[Rd] Vectorize library() to improve handling of versioned deps [sprint project proposal]

[Gabriel Becker]

Hi All,

This is a proposal for a project which could be worked on during the R
development Sprint at the end of this month; it was requested that we start
a discussion here to see what R-core's thoughts on it were before we
officially add it to the docket.


AFAIK, R officially supports both versioned dependencies (almost
exclusively of the >=version variety) and library paths with more than one
directory. Further, I believe if at least de facto supports the same
package being installed in different directories along the lib path. The
most common of these, I'd bet, would be different versions of the same
library being installed in a site library and in a user's personal library,
though that's not the only way this can happen.

The combination of these two features, however can give rise to
packages which are all correctly installed and all loadable individually,
but which must be loaded in a particular order when used together or the
loading of some of them will fail.

Consider the following dependency structure between packages

pkgA: pkgB (>= 0.5.0)

pkgC: pkgB (>= 0.6.0)

Consider the following multi-libpath setup:

~/pth1/: pkgA, pkg B [0.5.1]
~/pth2/: pkg C, pkg B [0.6.5]

And consider that we have the libpath c("~/pth1/", "~/pth2").

If we do

library(pkgA)

Things will work great.

Same if we do

library(pkgC)

BUT, if we do

library(pkgA)
library(pkgC)

pkgC will not be able to be loaded, because an insufficient version of
pkgB will
already be loaded.

I propose that library be modified to be able to take a character vector of
package names, when it does, it performs the dependency calculations to
determine how all packages in the vector can be loaded (in the order they
appear). In the example above, this would mean that if we did

library(c("pkgA", "pkgC"))

It would determine that pkgB version 0.6.5 was needed (or alternatively,
that version 0.5.1 was insufficient) and use that *when loading the
dependencies of pkgA*.

The proposal issue for the sprint itself is here:
https://github.com/r-devel/r-project-sprint-2023/discussions/15

Thoughts?

~G

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