Modernize the docs for args.

pkg/args/README.md

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[nweiz, 2014/05/12 20:15:30]

Empty line.

[Bob Nystrom, 2014/05/12 21:29:26]

Done.

+Parses raw command-line arguments into a set of options and values.
+
+This library supports [GNU][] and [POSIX][] style options, and it works
+in both server-side and client-side apps.
+
+For information on installing this library, see the
+[args package on pub.dartlang.org](http://pub.dartlang.org/packages/args).
+Import it like so:
+
+    import 'package:args/args.dart';

[nweiz, 2014/05/12 20:15:30]

I think at this point installation instructions produce more clutter than
benefit.

[Bob Nystrom, 2014/05/12 21:29:26]

Done.

+
+## Defining options
+
+First create an [ArgParser][]:
+
+    var parser = new ArgParser();
+
+Then define a set of options on that parser using [addOption()][addOption] and
+[addFlag()][addFlag]. Here's the minimal way to create an option named "name":
+
+    parser.addOption('name');
+
+When an option can only be set or unset (as opposed to taking a string value),
+use a flag:
+
+    parser.addFlag('name');
+
+Flag options, by default, accept a 'no-' prefix to negate the option. You can
+disable the 'no-' prefix using the `negatable` parameter:
+
+    parser.addFlag('name', negatable: false);
+
+*Note:* From here on out, "option" refers to both regular options and flags. In
+cases where the distinction matters, we'll use "non-flag option."
+
+Options can have an optional single-character abbreviation, specified with the
+`abbr` parameter:
+
+    parser.addOption('mode', abbr: 'm');
+    parser.addFlag('verbose', abbr: 'v');
+
+Options can also have a default value, specified with the `defaultsTo`
+parameter. The default value is used when arguments don't specify the option.
+
+    parser.addOption('mode', defaultsTo: 'debug');
+    parser.addFlag('verbose', defaultsTo: false);
+
+The default value for non-flag options can be any string. For flags, it must
+be a `bool`.
+
+To validate a non-flag option, you can use the `allowed` parameter to provide an
+allowed set of values. When you do, the parser throws a [FormatException] if the
+value for an option is not in the allowed set. Here's an example of specifying
+allowed values:
+
+    parser.addOption('mode', allowed: ['debug', 'release']);
+
+You can use the `callback` parameter to associate a function with an option.
+Later, when parsing occurs, the callback function is invoked with the value of
+the option:
+
+    parser.addOption('mode', callback: (mode) => print('Got mode $mode));
+    parser.addFlag('verbose', callback: (verbose) {
+      if (verbose) print('Verbose');
+    });
+
+The callbacks for all options are called whenever a set of arguments is parsed.
+If an option isn't provided in the args, its callback is passed the default
+value, or `null` if no default value is set.
+
+## Parsing arguments
+
+Once you have an [ArgParser][] set up with some options and flags, you use it by
+calling [ArgParser.parse()][parse] with a set of arguments:
+
+    var results = parser.parse(['some', 'command', 'line', 'args']);
+
+These arguments usually come from the arguments to `main()`, but you can pass in
+any list of strings. The `parse()` method returns an instance of [ArgResults][],
+a map-like object that contains the values of the parsed options.
+
+    var parser = new ArgParser();
+    parser.addOption('mode');
+    parser.addFlag('verbose', defaultsTo: true);
+    var results = parser.parse(['--mode', 'debug', 'something', 'else']);
+
+    print(results['mode']); // debug
+    print(results['verbose']); // true
+
+By default, the `parse()` method stops as soon as it reaches `--` by itself or
+anything that the parser doesn't recognize as an option, flag, or option value.
+If arguments still remain, they go into [ArgResults.rest][rest].
+
+    print(results.rest); // ['something', 'else']
+
+To continue to parse options found after non-option arguments, pass
+`allowTrailingOptions: true` when creating the [ArgParser][].
+
+## Specifying options
+
+To actually pass in options and flags on the command line, use GNU or POSIX
+style. Consider this option:
+
+    parser.addOption('name', abbr: 'n');
+
+You can specify its value on the command line using any of the following:
+
+    --name=somevalue
+    --name somevalue
+    -nsomevalue
+    -n somevalue
+
+Consider this flag:
+
+    parser.addFlag('name', abbr: 'n');
+
+You can set it to true using one of the following:
+
+    --name
+    -n
+
+You can set it to false using the following:
+
+    --no-name
+
+Multiple flag abbreviations can be collapsed into a single argument. Say you
+define these flags:
+
+    parser.addFlag('verbose', abbr: 'v');
+    parser.addFlag('french', abbr: 'f');
+    parser.addFlag('iambic-pentameter', abbr: 'i');
+
+You can set all three flags at once:
+
+    -vfi
+
