.  I could not find anything in the documentation that would
explain this behaviour, or indicate that this is the intended
behaviour.  

Enlightening comments and/or pointers to where this behaviour is
documented would be welcome.

Cheers,

	Berwin

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