[R-pkg-devel] Using parallel:::sendMaster &c. despite warnings

[David Norris]

I can confirm that getFromNamespace() does indeed circumvent the WARNING.
I might otherwise prefer ':::', however, for making its 'bad intentions' so clear.
I was very much aware of the cautionary language under `?readChild`, but wondered whether *in practice* more liberal community policies might be in force pending the maturation of the futureverse.
Although the sparse history of the relevant file hardly supports "might-break-at-any-time" alarmism, I do note that the last commit (in May 2020) introduced an argument sendMaster(raw.asis=TRUE) that I am actually using.
https://github.com/wch/r-source/commits/trunk/src/library/parallel/R/unix/mcfork.R

(Also agree with Yohann that ideally one might support Windows with a warning such as data.table issues regarding my Mac's non-support for OpenMP.)

Best, David

From: Sebastian Meyer <seb.meyer using fau.de>
Organization: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Date: Friday, July 23, 2021 at 8:05 AM
To: David Norris <david using precisionmethods.guru>, "r-package-devel using r-project.org" <r-package-devel using r-project.org>
Subject: Re: [R-pkg-devel] Using parallel:::sendMaster &c. despite warnings

Am 23.07.21 um 13:19 schrieb David Norris:
Because parallelized progress reporting in the futureverse.org incurs latencies too great for my application (https://github.com/HenrikBengtsson/progressr/issues/118), I have found it necessary to implement my own progress reporting using some of the non-exported functionality from `parallel`. (I do appreciate that Windows lacks the fork() system call, and will not support this. But am willing to make this an OS_type: unix-only package.)

Of course, I get a WARNING for this:

── R CMD check results ──────────────────────────────── precautionary 0.2.6 ────
Duration: 6m 41.8s

❯ checking dependencies in R code ... WARNING
    Unexported objects imported by ':::' calls:
      ‘parallel:::readChild’ ‘parallel:::selectChildren’
      ‘parallel:::sendMaster’
      See the note in ?`:::` about the use of this operator.
      Including base/recommended package(s):
      ‘parallel’

Is this warning an absolute deal-killer on CRAN? Is there a 'correct' way to do `:::` that avoids the warning altogether?

The 'parallel' functions your package intends to access seem to be 
intentionally unexported. Their help page says: "They are not available 
on Windows, and not exported from the namespace", and "This is a very 
low-level interface for expert use only: it not regarded as part of the 
R API and subject to change without notice."

Correspondingly, the CRAN Repository Policy says

Also, ::: should not be used to access undocumented/internal objects in base packages (nor should other means of access be employed). Such usages can cause packages to break at any time, even in patched versions of R.

which kind of answers both of your questions. The policy thus implicitly 
advises against using getFromNamespace().

Best regards,

	Sebastian Meyer


Kind regards,
David Norris

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