[Rd] Question about selective importing of package functions...

[Paul Gilbert]

On 13-10-20 04:58 PM, Gabor Grothendieck wrote:
> On Sun, Oct 20, 2013 at 4:49 PM, Duncan Murdoch
> <murdoch.duncan at gmail.com> wrote:
>> On 13-10-20 4:43 PM, Jonathan Greenberg wrote:
>>>
>>> I'm working on an update for my CRAN package "spatial.tools" and I noticed
>>> a new warning when running R CMD CHECK --as-cran:
>>>
>>> * checking CRAN incoming feasibility ... NOTE
>>> Maintainer: 'Jonathan Asher Greenberg <spatial-tools at estarcion.net>'
>>> Depends: includes the non-default packages:
>>>     'sp' 'raster' 'rgdal' 'mmap' 'abind' 'parallel' 'foreach'
>>>     'doParallel' 'rgeos'
>>> Adding so many packages to the search path is excessive
>>> and importing selectively is preferable.
>>>
>>> Is this a warning that would need to be fixed pre-CRAN (not really sure
>>> how, since I need functions from all of those packages)?  Is there a way
>>> to
>>> import only a single function from a package, if that function is a
>>> dependency?
>>
>>
>> You really want to use imports.  Those are defined in the NAMESPACE file;
>> you can import everything from a package if you want, but the best style is
>> in fact to just import exactly what you need.  This is more robust than
>> using Depends, and it doesn't add so much to the user's search path, so it's
>> less likely to break something else (e.g. by putting a package on the path
>> that masks some function the user already had there.)
>
> That may answer the specific case of the poster but how does one
> handle the case
> where one wants the user to be able to access the functions in the
> dependent package.

There are two answers to this, depending on how much of the dependent 
package you want to make available to the user. If you want most of that 
package to be available then this is the (only?) exception to the rule. 
 From Writing R Extensions:

   Field ‘Depends’ should nowadays be used rarely, only for packages
   which are intended to be put on the search path to make their
   facilities available to the end user (and not to the package itself):
   for example it makes sense that a user of package latticeExtra would
    want the functions of package lattice made available.

If you really only want to make a couple of functions available then you 
can import and export the functions. Currently this has the unfortunate 
side effect that you need to document the functions, you cannot just 
re-direct to the documentation in the imported package, at least, I have 
not figured out how to do that.

Paul

>
> For example, sqldf depends on gsubfn which provides fn which is used
> with sqldf to
> perform substitutions in the SQL string.
>
> library(sqldf)
> tt <- 3
> fn$sqldf("select * from BOD where Time > $tt")
>
> I don't want to ask the user to tediously issue a library(gsubfn) too since
> fn is frequently needed and for literally years this has not been necessary.
> Also I don't want to duplicate fn's code in sqldf since that makes the whole
> thing less modular -- it would imply having to change fn in two places
> if anything
> in fn changed.
>