[Rd] declaring package dependencies

[Duncan Murdoch]

On 13/09/2013 8:31 AM, Michael Friendly wrote:
> On 9/12/2013 1:37 PM, Duncan Murdoch wrote:
> >
> > I think this one would be fine if you had library(MASS) or
> > require(MASS) or (probably best) used MASS::loglm explicitly.  It may
> > be that in the past some other package put MASS on the search list,
> > and that's why it worked before.
> >
> > The distinction is between "loading" and "attaching" a package.
> > Loading it (which would be done if you had MASS::loglm, or imported
> > it) guarantees that the package is initialized and in memory, but
> > doesn't make it visible to the user without the explicit MASS::
> > prefix.  Attaching it first loads it, then modifies the user's search
> > list so the user can see it.
> >
> > Loading is less intrusive, so it's preferred over attaching.  Both
> > library() and require() would attach it.
> >
> Thanks for this explanation, but I'm still confused about how to avoid
> the wrath of the CRAN-devel
> daemon, whose appetite for new morsels of developer flesh seems ever
> increasing and makes
> keeping even a stable package up-to-date a moving target.  Perhaps such
> changes in R devel
> should be announced on this list for public comment before they are
> enforced in R CMD check.
Changes are generally announced in the NEWS.Rd file long before release, 
but R-devel is an unreleased version, so you won't see the news until it 
is there.  Announcing things that nobody can try leads to fewer useful 
comments than putting them into R-devel where at least people can see 
what is really happening.
>
> In vcdExtra, I use MASS::loglm in ~ 6 .Rd examples, a vignette, and also
> provide R code that
> extends *.loglm methods.  All of this previously worked by including
> Suggests: MASS, ...
> Changing this to Imports: MASS seems rather heavy-handed; I don't really
> want/need all of MASS
> in my namespace, and using MASS::loglm in examples seems ugly.  For now,
> I'll use
> require(MASS) in each example.

If you need a small number of things from MASS in your package code, 
then importing them explicitly is definitely the way to go, e.g. 
importFrom(MASS, loglm).  If you only use them in examples, I wouldn't 
do that, I'd recommend Suggests and use the MASS:: prefix. Whether that 
is ugly is a matter of taste:  it makes it clear to a user where that 
function came from, and doesn't potentially hide objects from other 
packages later in the search path.

On the other hand, require(pkg) is really simple; we have no equivalent 
function that only does the loading, without attaching.
So it's hard to write

if (requireLoadable(MASS)) {
   MASS::loglm( ... )
}

>
> > Attaching it first loads it, then modifies the user's search list so
> > the user can see it.
> Which directive does this?

library() and require() both do it.

Duncan Murdoch
>