R equivalent of python module structure and functionality?

how to structure an R file such that it can be both

1. used as a script via, e.g., (from OS commandline)

$ Rscript foo.r bar=baz

2. imported and called as a function via, e.g. (from R commandline)

source('./foo.r')

or otherwise loaded, then called, e.g.

foo(bar='baz')

? I'm looking for the 'R equivalent' of how python supports this

The optparse package is what you would want [to parse args for] a
script i.e.``Rscript foo.R --bar=baz`` in a pythonic manner.

The other piece of the puzzle is the ``interactive()`` function
which lets you know if you are are calling from the "R commandline".

Example:

### begin foo.R #####

# define foo function
foo <- function(bar) {
   print(bar)
}

# if not interactive we are calling from OS command line
# parse args and call function foo
if(!interactive()) {
   suppressPackageStartupMessages(library("optparse"))
  option_list <- list(
     make_option(c("-b", "--bar"), default="hello world")
  )
  opt <- parse_args(OptionParser(option_list=option_list))
  foo(bar=opt$bar)
}

##### end foo.R  ######