Clap: arbitraty function to validate argument

• value_parser

use clap::Parser;
use std::error::Error;

#[derive(Parser, Debug)]
struct Cli {
    #[arg(long, value_parser = parse_even_number)]
    number: u32,
}

fn parse_even_number(number: &str) -> Result<u32, Box<dyn Error + Send + Sync + 'static>> {
    let number = number.parse::<u32>()?;
    if number % 2 == 0 {
        return Ok(number);
    }
    Err(Box::<dyn Error + Send + Sync>::from(
        "An even number is expected",
    ))
}

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

$ cargo run -q -- --number 22
Cli { number: 22 }

$ cargo run -q -- --number 23
error: invalid value '23' for '--number <NUMBER>': An even number is expected

For more information, try '--help'.



$ cargo run -q -- --number ad
error: invalid value 'ad' for '--number <NUMBER>': invalid digit found in string

For more information, try '--help'.