inst/README.markdown

In pkgconfig: Private Configuration for 'R' Packages

Private configuration for R packages

Easy way to create configuration parameters in your R package. Configuration values set in different packages are independent.

Call set_config() to set a configuration parameter. Call get_config() to query it.

Installation

install.packages("pkgconfig")

Typical usage

Note: this is a real example, but it is not yet implemented in the CRAN version of the igraph package.

The igraph package has two ways of returning a set of vertices. Before version 1.0.0, it simply returned a numeric vector. From version 1.0.0 it sets an S3 class on this vector by default, but it has an option called return.vs.es that can be set to FALSE to request the old behavior.

The problem with the return.vs.es option is that it is global. Once set to FALSE (interactively or from a package), R will use that setting in all packages, which breaks packages that expect the new behavior.

pkgconfig solves this problem, by providing configuration settings that are private to packages. Setting a configuration key from a given package will only apply to that package.

Workflow

Let's assume that two packages, pkgA and pkgB, both set the igraph option return.vs.es, but pkgA sets it to TRUE, and pkgB sets it to FALSE. Here is how their code will look.

pkgA

pkgA imports set_config from the pkgconfig package, and sets the return.vs.es option from it's .onLoad function:

.onLoad <- function(lib, pkg) {
    pkgconfig::set_config("igraph::return.vs.es" = TRUE)
}

pkgB

pkgB is similar, but it sets the option to FALSE:

.onLoad <- function(lib, pkg) {
    pkgconfig::set_config("igraph::return.vs.es" = FALSE)
}

igraph

The igraph package will use get_config to query the option, and will supply a fallback value for the cases when it is not set:

return_vs_es_default <- TRUE
# ...
igraph_func <- function() {
    # ...
    pkgconfig::get_config("igraph::return.vs.es", return_vs_es_default)
    # ...
}

If igraph_func is called from pkgA (maybe through other packages), get_config will return TRUE, and if it is called from pkgB, get_config will return FALSE. If no package on the call stack sets the igraph::return.vs.es option, then its default value is used, as specified in igraph.

What if pkgA calls pkgB?

It might happen that both pkgA and pkgB set an option, and pkgA also calls functions from pkgB, which in turn, might call igraph. In this case the package that is further down the call stack wins. In other words, if the call sequence looks like this:

... -> pkgA -> ... -> pkgB -> ... -> igraph

then pkgB's value is used in igraph. (Assuming the last ... does not contain a call to pkgA of course.)

Feedback

Please comment in the Github issue tracker of the project.

License

MIT © Gábor Csárdi

pkgconfig documentation built on Sept. 22, 2019, 5:05 p.m.