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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)
// You can pass in a closure, or a function
.validator(is_png))
.get_matches();
// Here we can call .unwrap() because the argument is required.
println!("The .PNG file is: {}", matches.value_of("input").unwrap());
}
fn is_png(val: String) -> Result<(), String> {
// 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.
}
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