[Rd] Depends vs Imports

Paul Gilbert pgilbert902 at gmail.com
Thu Aug 1 05:12:27 CEST 2013


Thanks, that helps a lot, but see below ..

On 13-07-31 08:35 PM, Simon Urbanek wrote:
> On Jul 31, 2013, at 7:14 PM, Paul Gilbert wrote:
>> I am being asked to modernize the Depends line in the DESCRIPTION
>> file of some packages. Writing R Extensions says:
>> The general rules are
>> Packages whose namespace only is needed to load the package using
>> library(pkgname) must be listed in the ‘Imports’ field and not in
>> the ‘Depends’ field. Packages listed in imports or importFrom
>> directives in the NAMESPACE file should almost always be in
>> ‘Imports’ and not ‘Depends’.
>> Packages that need to be attached to successfully load the package
>> using library(pkgname) must be listed in the ‘Depends’ field,
>> only.
>> Could someone please explain a few points I thought I understood
>> but obviously do not, or point to where these are explained:
>> -What does it mean for the namespace only to be needed? I thought
>> the namespace was needed if the package or some of its functions
>> were mentioned in the NAMESPACE file, and that only the namespace
>> was needed if only the generics were called, and not other
>> functions. The above suggests that I may be wrong about this. If
>> so, that is, Imports will usually suffice, then when would Depends
>> ever be needed when a package is mentioned in the NAMESPACE file?
> In the namespace era Depends is never really needed. All modern
> packages have no technical need for Depends anymore. Loosely speaking
> the only purpose of Depends today is to expose other package's
> functions to the user without re-exporting them.

This seems to mostly work, except in the situation where a package is 
used that enhances an imported package. For example, I Import DBI but 
the call dbDriver("MySQL") fails looking for MySQL in package RMySQL if 
I only import that and do not list it in Depends. Am I missing something?

Similarly, I have a package tframePlus that provides extra methods (for 
zoo and xts) for my package tframe. Since tframe does not depend or 
import tframePlus (in fact, the reverse), I seem to need tframePlus in 
Depends not Imports of another package that Imports tframe. Does this 
sound right or am I missing something else?

Also, I have a package TSMySQL which enhances my package TSdbi. When a 
user uses TSMySQL they will want to use many functions in TSdbi. Here 
again, I seem to need TSMySQL to Depend on TSdbi, for the reason you 
mention, exposing all the functions to the user.

(I'm glad this is simple, I have trouble when things are difficult.)

Thanks again,
>> -Should the package DESCRIPTION make any accommodation for the
>> situation where users will probably need to directly call functions
>> in the imported package, even though the package itself does not?
>> -What does "need to be attached" mean? Is there a distinction
>> between a package being attached and a namespace being attached.
> No, the distinction is between loaded and attached (namespace/package
> is synonymous here).
>> -Does "successfully load" mean something different from actually
>> using the package? That is, can we assume that if the package loads
>> then all the functions to run things will actually be found?
> Define "found" - they will not be attached to the search path, so
> they will be found if you address them fully via myPackage::myFn but
> not just via myFn (except for another package that imports
> myPackage).
>> -If pkg1 uses a function foo in pkg3 indirectly, by a call to a
>> function in  pkg2 which then uses foo, how should pkg1 indicate the
>> relationship with foo's pkg3, or is there no need to indicate any
>> relationship with pkg3 because that is all looked after by pkg2?
> There is no need - how would you imagine being responsible for code
> that you did not write? pkg2 will import function from pkg1, but
> you're not importing them in pkg3, you don't even care about them so
> you have no direct relationship with pkg1 (imagine pkg2 switched to
> use pkg4 instead of pkg1).
> IMHO it's all really simple:
> load = functions exported in myPkg are available to interested
> parties as myPkg::foo or via direct imports - essentially this means
> the package can now be used
> attach = the namespace (and thus all exported functions) is attached
> to the search path - the only effect is that you have now added the
> exported functions to the global pool of functions - sort of like
> dumping them in the workspace (for all practical purposes, not
> technically)
> import a function into a package = make sure that this function works
> in my package regardless of the search path (so I can write fn1
> instead of pkg1::fn1 and still know it will come from pkg1 and not
> someone's workspace or other package that chose the same name)
> Cheers, Simon

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