[Rd] 'R CMD check' fails when suggested package is not available

[Herve Pages]

Hi there,

I was wondering why I get the following error message:

  * checking package dependencies ... ERROR
  Packages required but not available:
    Rmpi

when I run 'R CMD check' on a package that _suggests_ Rmpi?
Why isn't it OK to not have all the suggested packages installed?

Maybe one of the 3 following behaviours would be more appropriate:

  a) Having the error saying something like:

    Package suggested but not available:
      Rmpi

  b) Make this a warning instead of an error.

  c) Don't do anything at all for suggested packages.

This issue showed up today while I was checking a new Bioconductor package:
the package suggests Rmpi but the vignette and the examples don't use it. If I remove
Rmpi from the Suggests field then 'R CMD check' runs all the examples and re-create
the vignette with no problem. Most users will not have Rmpi on their machine neither
will they be interested in getting into the trouble of installing it.
The package I was checking suggests Rmpi only because it contains 1 function that tries
to use it if it's installed but will work perfectly fine otherwise.
In this case it seems reasonable to have Rmpi in the Suggests field but this will
make 'R CMD check' to fail which is problematic in the context of automated builds :-/
If 'R CMD check' can't be a little bit more relaxed about this, then I guess we will
need to remove Rmpi from the Suggests field, but then 'R CMD check' will complain that:

  * checking for unstated dependencies in R code ... WARNING
  'library' or 'require' calls not declared from:
    Rmpi

which is always better than getting an ERROR.

Thanks!

H.