We want users to be able to install new versions of R
as soon as they are out, without us having to do anything
special in repo2docker. So we can't actually check for 'invalid'
R versions. This is ok, as we don't have to special case apt repos
or similar when installing R from rstudio's debs
r-recommended is a collection of common CRAN packages,
which cause conflicts when trying to install older R.
These same packages can be regular dependencies retrieved from CRAN.
PPA is a specific kind of apt repository, hosted on
launchpad.net. We use https://cran.r-project.org/bin/linux/ubuntu/,
which is just a regular apt repository. The PPA terminology
always confused me, so this just clears that up
- MRAN doesn't seem to have R 4.1 specific snapshots, so let's
default to RSPM for anything 4.1+.
- Otherwise, snapshot dates in 2022 will result in using rspm