Introduce Directive symbol type

[adamsitnik]

From #2046:

Currently the only way for the users to implement custom directives is enabling directives parsing in the config, using this property to get parsed output:

command-line-api/src/System.CommandLine/ParseResult.cs

Line 86 in 76437b0

public IReadOnlyDictionary<string, IReadOnlyList<string>> Directives => _directives ??= new ();

and provide a custom, expensive Middleware.

We need to introduce a dedicated Directive type that derives from Symbol.

Initial requirements:

• introducing new Directive symbol type, with a possibility to define an Action<ParseResult, string> which should be executed when directive is parsed (the list of arguments for the action is an open question, when implementing the feature it should became more obvious)
• introducing Command.Directives property to allow users for defining directives
• turning existing code into new directives

  command-line-api/src/System.CommandLine/Parsing/ParseOperation.cs

  Lines 350 to 378 in 76437b0

  	private void OnDirectiveParsed(string directiveKey, string? parsedValues)
  	{
  	if (_configuration.EnableEnvironmentVariableDirective && directiveKey == "env")
  	{
  	if (parsedValues is not null)
  	{
  	var components = parsedValues.Split(new[] { '=' }, count: 2);
  	var variable = components.Length > 0 ? components[0].Trim() : string.Empty;
  	if (string.IsNullOrEmpty(variable) || components.Length < 2)
  	{
  	return;
  	}
  	var value = components[1].Trim();
  	Environment.SetEnvironmentVariable(variable, value);
  	}
  	}
  	else if (_configuration.ParseDirectiveExitCode.HasValue && directiveKey == "parse")
  	{
  	_isParseRequested = true;
  	_handler = new AnonymousCommandHandler(ParseDirectiveResult.Apply);
  	}
  	else if (_configuration.EnableSuggestDirective && directiveKey == "suggest")
  	{
  	int position = parsedValues is not null ? int.Parse(parsedValues) : _rawInput?.Length ?? 0;
  	_handler = new AnonymousCommandHandler(ctx => SuggestDirectiveResult.Apply(ctx, position));
  	}
  	}

• extending tokenization and parsing with the concept of Directive

[KalleOlaviNiemitalo]

I'm surprised that you want to place it in Command.Directives rather than in the config.
If a subcommand defines directives, then do users still place those directives at the start of the command line, or after the name of the subcommand?

[adamsitnik]

If a subcommand defines directives, then do users still place those directives at the start of the command line, or after the name of the subcommand?

They will still be supported only at the start. The problem with config is that it does not belong to symbol tree. But RootCommand does, so perhaps it should be allowed only for RootCommand?

[adamsitnik]

#2059 was supposed to close #2058, not this usse

[jonsequitur]

But RootCommand does, so perhaps it should be allowed only for RootCommand?

I think this is important.

We explicitly decided not to support directives anywhere on the command line after the first occurrence of a non-directive token.

[KalleOlaviNiemitalo]

We need to introduce a dedicated Directive type that derives from Symbol.

I'm not convinced that deriving Directive from Symbol is the best way to support custom directives. I'm thinking some IDictionary<string, Action<DirectiveContext>> in CommandLineConfiguration, instead. Because directives don't appear in help, cannot be placed in subcommands, and do not have names in the same namespace as subcommands and options.

If directive completion were a feature, then that could be a reason to derive from Symbol.

[adamsitnik]

Because directives don't appear in help

I believe that we should change that, otherwise how users are going to find out about their existence and capabilities?

If directive completion were a feature

We should consider that too.

[jonsequitur]

I think most end users will not care about directives (e.g. [suggest] is intended to be called from tooling, not directly by users) and many CLI authors won't want them in their help output. Maybe this is a use case for --help verbose or something similar.