[Rd] Conditional dependency between packages

[Jon Olav Skoien]

Hi Seth,

And thanks for your suggestion! I was not able to do exactly what you 
described (I have no earlier experience with using environments), but 
you mentioning .onLoad was a good starting point. I have now removed all 
references to pkg1 from the NAMESPACE, and wrote the following .onLoad 
function:

.onLoad <- function(libname, pkgname) {
  if ("pkg1" %in% rownames(utils:::installed.packages()) ) {
    library(pkg1)
    info = matrix(c("fun1", "fun2", "fun3", rep("pkg2", 3), rep(NA,3)), 
ncol = 3)
    registerS3methods(info, package = "pkg1", env = environment(funInPkg2))
  }
}

New methods for functions fun1, fun2 and fun3 seem to be available if 
pkg1 is installed, while they are ignored if pkg1 is not installed. The 
function above loads pkg1 automatically if installed (I would prefer 
this to be optional), but at least it will not be necessary to download 
pkg1 (with all its dependencies) for users without interest in it.

I have not found any description of registerS3methods (except from an 
old .Rd file stating that it is not intended to be called directly), so 
there might be better ways of doing this... And I am sure there is a 
better way of assigning the right environment...

Thanks again,
Jon



Seth Falcon wrote:
> Hi Jon,
>
> * On 2009-06-30 at 15:27 +0200 Jon Olav Skoien wrote:
>   
>> I work on two packages, pkg1 and pkg2 (in two different projects). pkg1 is 
>> quite generic, pkg2 tries to solve a particular problem within same field 
>> (geostatistics). Therefore, there might be users who want to use pkg2 as an 
>> add-on package to increase the functionality of pkg1. In other words, 
>> functions in pkg1 are based on the S3 class system, and I want pkg2 to 
>> offer methods for pkg2-objects to functions defined in pkg1, for users 
>> having both packages installed. Merging the packages or making pkg2 always 
>> depend pkg1 would be the easiest solution, but it is not preferred as most 
>> users will only be interested in one of the packages.
>>     
>
> I'm not sure I understand the above, I think you may have a pkg2 where
> you meant pkg1, but I'm not sure it matters.
>
> I think the short version is, pkg2 can be used on its own but will do
> more if pkg1 is available.  I don't think R's packaging system
> currently supports conditional dependencies as you might like.
> However, I think you can get the behavior you want by following a
> recipe like:
>
> * In pkg2 DESCRIPTION, list Suggests: pkg1.
>
> * In pkg2 code, you might define a package-level environment and 
>   in .onLoad check to see if pkg1 is available.
>
>      PKG_INFO <- new.env(parent=emptyenv())
>      .onLoad <- function(libname, pkgname) {
>          if (check if pkg1 is available) {
>             PKG_INFO[["pkg1"]] <- TRUE
>          }
>      }
>
> * Then your methods can check PKG_INFO[["pkg1"]].
>
>
>      
> + seth
>