clap/examples/15_custom_validator.rs

extern crate clap;

use clap::{App, Arg};

fn main() {
    // You can define a function (or a closure) to use as a validator to argument values. The
    // function must accept a String and return Result<(), String> where Err(String) is the message
    // displayed to the user.

    let matches = App::new("myapp")
                        // Application logic goes here...
                        .arg(Arg::with_name("input")
                                     .help("the input file to use")
                                     .index(1)
                                     .required(true)

                                     .validator(|val| {
                                         // val is the argument value passed in by the user
                                         // val has type of String.

                                         if val.ends_with(".png") {
                                             Ok(())
                                         } else {
                                             // clap automatically adds "error: " to the beginning
                                             // of the message.
                                             Err(String::from("the file format must be png."))
                                         }

                                         // Of course, you can do more complicated validation as
                                         // well, but for the simplicity, this example only checks
                                         // if the value passed in ends with ".png" or not.
                                     }))
                        .get_matches();

    // Here we can call .unwrap() because the argument is required.
    println!("The .PNG file is: {}", matches.value_of("input").unwrap());
}