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docopt creates beautiful command-line interfaces

This is a port of docopt to Nim. Visit for more information.

let doc = """
Naval Fate.

  naval_fate ship new <name>...
  naval_fate ship <name> move <x> <y> [--speed=<kn>]
  naval_fate ship shoot <x> <y>
  naval_fate mine (set|remove) <x> <y> [--moored | --drifting]
  naval_fate (-h | --help)
  naval_fate --version

  -h --help     Show this screen.
  --version     Show version.
  --speed=<kn>  Speed in knots [default: 10].
  --moored      Moored (anchored) mine.
  --drifting    Drifting mine.

import strutils
import docopt

let args = docopt(doc, version = "Naval Fate 2.0")

if args["move"]:
  echo "Moving ship $# to ($#, $#) at $# kn".format(
    args["<name>"], args["<x>"], args["<y>"], args["--speed"])
    parseFloat($args["<x>"]), parseFloat($args["<y>"]),
    speed = parseFloat($args["--speed"]))

if args["new"]: 
  for name in @(args["<name>"]): 
    echo "Creating ship $#" % name 

The option parser is generated based on the docstring above that is passed to docopt function. docopt parses the usage pattern ("Usage: ...") and option descriptions (lines starting with dash "-") and ensures that the program invocation matches the usage pattern; it parses options, arguments and commands based on that. The basic idea is that a good help message has all necessary information in it to make a parser.


proc docopt(doc: string, argv: seq[string] = nil,
            help = true, version: string = nil,
            optionsFirst = false, quit = true): Table[string, Value]

docopt takes 1 required and 5 optional arguments:

  • doc is a string that contains a help message that will be parsed to create the option parser. The simple rules of how to write such a help message are described at Here is a quick example of such a string:

      Usage: my_program [-hso FILE] [--quiet | --verbose] [INPUT ...]
      -h --help    show this
      -s --sorted  sorted output
      -o FILE      specify output file [default: ./test.txt]
      --quiet      print less text
      --verbose    print more text
  • argv is an optional argument vector; by default docopt uses the argument vector passed to your program (commandLineParams()). Alternatively you can supply a list of strings like @["--verbose", "-o", "hai.txt"].

  • help, by default true, specifies whether the parser should automatically print the help message (supplied as doc) and terminate, in case -h or --help option is encountered (options should exist in usage pattern). If you want to handle -h or --help options manually (as other options), set help = false.

  • version, by default nil, is an optional argument that specifies the version of your program. If supplied, then, (assuming --version option is mentioned in usage pattern) when parser encounters the --version option, it will print the supplied version and terminate. version can be any string, e.g. "2.1.0rc1".

    Note, when docopt is set to automatically handle -h, --help and --version options, you still need to mention them in usage pattern for this to work. Also, for your users to know about them.

  • optionsFirst, by default false. If set to true will disallow mixing options and positional arguments. I.e. after first positional argument, all arguments will be interpreted as positional even if the look like options. This can be used for strict compatibility with POSIX, or if you want to dispatch your arguments to other programs.

  • quit, by default true, specifies whether quit() should be called after encountering invalid arguments or printing the help message (see help). Setting this to false will allow docopt to raise a DocoptExit exception (with the usage member set) instead.

If the doc string is invalid, DocoptLanguageError will be raised.

The return value is a Table with options, arguments and commands as keys, spelled exactly like in your help message. Long versions of options are given priority. For example, if you invoke the top example as:

naval_fate ship Guardian move 100 150 --speed=15

the result will be:

{"--drifting": false,     "mine": false,
 "--help": false,         "move": true,
 "--moored": false,       "new": false,
 "--speed": "15",         "remove": false,
 "--version": false,      "set": false,
 "<name>": @["Guardian"], "ship": true,
 "<x>": "100",            "shoot": false,
 "<y>": "150"}

Note that this is not how the values are actually stored, because a Table can hold values of only one type. For that reason, a variant Value type is needed. Value's only accessible member is kind: ValueKind (which shouldn't be needed anyway, because it is known beforehand). ValueKind is one of:

  • vkNone (No value)

    This kind of Value appears when there is an option which hasn't been set and has no default. It is false when converted toBool.

  • vkBool (A boolean)

    This represents whether a boolean flag has been set or not. Just use it in a boolean context (conversion toBool is present).

  • vkInt (An integer)

    An integer represents how many times a flag has been repeated (if it is possible to supply it multiple times). Use value.len to obtain this int, or just use the value in a boolean context to find out whether this flag is present at least once.

  • vkStr (A string)

    Any option that has a user-supplied value will be represented as a string (conversion to integers, etc, does not happen). To obtain this string, use $value.

  • vkList (A list of strings)

    Any value that can be supplied multiple times will be represented by a seq[string], even if the user provides just one. To obtain this seq, use @value. To obtain its length, use value.len or @value.len. To obtain the n-th value (0-indexed), both value[i] and @value[i] will work. If you are sure there is exactly one value, $value is the same as value[0].

Note that you can use any kind of value in a boolean context and convert any value to string.

Look in the source code to find out more about these conversions.

As of version 0.7.0 docopt also includes a dispatch mechanism for automatically running procedures and converting arguments. This works by a simple macro that inspects the signature of the given procedure. The macro then returns code that will inspect the parsed arguments and if a list of supplied conditions are true the matched arguments from the signature will be extracted from the arguments and converted to the correct type before the procedure is called. A simple example would be something like this (a longer example can be found in the examples folder):

let doc = """
Naval Fate Lite

  naval_fate ship new <name>...
  naval_fate ship <name> move <x> <y> [--speed=<kn>]
  naval_fate (-h | --help)
  naval_fate --version

  -h --help     Show this screen.
  --version     Show version.
  --speed=<kn>  Speed in knots [default: 10].

import strutils
import docopt
import docopt/dispatch
import sequtils

let args = docopt(doc, version = "Naval Fate Lite")

# Define procedures with parameters named the same as the arguments
proc newShip(name: seq[string]) =
  for ship in name:
    echo "Creating ship $#" % ship

proc moveShip(name: string, x, y: int, speed: int) =
  echo "Moving ship $# to ($#, $#) at $# kn".format(
    name, x, y, speed)

if args.dispatchProc(newShip, "ship", "new") or # Runs newShip if "ship" and "new" is set
  args.dispatchProc(moveShip, "ship", "move"): # Runs newShip if "ship" and "move" is set
  echo "Ran something"
  echo doc


See examples folder.

For more examples of docopt language see examples.


nimble install docopt

This library has no dependencies outside the standard library. An impure re library is used.


Command line arguments parser that will make you smile (port of docopt to Nim)








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