How to use ArgAction::Append with derive instead of builder?

[sloganking]

Hello. I am attempting to receive multiple values passed after one flag, instead of having to repeat the flag every time I want to take another argument for it. Something like --flag val1 val2 instead of --flag val1 --flag val2

It looks like ArgAction::Append can do this. However the documentation for it is always under the builder section of the docs.
https://docs.rs/clap/latest/clap/builder/enum.ArgAction.html#variant.Append

I see I can pass action values in the derive syntax via:

#[clap(long, default_values = &["Solarized (dark)"], action = clap::ArgAction::Append)]
pub theme: Vec<String>,

But upon running my command with --theme "Solarized (dark)" "Solarized (light)", I still get

error: Found argument 'Solarized (light)' which wasn't expected, or isn't valid in this context

[epage]

Something like --flag val1 val2 instead of --flag val1 --flag val2

That is the opposite of ArgAction::Append. Assuming you are using clap v4, you want num_args.

Keep in mind what clap automatically does based on the field's type](https://docs.rs/clap/latest/clap/_derive/index.html#arg-types) as you will want to consider how much you want to undo.

It looks like ArgAction::Append can do this. However the documentation for it is always under the builder section of the docs.

The derive reference explains raw attributes and delegates documentation for them to the builder API.

See also #4090

[sloganking]

So to take between 0 and infinite args, I would use .num_args(0..)?

For now I have solved my issue by using value_delimiter = ',', used like so #[clap(long, value_delimiter = ',')]. But I am wondering if there is an advantage to using .num_args(0..) instead of value_delimiter = ',' or value_delimiter = ' '.

[epage]

Note that value_delimiter = ' ' requires quoting, like cargo check --features "foo bar"

Generally, Append and/or value_delimiter are encouraged over num_args

[sloganking]

Generally, Append and/or value_delimiter are encouraged over num_args

Do you have a reason why this is? value_delimiter = ',' works fine unless you want to have the , character as part of your input value.

For instance feeding "hello,world",foo to value_delimiter = ',' would result in ["hello", "world", "foo"]. But feeding it to num_args = 0.. would result in ["hello,world", "foo"], which seems more intuitive given the quoting behavior.

Both seem workable, but I'm just trying to iron out the whys here.

[epage]

I'd need more details to know have an idea of why it is working that way. I would need a reproduction case, what shell you are running, etc.