[Rd] Exiting R and package detachment?

[Henrik Bengtsson]

Hi,

is there away to assure that a package is detached when R quits?  I know 
about .Last(), but that requires the user to setup that function.  What 
I am looking for is a way for the package to do this itself, without 
asking the user to edit "their" .Last().  From ?setHook I know that:

   "...when an R is finished, packages are not detached and namespaces 
are not unloaded, so the corresponding hooks will not be run."

I am going to use this to load settings from file when a package loads 
and automatically save (by optionally prompting the user) them back to 
file when the package is no longer available (==detached/unloaded/R 
quits).  I am currently loading the settings in .First.lib() and have 
code in .Last.lib() to save them.

Are there other ways to assure functions to be called when R quits?  The 
best I can think of now is to "hack" .Last() by doing something like

if (!exists(".LastOriginal", mode="function")) {
   .LastOriginal <<- get(".Last", envir=.GlobalEnv);

   .Last <<- function(..., envir=parent.frame()) {
     for (fun in getHook(".Last")) {
       if (is.character(fun))
         fun <- get(fun, mode="function")
       try(fun());
     }
     eval(.LastOriginal(...), envir=envir);
   } # .Last()
}

Then in package <pkg>:
.First.lib <- function(libname, pkgname) {
   # Detach package when R finishes.
   setHook(".Last", function(...) {
     pos <- match(paste("package:", pkgname, sep=""), search());
     if (!is.na(pos))
       detach(pos=pos);
   })
}

However, this will be broken if user redefines .Last().  What about 
defining a hook "onSessionExit" to be called before (after?) .Last() is 
called.  In analogue to on.exit() one could then define

onSessionExit <- function(fcn, ...) {
   setHook("onSessionExit", fcn, ...);
}


Just curious, the above quote makes me wonder what is the rational for 
the behavior?  Was it made on purpose or is it simply just easier for R 
to finish without detaching/unloading packages first?  In what 
situations to you have "clean-up" code for a package that is only called 
when detach("package:<pkg>") is used?  One situation I can imaging is 
when a bundle of packages are loaded and when you detach the package 
that all other packages requires, the other packages are also detached 
for conveniency.

Best wishes

Henrik