add_argument: Add an argument to a parser.

Description

This function adds an argument to an arg.parser object and returns the modified object.

Usage

add_argument(
  parser,
  arg,
  help,
  default = NULL,
  type = NULL,
  nargs = NULL,
  flag = NULL,
  short = NULL
)

Value

an arg.parser object with the argument added

Arguments

parser: an arg.parser object
arg: argument name (use no prefix for positional arguments, -- or - prefix for optional arguments or flags)
help: help description for the argument
default: default value for the argument [default: NA]
type: variable type of the argument (which can be inferred from default); assumed to be character otherwise. See details for more information.
nargs: number of argument values (which can be inferred from default); set to Inf for an indefinite number; an optional argument with an indefinite number of values may need to be followed by another optional argument or flag (e.g. --) to separate the indefinite optional argument from possible position arguments
flag: whether argument is a flag (and does not consume a value) [default: FALSE]; during argument parsing, a flag argument is FALSE by default if it is not set
short: short-form for flags and positional arguments; short-forms can be assigned automatically based on the first character of the argument name, unless a conflict arises with an existing short-form; to avoid conflicts, add the argument as early as possible

Details

This function supports multiple arguments in a vector. To ensure that the argument variable type is set correctly, either specify type directly or supply default argument values as a list. Custom types are supported by defining a new class and a S4 method for coerce, see the examples section.

Examples

Run this code

p <- arg_parser("A text file modifying program")

# Add a positional argument
p <- add_argument(p, "input", help="input file")

# Add an optional argument
p <- add_argument(p, "--output", help="output file", default="output.txt")

# Add a flag
p <- add_argument(p, "--append", help="append to file", flag=TRUE)

# Add multiple arguments together
p <- add_argument(p,
    c("ref", "--date", "--sort"),
    help = c("reference file", "date stamp to use", "sort lines"),
    flag = c(FALSE, FALSE, TRUE))

# Print the help message
print(p)

# Example of custom type, using the example from pythons argparse
setClass("perfectSquare")
setMethod("coerce", c(from = "ANY", to = "perfectSquare"),
    function(from, to) {
      from <- as.numeric(from)
      if (!all.equal(from, as.integer(from))) {
        stop("Type error: ", from, " is not an integer!")
      }
      sqt <- sqrt(from)
      if (sqt != as.integer(sqt)) {
         stop("Type error: ", from, " is not a perfect square!")
      }
      from
    }
)

p2 <- arg_parser("Perfect square checker")
p2 <- add_argument(p2, arg = c("--perfect-square"), 
                   help = "A perfect square integer",
                   type = "perfectSquare")

parse_args(p2, c("--perfect-square", 144))