help requested behaviour of parser_args doesn't match behaviour in README

[alexpenson]

suppressPackageStartupMessages(library("optparse"))
option_list <- list(
  make_option(c("-v", "--verbose"), action="store_true", default=TRUE,
              help="Print extra output [default]"),
  make_option(c("-q", "--quietly"), action="store_false",
              dest="verbose", help="Print little output"),
  make_option(c("-c", "--count"), type="integer", default=5,
              help="Number of random normals to generate [default %default]",
              metavar="number")
)
parser <- OptionParser(option_list=option_list)
parse_args(
  parser,
  args = c("--verbose", "--count=11"))
parse_args(parser, args = c("--help"))

> sessionInfo()
R version 3.5.0 (2018-04-23)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: CentOS release 6.9 (Final)

Matrix products: default
BLAS: /opt/common/CentOS_6-dev/R/R-3.5.0/lib64/R/lib/libRblas.so
LAPACK: /opt/common/CentOS_6-dev/R/R-3.5.0/lib64/R/lib/libRlapack.so

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
 [9] LC_ADDRESS=C               LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] optparse_1.6.0

loaded via a namespace (and not attached):
[1] compiler_3.5.0 getopt_1.20.2

[trevorld]

What exactly is the issue? This is the expected behaviour assuming interactive() is TRUE and you didn't manually set the print_help_and_exit argument of parse_args to FALSE. If you didn't manually set print_help_and_exit to FALSE and interactive() is FALSE it will instead call quit instead of throwing an error. If print_help_and_exit is manually set to FALSE if won't print out a help message nor throw an error/quit. optparse is intended primarily for writing Rscripts where printing a usage message and then killing scripts early if a help flag is observed is the desired behaviour of 99% of users but perhaps you might be interested instead of manually setting print_help_and_exit to FALSE and then manually calling the print_help function and then manually calling quit / killing the script early yourself instead of having parse_args doing that for you.

> print_help(parser)
Usage: %prog [options]


Options:
	-v, --verbose
		Print extra output [default]

	-q, --quietly
		Print little output

	-c NUMBER, --count=NUMBER
		Number of random normals to generate [default 5]

	-h, --help
		Show this help message and exit

[trevorld]

Although the behaviour is correct the documentation in the README does not fully match the behaviour and needs to be updated to match the behaviour.