+By default, an option has only a single value, with later option values
+overriding earlier ones; for example:
+
+    var parser = new ArgParser();
+    parser.addOption('mode');
+    var results = parser.parse(['--mode', 'on', '--mode', 'off']);
+    print(results['mode']); // prints 'off'
+
+If you need multiple values, set the `allowMultiple` parameter. In that case the
+option can occur multiple times, and the `parse()` method returns a list of
+values:
+
+    var parser = new ArgParser();
+    parser.addOption('mode', allowMultiple: true);
+    var results = parser.parse(['--mode', 'on', '--mode', 'off']);
+    print(results['mode']); // prints '[on, off]'
+
+## Defining commands ##
+
+In addition to *options*, you can also define *commands*. A command is a named
+argument that has its own set of options. For example, consider this shell
+command:
+
+    $ git commit -a
+
+The executable is `git`, the command is `commit`, and the `-a` option is an
+option passed to the command. You can add a command using the [addCommand][]
+method:
+
+    var parser = new ArgParser();
+    var command = parser.addCommand('commit');
+
+It returns another [ArgParser][], which you can then use to define options
+specific to that command. If you already have an [ArgParser][] for the command's
+options, you can pass it in:
+
+    var parser = new ArgParser();
+    var command = new ArgParser();
+    parser.addCommand('commit', command);
+
+The [ArgParser][] for a command can then define options or flags:
+
+    command.addFlag('all', abbr: 'a');
+
+You can add multiple commands to the same parser so that a user can select one
+from a range of possible commands. When parsing an argument list, you can then
+determine which command was entered and what options were provided for it.
+
+    var results = parser.parse(['commit', '-a']);
+    print(results.command.name);   // "commit"
+    print(results.command['all']); // true
+
+Options for a command must appear after the command in the argument list. For
+example, given the above parser, `"git -a commit"` is *not* valid. The parser
+tries to find the right-most command that accepts an option. For example:
+
+    var parser = new ArgParser();
+    parser.addFlag('all', abbr: 'a');
+    var command = parser.addCommand('commit');
+    command.addFlag('all', abbr: 'a');
+
+    var results = parser.parse(['commit', '-a']);
+    print(results.command['all']); // true
+
+Here, both the top-level parser and the `"commit"` command can accept a `"-a"`
+(which is probably a bad command line interface, admittedly). In that case, when
+`"-a"` appears after `"commit"`, it is applied to that command. If it appears to
+the left of `"commit"`, it is given to the top-level parser.
+
+## Displaying usage
+
+You can automatically generate nice help text, suitable for use as the output of
+`--help`. To display good usage information, you should provide some help text
+when you create your options.
+
+To define help text for an entire option, use the `help:` parameter:
+
+    parser.addOption('mode', help: 'The compiler configuration',
+        allowed: ['debug', 'release']);
+    parser.addFlag('verbose', help: 'Show additional diagnostic info');
+
+For non-flag options, you can also provide detailed help for each expected value
+by using the `allowedHelp:` parameter:
+
+    parser.addOption('arch', help: 'The architecture to compile for',
+        allowedHelp: {
+          'ia32': 'Intel x86',
+          'arm': 'ARM Holding 32-bit chip'
+        });
+
+To display the help, use the [getUsage()][getUsage] method:
+
+    print(parser.getUsage());
+
+The resulting string looks something like this:
+
+    --mode            The compiler configuration
+                      [debug, release]
+
+    --[no-]verbose    Show additional diagnostic info
+    --arch            The architecture to compile for
+
+          [arm]       ARM Holding 32-bit chip
+          [ia32]      Intel x86
+
+To assist the formatting of the usage help, single-line help text is followed by
+a single new line. Options with multi-line help text are followed by two new
+lines. This provides spatial diversity between options.
+
+[posix]: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap12.html#tag_12_02
+[gnu]: http://www.gnu.org/prep/standards/standards.html#Command_002dLine-Interfaces
+[ArgParser]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser
+[ArgResults]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgResults
+[addOption]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_addOption
+[addFlag]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_addFlag
+[parse]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_parse
+[rest]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgResults#id_rest
+[addCommand]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_addCommand
+[getUsage]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_getUsage