Simon
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,
Paul
-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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