Is it possible to get the default value from the value of another flag?

[bjeanes]

This is similar to #1879, except with the added complexity of values.

I basically would like to define a relationship where by --foo=42 would imply --bar=42, unless --bar was explicitly given it's own value.

If --foo has a default value of 99, I'd be after something like:

$ cmd                  # --foo=99 and --bar=99 implied
$ cmd --foo=42         # --bar=42 implied
$ cmd --foo=42 --bar=0 # still possible
$ cmd --bar=0          # --foo=99 because --foo sets default for --bar, not vice versa

[CreepySkeleton]

You're probably looking for this one: https://docs.rs/clap/2.33.0/clap/struct.Arg.html#method.overrides_with

[CreepySkeleton]

Nope, wrong. I don't think this kind of relations is possible.

[kbknapp]

There isn't a current way to do that in clap code, however doing so in consumer code is fairly easy:

    let matches = App::new("test")
        .arg(Arg::with_name("foo").long("foo").default_value("99"))
        .arg(Arg::with_name("bar").long("bar").takes_value(true))
        .get_matches();

    let foo = matches.value_of("foo").unwrap();
    let bar = matches.value_of("bar").unwrap_or(foo);

Edit: simplified

[bjeanes]

Yep that's roughly what I am doing. The only reason I asked if it was built in is so that the [default: ...] part in generated docs (I'm using the derive macro) would reflect this in some way. But I suppose I could append [default: value of --foo] to the docstring 🤔 ...

[kbknapp]

Yes, unfortunately that's the best option right now. I don't know that having one arg mirror the default value of another arg is a general enough use case for us to add it to the feature list.

[bjeanes]

Yup, that's fair enough!