# Module Arg

    module Arg = struct ... end

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 Pervasives.ref`` | `(* Set the reference to true *)`
`| Clear of `bool Pervasives.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 : (string * spec * string) list -> (string -> unit) -> 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.
- `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 two unit options, `-help` and `--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` and `--help` options 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 : (string * spec * string) list -> 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 Pervasives.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.