Add logic to ArgPredicate

[jaskij]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

4.1.6

Describe your use case

I want to set an arguments default value, using default_value_ifs() depending on multiple other arguments.

For example, in the code below, I would like to change the value for destination depending on the value of data_type (since in actual usage the UDP port depends on the type of data we're sending).

#[derive(Debug, clap::Parser)]
#[command(
    long_about = None,
    long_version(crate::build_info::BUILD_INFO.long_version_string(),
))]
pub(crate) struct Cli {
    #[arg(value_enum)]
    // Which data type to send
    pub(crate) data_type: DataType,

    #[arg(value_enum)]
    // How to send
    pub(crate) sender: Sender,

    #[arg(
        short,
        long,
        default_value_ifs([
            ("sender", clap::builder::ArgPredicate::Equals("nats".into()), "localhost:4222"),
            ("sender", clap::builder::ArgPredicate::Equals("udp".into()), "localhost:8585")
        ]),
    )]
    /// Where to send
    pub destination: String,
}

Describe the solution you'd like

The idea is to expand ArgPredicate to look something like this:

pub enum ArgPredicate {
    IsPresent,
    Equals(OsStr),
    And(ArgPredicate, ArgPredicate),
    Or(ArgPredicate, ArgPredicate),
}

Alternatives, if applicable

No idea

Additional Context

No response

[epage]

We've talked of generalizing a lot of this logic in #3476.

For this specific proposal, I'm not seeing how it would work as it sounds like you want a default for destination to read both sender and data_type but ArgPredicate doesn't support that, so adding And and Or wouldn't do much.

[jaskij]

Argh, my bad. Yes, it does not read multiple arguments. So the change would require moving the argument name into the predicate, like so:

pub enum ArgPredicate {
    IsPresent,
    Equals(OsStr, OsStr),
    And(ArgPredicate, ArgPredicate),
    Or(ArgPredicate, ArgPredicate),
}

But, I just realized that this would lead to a full-blown logic system and end up with an inner platform effect. So might as wall ditch the predicate, accept something like Fn(&HashMap<OsStr, OsStr>) -> OsStr and let clap's user worry about it.