[Rd] Conventions: Use of globals and main functions

[Gábor Csárdi]

This is what I usually put in scripts:

if (is.null(sys.calls())) {
  main()
}

This is mostly equivalent to the Python idiom. It the script runs from
Rscript, then it will run main(). It also lets you source() the
script, and debug its functions, test them, etc. It works best if all
the code in the script is organized into functions.

Gabor

On Sun, Aug 25, 2019 at 6:11 AM Cyclic Group Z_1 via R-devel
<r-devel using r-project.org> wrote:
>
> In R scripts (as opposed to packages), even in reproducible scripts, it seems fairly conventional to use the global workspace as a sort of main function, and thus R scripts often populate the global environment with many variables, which may be mutated. Although this makes sense given R has historically been used interactively and this practice is common for scripting languages, this appears to disagree with the software-engineering principle of avoiding a mutating global state. Although this is just a rule of thumb, in R scripts, the frequent use of global variables is much more pronounced than in other languages.
>
> On the other hand, in Python, it is common to use a main function (through the `def main():` and  `if __name__ == "__main__":` idioms). This is mentioned both in the documentation as well as in the writing of Python's main creator. Although this is more beneficial in Python than in R because Python code is structured into modules, which serve as both scripts and packages, whereas R separates these conceptually, a similar practice of creating a main function would help avoid the issues from mutating global state common to other languages and facilitate maintainability, especially for longer scripts.
>
> Although many great R texts (Advanced R, Art of R Programming, etc.) caution against assignment in a parent enclosure (e.g., using `<<-`, or `assign`), I have not seen many promote the use of a main function and avoiding mutating global variables from top level.
>
> Would it be a good idea to promote use of main functions and limiting global-state mutation for longer scripts and dedicated applications (not one-off scripts)? Should these practices be mentioned in the standard documentation?
>
> This question was motivated largely by this discussion on Reddit: https://www.reddit.com/r/rstats/comments/cp3kva/is_mutating_global_state_acceptable_in_r/ . Apologies beforehand if any of these (partially subjective) assessments are in error.
>
> Best,
> CG
>
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