args: define args using dart method signature

[DartBot]

<img src="https://avatars.githubusercontent.com/u/444270?v=3" align="left" width="96" height="96"hspace="10"> Issue by seaneagan
Originally opened as dart-lang/sdk#12272

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The args of a command line script seem to map nicely to the args of a dart method. It would be nice to be able to define script args using the same techniques as dart methods, and be able to automatically forward script args to said method. Presumably both of these could be done using a mirror on the method.

Say we have a script called 'greet.dart':

    import 'package:args/args.dart' as args;
    
    main() => args.forward(_main);

    greet(String salutation, String name, [String punctuation = '']) =>
        print('$salutation $name$punctuation);

where _main could be defined using String positional arguments:

    // example:
    // some_script.dart John Hi
    _main(String name, [String salutation = 'Hello']) =>
        greet(salutation, name);

or String or bool named arguments:

    // example:
    // some_script.dart --salutation Hi --exclaim John
    _main(String name, {String salutation: 'Hello ', bool exclaim: false}) =>
        greet(salutation, name, exclaim ? '!' : '');

or with the trailing positional parameter being a List<String>:

    // example:
    // some_script.dart --salutation Hi Bob Alice
    _main(List<String> rest, {String salutation: 'Hello '}) =>
        greet(salutation, rest.join(', '));

If dart ever gets true rest arguments those could be used instead.

To add "help", comments on each arg would make sense, but that is against the dart style guide. For this and other arg metadata, dart's metadata could be used:

    _main(
        String name, {
        @­Arg(abbr: 's', 'How to greet them')
        String salutation: 'Hello ',
        @­Arg(abbr: 'p', help: 'Whether to includ a "!"', allowedHelp: const {'': 'none', '!': 'exclamation'})
        String punctuation = ''}) =>
        greet(salutation, name, punctuation);

[DartBot]

<img src="https://avatars.githubusercontent.com/u/5479?v=3" align="left" width="48" height="48"hspace="10"> Comment by sethladd

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Removed Type-Defect label.
Added Type-Enhancement, Area-Pkg, Library-Args, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/444270?v=3" align="left" width="48" height="48"hspace="10"> Comment by seaneagan

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This is now implemented as:

http://pub.dartlang.org/packages/unscripted

I think it is probably beyond the scope of the "args" package, in which case this bug can be closed.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3276024?v=3" align="left" width="48" height="48"hspace="10"> Comment by anders-sandholm

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Removed Library-Args label.
Added Pkg-Args label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Yeah, this is a swell idea but I want to avoid using mirrors in args.

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Added WontFix label.