Without having read all of the comments already made here, but my
understanding why ::: is not allowed is because you are reaching into
the internal API that the package owner does not guarantee will exist
in the next release. If you rely on the internal code of another CRAN
package in your CRAN package, your CRAN package might break without
your control. This might release an avalanche of reverse package
dependencies failing on CRAN.
The only thing you can safely rely on is the API that is explicitly
*exported* by an R package. In order for the maintainer to break that
API for reverse dependent packages, they need to go through a process
of deprecating and defuncting what they want to break/remove - a
process that involves multiple releases and often reaching out to
package maintainers and asking them to update accordingly. CRAN runs
reverse package dependency checks making sure that a package does not
break its exported API. If it does, it will not roll out on CRAN.
So, in that sense CRAN helps uphold the contract of the exported APIs.
In contrast, a maintainer can do whatever they want whenever they want
with their internal code/API.
With more and more packages being infrastructure packages, I think
there is room for "protected" API, which is not exported to avoid
cluttering up the search path for end-users while it yet provides a
contract toward package developers relying on it. There are various
ways to emulate such protected APIs but we don't have a standard and
there's a risk that 'R CMD check' fails to detect when the contract is
broken (resulting in delayed run-time errors on the user end).
My $.02
Henrik
On Mon, Sep 14, 2020 at 12:06 PM David Kepplinger
<david.kepplinger at gmail.com> wrote:
Yes, my view is certainly rigid and I agree that in the cases where the
function is actually used directly by the user, exporting it is the correct
step.
However, it seems some packages actually need to access internal functions
from an outside context, but the code that accesses the function is
logically contained completely inside the package. In these cases, package
maintainers seem to be looking for alternatives to `:::` for the sake of
avoiding the R CMD check note. I argue that the work arounds, however,
either (a) achieve the exact same result as `:::`, but in a less
transparent and likely more error prone way, or (b) unnecessarily making an
internal function available to the user.
I also agree with the CRAN team that package maintainers need to be made
aware of the issue when using `:::` inside their package as it is most
likely unnecessary. But the phrasing of the note ("almost never needs to
use :::") combined with a lack of transparent guidelines on when it is
acceptable leads to maintainers looking for alternatives mimicking the
behavior of `:::`. I haven't found any official instructions in Writing R
extensions or on the mailing list under what circumstances `:::` is deemed
to be acceptable by the CRAN team (I have to admit searching for `:::` in
the archives yields so many results I haven't looked at all of them). It's
probably impossible to conceive every possible use case for `:::`, but a
good start may be to have something in the documentation explicitly
mentioning commonly observed patterns where `:::` is not acceptable, and
the common exceptions to the rule (if there are any).
Maybe this issue is so miniscule and almost never comes up that it's not
worth mentioning in the documentation.
Best,
David
On Mon, Sep 14, 2020 at 3:19 AM Georgi Boshnakov <
georgi.boshnakov at manchester.ac.uk> wrote:
You may have a case to argue to CRAN that you can get the "almost"
exemption (can't say without details) but your views look overly rigid.
Exporting an object and marking it as internal is not a "work around",
even less a "dirty trick".
Export makes the object available outside the package's namespace and
makes it clear that this is intentional.
If you can't drop the 'package:::' prefix in your use case, this means
that this is what you actually do (i.e. use those objects outside the
namespace of the package). I would be grateful to CRAN for asking me to
export and hence document this.
Georgi Boshnakov
PS Note that there is no such thing as "public namespace".
-----Original Message-----
From: R-package-devel <r-package-devel-bounces at r-project.org> On Behalf
Of David Kepplinger
Sent: 13 September 2020 20:52
To: R Package Devel <r-package-devel at r-project.org>
Subject: [R-pkg-devel] Use of `:::` in a package for code run in a
parallel cluster
Dear list members,
I submitted an update for my package and got automatically rejected by the
incoming checks (as expected from my own checks) for using `:::` calls to
access the package's namespace.
"There are ::: calls to the package's namespace in its code. A package
*almost* never needs to use ::: for its own objects:?" (emphasis mine)
This was a conscious decision on my part as the package runs code on a
user-supplied parallel cluster and I consider cluster-exporting the
required functions a no-go as it would potentially overwrite objects in the
clusters R sessions. The package code does not own the cluster and hence
the R sessions. Therefore overwriting objects could potentially lead to
unintended behaviour which is opaque to the user and difficult to debug.
Another solution to circumvent the R CMD check note is to export the
functions to the public namespace but mark them as internal. This was also
suggested in another thread on this mailing list (c.f. "Etiquette for
package submissions that do not automatically pass checks?"). I do not
agree with this work-around as the methods are indeed internal and should
never be used by users. Exporting truly internal functions for the sake of
satisfying R CMD check is a bad argument, in particular if there is a
clean, well-documented, solution by using `:::`.
I argue `:::` is the only clean solution to this problem and no dirty
work-arounds are necessary. This is a prime example of where `:::` is
actually useful and needed inside a package. If the R community disagrees,
I think R CMD check should at least emit a WARNING instead of a NOTE and
elaborate on the problem and accepted work-arounds in "Writing R
extensions". Or keep emitting a NOTE but listing those nebulous reasons
where `:::` would be tolerated inside a package. Having more transparent
criteria for submitting to CRAN would be really helpful to the entire R
community and probably also reduce the traffic on this mailing list.
Best,
David
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