[Rd] optional package dependency

[Ross Boylan]

I have a package that can use rmpi, but works fine without it.  None of
the automatic test code invokes rmpi functionality.  (One test file
illustrates how to use it, but has quit() as its first command.)

What's the best way to handle this?  In particular, what is the
appropriate form for upload to CRAN?

When I omitted rmpi from the DESCRITPION file R CMD check gave 
<quote>
* checking R code for possible problems ... NOTE
alldone: no visible global function definition for ‘mpi.bcast.Robj’
alldone: no visible global function definition for ‘mpi.exit’
<quote>
followed by many more warnings.

When I add
Suggests: Rmpi
in DESCRIPTION the check stops if the package is not installed:
<quote>
* checking package dependencies ... ERROR
Packages required but not available:
  Rmpi
</quote>
Rmpi is not required, but I gather from previous discussion on this list
that suggests basically means required for R CMD check.

NAMESPACE seems to raise similar issues; I don't see any mechanism for
optional imports.  Also, I have not used namespaces, and am not eager to
destabilize things so close to release.  At least, I hope I'm close to
release :)

Thanks for any advice.

Ross Boylan

P.S. Thanks, Duncan, for your recent advice on my help format problem
with R 2.7.  I removed the nested \description, and now things look OK.