# Module Arg

    module Arg: sig .. 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`, `Bool`, `Set`, `Clear`, `String`, `Set_string`, `Int`, `Set_int`, `Float`, `Set_float`, `Tuple`, `Symbol`, and `Rest`. `Unit`, `Set` and `Clear` keywords take no argument. A `Rest` keyword takes the remaining of the command line as arguments. Every other keyword takes the following word on the command line as 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"`)
- `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 | `*)`
`|` | `Bool of `(bool -> unit)`` | `(*` | Call the function with a bool 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 | `*)`
`|` | `Set_string of `string ref`` | `(*` | Set the reference to the string argument | `*)`
`|` | `Int of `(int -> unit)`` | `(*` | Call the function with an int argument | `*)`
`|` | `Set_int of `int ref`` | `(*` | Set the reference to the int argument | `*)`
`|` | `Float of `(float -> unit)`` | `(*` | Call the function with a float argument | `*)`
`|` | `Set_float of `float ref`` | `(*` | Set the reference to the float argument | `*)`
`|` | `Tuple of `spec list`` | `(*` | Take several arguments according to the spec list | `*)`
`|` | `Symbol of `string list * (string -> unit)`` | `(*` | Take one of the symbols as argument and call the function with the symbol | `*)`
`|` | `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.

    type key = string

    type doc = string

    type usage_msg = string

    type anon_fun = string -> unit

    val parse : (key * spec * doc) list -> anon_fun -> usage_msg -> unit

`Arg.parse speclist anon_fun 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. `anon_fun` is called on anonymous arguments. The functions in `spec` and `anon_fun` 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 to standard error 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 anon_fun, doc)` in `speclist`.

By default, `parse` recognizes two unit options, `-help` and `--help`, which will print to standard output `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`.

    val parse_argv : ?current:int ref ->
           string array ->
           (key * spec * doc) list -> anon_fun -> usage_msg -> unit

`Arg.parse_argv ~current args speclist anon_fun usage_msg` parses the array `args` as if it were the command line. It uses and updates the value of `~current` (if given), or `Arg.current`. You must set it before calling `parse_argv`. The initial value of `current` is the index of the program name (argument 0) in the array. If an error occurs, `Arg.parse_argv` raises `Arg.Bad` with the error message as argument. If option `-help` or `--help` is given, `Arg.parse_argv` raises `Arg.Help` with the help message as argument.

    exception Help of string

Raised by `Arg.parse_argv` when the user asks for help.

    exception Bad of string

Functions in `spec` or `anon_fun` can raise `Arg.Bad` with an error message to reject invalid arguments. `Arg.Bad` is also raised by `Arg.parse_argv` in case of an error.

    val usage : (key * spec * doc) list -> usage_msg -> unit

`Arg.usage speclist usage_msg` prints to standard error an error message that includes 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 usage_string : (key * spec * doc) list -> usage_msg -> string

Returns the message that would have been printed by `Arg.usage`, if provided with the same parameters.

    val align : (key * spec * doc) list -> (key * spec * doc) list

Align the documentation strings by inserting spaces at the first space, according to the length of the keyword. Use a space as the first character in a doc string if you want to align the whole string. The doc strings corresponding to `Symbol` arguments are aligned on the next line.

    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. `Arg.parse` uses the initial value of `Arg.current` as the index of argument 0 (the program name) and starts parsing arguments at the next element.