[R-pkg-devel] Handling Not-Always-Needed Dependencies?

[Dirk Eddelbuettel]

On 2 August 2016 at 11:00, Kevin Ushey wrote:
| You could abuse the LinkingTo: field of the DESCRIPTION file, to
| specify 'packages you want installed alongside your package that you
| won't necessarily use at runtime'. (It's possible that 'R CMD check'
| might warn about strange usages of LinkingTo:, but I'm not sure)

I'd rather not abuse LinkingTo: any further. I fear its feelings are
sufficiently hurt being mostly used for 'IncludeFrom:' and no linking.

Kidding aside I think we have room for new tags in DESCRIPTION. We may as
well do it right.
 
| You could also just place these packages in Suggests, but guard their
| usages with a function that attempts to download, install and load
| those packages. E.g. (pseudo-ish code)
| 
|     ensure_package_loaded <- function(package) {
|       if (requireNamespace(package))
|         return(TRUE)

Nice.  [ And erratum in my previus email where I meant requireNamespace() ]

We could even go one further and store this in a per-package environment,
being filled through .onLoad() or .onAttach().

|       if (interactive())
|        # prompt user to download + install package, then try to load again
|       stop("routine requires package but is not installed")
|     }

I don't see why this should work only when interactive(). I think it should
work all the time.

I also don't think it needs to stop. It is after all just some auxiliary
functionality that Suggests provides.  Eg extra regression tests, say with or
without a certain backend -- and those tests then need the yes/no toggle and
skip if unavailable.
 
| and use it as:
| 
|     uses_foo <- function() {
|       ensure_package_loaded("foo")
|       foo::bar()
|     }

Sure.
 
| This would at least minimize the amount of time spent 'annoying' the
| user re: installation of package dependencies (it would only happen
| the first time they try to use those functions)

I don't really want other packages going off installing autonomously.
We pretty much make this explicit in DESCRIPTION so that the user can make an
informed choice.
 
| Overall, I think the best resolution here is through clever use of
| Suggests + on-demand installation of required packages, rather than
| adding another field to the DESCRIPTION file.

Wholly agreed on some part, less so on the remainders.

Dirk

-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org