[R-pkg-devel] Handling Not-Always-Needed Dependencies?

[Kevin Ushey]

You could abuse the LinkingTo: field of the DESCRIPTION file, to
specify 'packages you want installed alongside your package that you
won't necessarily use at runtime'. (It's possible that 'R CMD check'
might warn about strange usages of LinkingTo:, but I'm not sure)

You could also just place these packages in Suggests, but guard their
usages with a function that attempts to download, install and load
those packages. E.g. (pseudo-ish code)

    ensure_package_loaded <- function(package) {
      if (requireNamespace(package))
        return(TRUE)
      if (interactive())
       # prompt user to download + install package, then try to load again
      stop("routine requires package but is not installed")
    }

and use it as:

    uses_foo <- function() {
      ensure_package_loaded("foo")
      foo::bar()
    }

This would at least minimize the amount of time spent 'annoying' the
user re: installation of package dependencies (it would only happen
the first time they try to use those functions)

Overall, I think the best resolution here is through clever use of
Suggests + on-demand installation of required packages, rather than
adding another field to the DESCRIPTION file.

Cheers,
Kevin

On Tue, Aug 2, 2016 at 9:26 AM, Thomas J. Leeper <thosjleeper at gmail.com> wrote:
> I have a fairly open-ended question about the handling of package
> dependencies that is inspired by the precise distinction between
> "Depends", "Imports", and "Suggests" listed in DESCRIPTION.
>
> Some background, as I understand it: Prior to requiring package
> namespaces, we listed package dependencies in "Depends" because the
> required package had to be (1) installed and (2) on the search path in
> order to access its functions. After introducing and requiring
> namespaces, there are very few circumstances in which "Depends" is
> needed because functions from the required package can either be
> imported (via "Imports" and NAMESPACE) and/or referred to using the
> `pkg::fun()` style. "Suggests" packages are maybe needed for examples,
> vignettes, etc. but are not installed by default via
> `install.packages()`, so are not guaranteed to be available.
>
> Some observations:
>
> 1. "Depends" seems to be less useful than before, except in rare cases
> where a package needs to be (a) installed, (b) loaded, and (c) on the
> search path. Imports covers most package dependency use cases.
>
> 2. There is a gap in functionality between "Imports" and "Suggests".
> Sometimes there is a need for functions that should be available
> (i.e., installed) but do not need to be loaded or imported because
> they are rarely used (e.g., graphing functions). Importing the
> functions adds bloat but only putting their package in "Suggests" does
> not guarantee availability when, for example, calling
> `requireNamespace()` or `require()`.
>
> Suggestion:
>
> Might it be useful to have a category between "Imports" and "Suggests"
> that covers packages that should be installed but not imported?
>
> This could be done by changing `install.packages()` to cover
> "Suggests" by default, changing the meaning of "Depends" and
> "Imports", or adding a new category to DESCRIPTION.
>
> I am interested in hearing your thoughts on this issue.
>
> Best,
> -Thomas
>
> Thomas J. Leeper
> http://www.thomasleeper.com
>
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