Convert flag to optional option

[devmatteini]

Hi!

I have this simple clap Parser:

#[derive(Debug, Parser)]
pub struct Cli {
    #[arg(short, long)]
    install: bool,

    repo: String,
}

which you can run like

$ cargo run -- any_repo
# Args: Cli { install: false, repo: "any_repo" }
$ cargo run -- --install any_repo
# Args: Cli { install: true, repo: "any_repo" }

Now I need to convert the flag to an optional option:

#[derive(Debug, Parser)]
pub struct Cli {
    #[arg(short, long)]
    install: Option<Option<String>>,

    repo: String,
}

But running the last example command, it doesn't work anymore:

$ cargo run -- --install any_repo
error: the following required arguments were not provided:
  <REPO>

Usage: clap-flag-to-option --install [<INSTALL>] <REPO>

For more information, try '--help'

Here, the value any_repo is interpreted as --install value instead of the required argument.

The only way to make it work is to run cargo run -- --install= any_repo, where the value is an empty string.

In this situation, do you have any advice on possible solutions?

The two solution I see are:

1. Ship this possible breaking change
2. Leave install as a flag, and create a new option install-foo. Then I could use group to avoid setting both at the same time

[epage]

Yes, this is implicitly a num_args(0..=1) which comes with a big warning in the docs.

One option not covered there is to use require_equals = true

#!/usr/bin/env nargo
---
[dependencies]
clap = { path = "../clap", features =["derive"] }
---

use clap::Parser;

#[derive(Debug, Parser)]
pub struct Cli {
    #[arg(short, long, require_equals = true)]
    install: Option<Option<String>>,

    repo: String,
}

fn main() {
    let cli = Cli::parse();
    println!("{cli:?}");
}

$ ./clap-5598.rs  --install a
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.09s
     Running `/home/epage/src/personal/cargo/target/debug/cargo -Zscript ./clap-5598.rs --install a`
warning: `package.edition` is unspecified, defaulting to `2021`
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.07s
     Running `/home/epage/.cargo/target/38/fa72cbe0999338/debug/clap-5598 --install a`
Cli { install: Some(None), repo: "a" }

$ ./clap-5598.rs  --install=b a
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.08s
     Running `/home/epage/src/personal/cargo/target/debug/cargo -Zscript ./clap-5598.rs --install=b a`
warning: `package.edition` is unspecified, defaulting to `2021`
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.07s
     Running `/home/epage/.cargo/target/38/fa72cbe0999338/debug/clap-5598 --install=b a`
Cli { install: Some(Some("b")), repo: "a" }

[devmatteini]

Sounds good, thank you very much for the amazing work on clap 😄