[Rd] Conditional dependency between packages

[Henrik Bengtsson]

if ("pkg1" %in% rownames(utils:::installed.packages()) ) {
  library(pkg1)
  ...
}

can be replaced by:

if (require("pkg1")) {
  ...
}

/Henrik

On Thu, Jul 2, 2009 at 5:29 AM, Jon Olav Skoien<j.skoien at geo.uu.nl> wrote:
> 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
>>
>
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