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. There are six types of keywords: Unit, Set, Clear, String, Int, and Float. Unit, Set and Clear keywords take no argument. String, Int, and Float keywords take the following word on the command line as an argument. 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")

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 *)

The concrete type describing the behavior associated with a keyword.

val parse : (string * spec * string) list -> (string -> unit) -> string -> unit

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, parse exits the program, after printing an error message as follows:
The reason for the error: unknown option, invalid or missing argument, etc.
usage_msg
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 Bad with an error message to reject invalid arguments.

val usage: (string * spec * string) list -> string -> unit

usage speclist usage_msg speclist and usage_msg are the same as for parse. usage prints the same error message that parse prints in case of error.

val current: int ref;;

Position (in Sys.argv) of the argument being processed. You can change this value, e.g. to force parse to skip some arguments.