19.1 Module Arg: parsing of command line arguments

This module provides a general mechanism for extracting options and arguments from the command line to the program.
Syntax of command lines: A keyword is a character string starting with a -. An option is a keyword alone or followed by an argument. The types of keywords are: Unit, Set, Clear, String, Int, Float, and Rest. Unit, Set and Clear keywords take no argument. String, Int, and Float keywords take the following word on the command line as an argument. A Rest keyword takes the remaining of the command line as (string) arguments. Arguments not preceded by a keyword are called anonymous arguments.
Examples (cmd is assumed to be the command name):
cmd -flag (a unit option)
cmd -int 1 (an int option with argument 1)
cmd -string foobar (a string option with argument "foobar")
cmd -float 12.34 (a float option with argument 12.34)
cmd a b c (three anonymous arguments: "a", "b", and "c")
cmd a b -- c d (two anonymous arguments and a rest option with
two arguments)
type spec =
  | Unit of (unit -> unit)     (* Call the function with unit argument *)
  | Set of bool ref            (* Set the reference to true *)
  | Clear of bool ref          (* Set the reference to false *)
  | String of (string -> unit) (* Call the function with a string argument *)
  | Int of (int -> unit)       (* Call the function with an int argument *)
  | Float of (float -> unit)   (* Call the function with a float argument *)
  | Rest of (string -> unit)   (* Stop interpreting keywords and call the
                                  function with each remaining argument *)
The concrete type describing the behavior associated with a keyword.
val parse : keywords:(string * spec * string) list ->
            others:(string -> unit) -> errmsg:string -> unit
Arg.parse speclist anonfun usage_msg parses the command line. speclist is a list of triples (key, spec, doc). key is the option keyword, it must start with a '-' character. spec gives the option type and the function to call when this option is found on the command line. doc is a one-line description of this option. anonfun is called on anonymous arguments. The functions in spec and anonfun are called in the same order as their arguments appear on the command line.

If an error occurs, Arg.parse exits the program, after printing an error message as follows:
The reason for the error: unknown option, invalid or missing argument, etc.
The list of options, each followed by the corresponding doc string.

For the user to be able to specify anonymous arguments starting with a -, include for example ("-", String anonfun, doc) in speclist.

By default, parse recognizes a unit option -help, which will display usage_msg and the list of options, and exit the program. You can override this behaviour by specifying your own -help option in speclist.
exception Bad of string
Functions in spec or anonfun can raise Arg.Bad with an error message to reject invalid arguments.
val usage : keywords:(string * spec * string) list -> errmsg:string -> unit
Arg.usage speclist usage_msg prints an error message including the list of valid options. This is the same message that Arg.parse prints in case of error. speclist and usage_msg are the same as for Arg.parse.
val current : int ref;;
Position (in Sys.argv) of the argument being processed. You can change this value, e.g. to force Arg.parse to skip some arguments.