Rules for positional options

[phlptp]

Positional arguments seem to be a topic of interest and figuring out how the matching should be done in relation to unlimited positional arguments and some other modifiers is a bit ad-hoc at the moment.

Positional rules
A positional argument will match the first option not filled searched in order

1. of Definition in the main app
2. By order of definition in any option_groups which are themselves ordered by order of definition.
3. positional arguments fall through to a parent if not otherwise matched.

if validate_positionals is active, then the additional consideration is that an argument must pass validation to match.

if positionals_at_end is active then positional options with the required flag will be filled with priority over non-required positionals options if insufficient arguments remain to fill any options beyond the required ones.

A question for anyone interested is are these rules sufficient or is more fine grained control necessary and if so what might that look like.

[phlptp]

based on some initial discussion in #306

[phlptp]

One specific issue is vector positionals are never filled unless they have a specific expected value set. So they consume all the positional arguments regardless of any other subsequent definitions.

[henryiii]

I think having the tutorial docs will help here, as we can map out what is currently done (and maybe inspire a plan for what should be done).

[kambala-decapitator]

I can't seem to understand how to make parsing fail when 0 positionals are given... Could you help?

Basic idea is: subcommand is required, positionals should be trailing (don't really like placing them at random places) and positionals have the same meaning for all subcommands. Sample call: app --param /some/file subcommand /path1 /path2

CLI::App app;
app.require_subcommand();
app.positionals_at_end();

std::string s;
app.add_option("--param", s, "")->required()->check(CLI::ExistingFile);

std::vector<std::string> filePaths;
// with required() parsing fails when positionals are given after subcommand
app.add_option("pos", filePaths, "Path(s) to file(s)")->expected(-1)->check(CLI::ExistingFile);

// adding positionals_at_end() has no effect
auto dumpCommand = app.add_subcommand("dump", "Dump mode")->fallthrough();

// subcommand params and other subcommands...

CLI11_PARSE(app, argc, argv);

[phlptp]

One possibility is to have the positional options enabled by the subcommand. Then they won't work unless the subcommand is called. it would also be possible to add required to the positional option.