The goal of {pptm} is to provide a way to install packages as the appeared on CRAN on a particular date. For instance to ensure that all packages work with the chosen version of R.
On a day-to-day basis it probably works well enough to use the fluent versions that comes with install.packages (that is, always installing the newest version) -- even though we potentially experience incompatible package versions.
But when using R more seriously I prefer to fix the versions of R as well as the packages in use.
For many moons I have relied on Microsoft's departed MRAN to provide such a time machine. However, this service has come to an end and I need an alternative. One could start making copies of CRAN just like MRAN or use Posit's Package Manager, but as its name suggests this package is for people who want a cheaper option.
For completeness I should mention that the {renv} package offer similar capabilities using Posit's public Package Manager. I learned of this from the "renv 1.0.0" blog post published after I made {pptm}.
{pptm} is only on GitHub and can be installed with the {remotes} package:
remotes::install_github('robertdj/pptm')On CRAN a package provides the current version in source code format as well as binaries for Windows and macOS. Furthermore, CRAN keeps an archive of previous versions, but only in source code format.
{pptm} looks through a package's archive and picks the version that was current at a specified date. We need to do this recursively by checking all dependencies of the package as well.
As an example, consider the {here} package on a date suitable for R version 4.1.1:
versions <- pptm::get_version('here', date = as.Date('2021-01-01'))
pptm::install_version(versions)Why choose the date 2021-11-01 for R version 4.1.1?
This is the date where R version 4.1.2 was released and hence the last day where all packages on CRAN ought to work with R version 4.1.1.
My usecase is always to use these "last day of R version X being the latest" and to facilitate this install_version can take a version of R as argument:
pptm::get_version('here', r_version = '4.1.1')For examples of how to use {pptm} in Docker images check out my Dockerfiles for R.
The package files downloaded by get_version are saved in the folder structure required to make it a CRAN.
To include the required metadata, run pptm::make_cran(versions).
After this, install.packages will "just work" if its repos argument is the download path -- this is what install_version does.
By construction, it is also a self-contained subset of CRAN, so no extra CRAN-like resources are needed.