Structopt: Default subcommand

[bb010g]

It would be nice if you could specify a default subcommand, like systemctl does with list-units. It could look like this:

#[derive(StructOpt)]
#[structopt(name = "systemctl", default_cmd = "list-units")]
enum Systemctl {
    #[structopt(name = "list-units")]
    ListUnits {},
    #[structopt(name = "status")]
    Status {},
}

[TeXitoi]

I don't get the feature.

You can have an optional subcommand:

#[macro_use] extern crate structopt;
use structopt::StructOpt;
#[derive(StructOpt, Debug)]
struct Opt {
    #[structopt(subcommand)]
    subcommand: Option<Cmd>
}
#[derive(StructOpt, Debug)]
enum Cmd {
    #[structopt(name = "list-units")]
    ListUnits,
    #[structopt(name = "status")]
    Status,
}
fn main() {
    let opt = Opt::from_args();
    println!("{:?}", opt);
}

Having a "default subcommand" is quite complicated: you need something to create a default variant. I need a real case example to understand the need.

[TeXitoi]

@bb010g ping

[bb010g]

I'd simply like to be able to nicely have a default command, like you can have defaults for values. You could argue that default_value doesn't need to exist because you can just use an Option<T> and create the default with a function or at use, but it's convenient to specify the default right there. I'm not sure what you'd want to help clarify.

[TeXitoi]

If you type cmd -a -f the flags a and f should go to the subcommand ?

[bb010g]

That would be an issue. It looks like systemctl doesn't really have any subcommand-specific flags, which probably helps them get away with this. I'd lean towards prioritizing main command options first, and then subcommands if that fails, but that would introduce a new level of complexity to parsing. So yeah, with that in mind, explicitly using an Option<Cmd> and forcing the library user to think about how they want to handle default subcommand options would probably be the best choice, unless you or anyone else can think of a better solution. (It might not hurt to add a note on this to the docs on why there's not a default subcommand feature, and what you'd need to watch out for if you want to roll your own.)

[TeXitoi]

I don't have any great idea about that.

Now, with the context, it should be a feature request to clap that parse the command line, not structopt.

[nixpulvis]

Is there an issue or resolution to this in clap?

[TeXitoi]

I can't find anything with a quick search

[nicoulaj]

The feature request on clap is clap-rs/clap#975 but it has also been closed...