[R] Suggests/Enhances or .onLoad for S3-methods for optional package

[Jon Olav Skoien]

Hi,

I am developing a package B that, among other things, also offers some 
extra S3-methods for functions in package A if the user has installed A. 
I do not want to list A under Depends of B, as the dependency list of A 
is rather long, and most potential users of B will not be interested in 
package A. Unfortunately I struggle with doing this right. So far I have 
listed A under Suggests, and had a .onLoad function in B with
if (require(A)) registerS3methods(newMethodsMatrix, package = A, env = 
environment(B))
But starting with R 2.13 or R 2.14, R CMD check complain about this 
require call in .onLoad, referring to the "good practice"-section of 
.onAttach. I guess this could also be a problem when uploading the 
package to CRAN, so I am trying to find another solution.

So far I have tried:
List A under Suggest of B, with a conditional import in NAMESPACE.
If I build a Windows-binary from this when A is installed, this package 
can be installed but not loaded on computers where A is not installed.

List A under Enhances of B.
This seems to be the right thing, as the R extensions manual says: "the 
‘Enhances’ field lists packages “enhanced” by the package at hand, e.g., 
by providing methods for classes from these packages".
However, although it seems I can install and load package B when I 
conditionally import package A in the NAMESPACE, R CMD check stops with 
the error: Namespace dependency not required: A
If I remove the import, R CMD check is happier, but I cannot load the 
package after installing.

I have read about the use of "Suggest", "Enhances" etc in "Writing R 
Extensions", but could not figure out the right way to do this. I am 
sure there is something I am missing here.

Thanks,
Jon

-- 
Jon Olav Skøien
Joint Research Centre - European Commission
Institute for Environment and Sustainability (IES)
Land Resource Management Unit

Via Fermi 2749, TP 440,  I-21027 Ispra (VA), ITALY

jon.skoien at jrc.ec.europa.eu
Tel:  +39 0332 789206

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