Command line applications for R

This is a work in progress for a sys module that supports command line applications in R. Inspired by Python, it implements a simple, opinionated paradigm for implementing command line tools.

To ensure that the code of the command line application is itself reusable, sys$run offers an entry point that is only invoked when the code is run self-contained from the command line, not when it’s included as a module (similar to Python’s if __name__ == '__main__'). The module also offers a self-contained command line parser that supports largely the same syntax as getopt and argparse, but with an R-like API.

This module supports writing tools using code such as the following:

#!/usr/bin/env Rscript

sys = modules::import('sys')

"A simple mathematical expression evaluator"
VERSION = '1.0'

sys$run({
    args = sys$cmd$parse(opt('n', 'allow-nan', 'allow NaN results (otherwise, the program exits with a failure)', FALSE),
                         opt('d', 'digits', 'number of digits to print', 0),
                         arg('expression', 'mathematical expression'))

    result = eval(parse(text = args$expression))
    if (! args$allow_nan && is.nan(result))
        sys$exit(1, 'NaN result invalid')
    sys$printf('%.*f', args$digits, result)
})

Saving the code as calc.r, making it executable and running it, via

./calc.r -d 2 "10 / 3"

yields the output 3.33. Running it with missing or invalid arguments displays an error message:

./calc.r

Usage: ./calc.r [--allow-nan] [--digits=DIGITS] expression
Error: Mandatory argument ‘expression’ not provided

Running it via ./calc.r --help displays a formatted help message, listing the available options, their description and default values:

A simple mathematical expression evaluator (version 1.0)

Usage: ./calc.r [--allow-nan] [--digits=DIGITS] expression

Positional arguments:
  expression         mathematical expression

Options:
  -n, --allow-nan    allow NaN results (otherwise, the program exits with a
                     failure) (default: FALSE)
  -d, --digits       number of digits to print (default: 0)

Status

This module already largely works but the API (in particular that of the command line parser) are still subject to change. Use at your own risk.