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R: sourcing files using a relative path

Sourcing files using a relative path is useful when dealing with large codebases. Other programming languages have well-defined mechanisms for sourcing files using a path relative to the directory of the file being sourced into. An example is Ruby's require_relative. What is a good way to implement relative path sourcing in R?

Below is what I pieced together a while back using various recipes and R forum posts. It's worked well for me for straight development but is not robust. For example, it breaks when the files are loaded via the testthat library, specifically auto_test(). rscript_stack() returns character(0).

# Returns the stack of RScript files
rscript_stack <- function() {
  Filter(Negate(is.null), lapply(sys.frames(), function(x) x$ofile))
}
 
# Returns the current RScript file path
rscript_current <- function() {
  stack <- rscript_stack()
  r <- as.character(stack[length(stack)])
  first_char <- substring(r, 1, 1)
  if (first_char != '~' && first_char != .Platform$file.sep) {
    r <- file.path(getwd(), r)
  }
  r
}
 
# Sources relative to the current script
source_relative <- function(relative_path, ...) {
  source(file.path(dirname(rscript_current()), relative_path), ...)
}

Do you know of a better source_relative implementation?