Implementing Boolean Negation Flags with Clap

Rust’s clap library is the language’s most popular crate for parsing command-line arguments. Though it has many useful features, programmers used to libraries like Python’s Click may find themselves struggling to implement one certain CLI convention: boolean flags --foo and --no-foo that undo each other. This article will show you how to implement such flags using clap’s Parser derivation mode — no need to dig into the far more verbose “builder” mode!

To implement two boolean flags --foo and --no-foo such that the field foo is true when --foo is given last and false when either no arguments are given or when --no-foo is given last, simply declare a --foo boolean option as usual, then add a separate --no-foo option, and declare that they override each other using overrides_with. The --foo option field will then end up holding the final boolean value, and the --no-foo field will be unused, so put an underscore at the start of its name so that clippy doesn’t complain. (If you don’t want any unused fields in your arguments struct, I’m afraid your only option is to turn to the more verbose builder mode.)

A minimal example of such code would look like this:

use clap::Parser;

#[derive(Debug, Parser)]
struct Arguments {
    /// Foo all the bars
    #[clap(long, overrides_with = "_no_foo")]
    foo: bool,

    /// Don't foo any bars [default]
    #[clap(long = "no-foo")]
    _no_foo: bool,
}

fn main() {
    for opts in [
        vec!["arg0"],
        vec!["arg0", "--foo"],
        vec!["arg0", "--no-foo"],
        vec!["arg0", "--no-foo", "--foo"],
        vec!["arg0", "--foo", "--no-foo"],
        vec!["arg0", "--no-foo", "--foo", "--no-foo"],
        vec!["arg0", "--foo", "--no-foo", "--foo"],
    ] {
        let args = Arguments::parse_from(&opts);
        println!("{opts:?} -> {args:?}");
    }
}

[Link to the code on the Rust Playground]

The output from the above code is:

["arg0"] -> Arguments { foo: false, _no_foo: false }
["arg0", "--foo"] -> Arguments { foo: true, _no_foo: false }
["arg0", "--no-foo"] -> Arguments { foo: false, _no_foo: true }
["arg0", "--no-foo", "--foo"] -> Arguments { foo: true, _no_foo: false }
["arg0", "--foo", "--no-foo"] -> Arguments { foo: false, _no_foo: true }
["arg0", "--no-foo", "--foo", "--no-foo"] -> Arguments { foo: false, _no_foo: true }
["arg0", "--foo", "--no-foo", "--foo"] -> Arguments { foo: true, _no_foo: false }

As you can see, the value of the foo field is true when --foo is the last --foo/--no-foo option given and false otherwise.

What if you want a pair of boolean flags where the default is --foo/true rather than --no-foo/false? This is doable, but the code can look quite a bit confusing. Starting with the setup above, add action = clap::builder::ArgAction::SetFalse to the --no-foo option; this will invert the values of the --no-foo field, causing it to be true whenever --foo is the option in effect and false otherwise. But wait — that’s what we want for the --foo field, isn’t it? So we then swap the names of the fields (along with any documentation comments) while preserving their long and/or short attributes (which must now be given explicit values), so that the field named foo, now corresponding to the --no-foo option, will be true when --foo is given last and also when no options are given.

Sample code:

use clap::builder::ArgAction;
use clap::Parser;

#[derive(Debug, Parser)]
struct Arguments {
    /// Don't foo any bars
    #[clap(long = "no-foo", action = ArgAction::SetFalse)]
    foo: bool,

    /// Foo all the bars [default]
    #[clap(long = "foo", overrides_with = "foo")]
    _no_foo: bool,
}

fn main() {
    for opts in [
        vec!["arg0"],
        vec!["arg0", "--foo"],
        vec!["arg0", "--no-foo"],
        vec!["arg0", "--no-foo", "--foo"],
        vec!["arg0", "--foo", "--no-foo"],
        vec!["arg0", "--no-foo", "--foo", "--no-foo"],
        vec!["arg0", "--foo", "--no-foo", "--foo"],
    ] {
        let args = Arguments::parse_from(&opts);
        println!("{opts:?} -> {args:?}");
    }
}

[Link to the code on the Rust Playground]

Output:

["arg0"] -> Arguments { foo: true, _no_foo: false }
["arg0", "--foo"] -> Arguments { foo: true, _no_foo: true }
["arg0", "--no-foo"] -> Arguments { foo: false, _no_foo: false }
["arg0", "--no-foo", "--foo"] -> Arguments { foo: true, _no_foo: true }
["arg0", "--foo", "--no-foo"] -> Arguments { foo: false, _no_foo: false }
["arg0", "--no-foo", "--foo", "--no-foo"] -> Arguments { foo: false, _no_foo: false }
["arg0", "--foo", "--no-foo", "--foo"] -> Arguments { foo: true, _no_foo: true }