package monomorphic

  1. Overview
  2. Docs
Module type
Class type

Parsing of command line arguments.

This module provides a general mechanism for extracting options and arguments from the command line to the program. For example:

let usage_msg = "append [-verbose] <file1> [<file2>] ... -o <output>"
let verbose = ref false
let input_files = ref []
let output_file = ref ""

let anon_fun filename =
  input_files := filename::!input_files

let speclist =
  [("-verbose", Arg.Set verbose, "Output debug information");
   ("-o", Arg.Set_string output_file, "Set output file name")]

let () =
  Arg.parse speclist anon_fun usage_msg;
  (* Main functionality here *)

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, Rest, Rest_all and Expand.

Unit, Set and Clear keywords take no argument.

A Rest or Rest_all keyword takes the remainder of the command line as arguments. (More explanations below.)

Every other keyword takes the following word on the command line as argument. For compatibility with GNU getopt_long, keyword=arg is also allowed. 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)

Rest takes a function that is called repeatedly for each remaining command line argument. Rest_all takes a function that is called once, with the list of all remaining arguments.

Note that if no arguments follow a Rest keyword then the function is not called at all whereas the function for a Rest_all keyword is called with an empty list.

type spec =
  1. | Unit of unit -> unit

    Call the function with unit argument

  2. | Bool of bool -> unit

    Call the function with a bool argument

  3. | Set of bool ref

    Set the reference to true

  4. | Clear of bool ref

    Set the reference to false

  5. | String of string -> unit

    Call the function with a string argument

  6. | Set_string of string ref

    Set the reference to the string argument

  7. | Int of int -> unit

    Call the function with an int argument

  8. | Set_int of int ref

    Set the reference to the int argument

  9. | Float of float -> unit

    Call the function with a float argument

  10. | Set_float of float ref

    Set the reference to the float argument

  11. | Tuple of spec list

    Take several arguments according to the spec list

  12. | Symbol of string list * string -> unit

    Take one of the symbols as argument and call the function with the symbol

  13. | Rest of string -> unit

    Stop interpreting keywords and call the function with each remaining argument

  14. | Rest_all of string list -> unit

    Stop interpreting keywords and call the function with all remaining arguments

  15. | Expand of string -> string array

    If the remaining arguments to process are of the form ["-foo"; "arg"] @ rest where "foo" is registered as Expand f, then the arguments f "arg" @ rest are processed. Only allowed in parse_and_expand_argv_dynamic.


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. Beware: options that have an empty doc string will not be included in the list.

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_dynamic : (key * spec * doc) list ref -> anon_fun -> usage_msg -> unit

Same as Arg.parse, except that the speclist argument is a reference and may be updated during the parsing. A typical use for this feature is to parse command lines of the form:

  • command subcommand options where the list of options depends on the value of the subcommand argument.
  • since 4.01.0
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.

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

Same as Arg.parse_argv, except that the speclist argument is a reference and may be updated during the parsing. See Arg.parse_dynamic.

  • since 4.01.0
val parse_and_expand_argv_dynamic : int ref -> string array ref -> (key * spec * doc) list ref -> anon_fun -> string -> unit

Same as Arg.parse_argv_dynamic, except that the argv argument is a reference and may be updated during the parsing of Expand arguments. See Arg.parse_argv_dynamic.

  • since 4.05.0
val parse_expand : (key * spec * doc) list -> anon_fun -> usage_msg -> unit

Same as Arg.parse, except that the Expand arguments are allowed and the current reference is not updated.

  • since 4.05.0
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 : ?limit:int -> (key * spec * doc) list -> (key * spec * doc) list

Align the documentation strings by inserting spaces at the first alignment separator (tab or, if tab is not found, space), according to the length of the keyword. Use a alignment separator 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.

  • parameter limit

    options with keyword and message longer than limit will not be used to compute the alignment.

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.

val read_arg : string -> string array

Arg.read_arg file reads newline-terminated command line arguments from file file.

  • since 4.05.0
val read_arg0 : string -> string array

Identical to Arg.read_arg but assumes null character terminated command line arguments.

  • since 4.05.0
val write_arg : string -> string array -> unit

Arg.write_arg file args writes the arguments args newline-terminated into the file file. If the any of the arguments in args contains a newline, use Arg.write_arg0 instead.

  • since 4.05.0
val write_arg0 : string -> string array -> unit

Identical to Arg.write_arg but uses the null character for terminator instead of newline.

  • since 4.05.0

Innovation. Community. Security.