How to refine parsing of associated value in enum variant?

[fritzrehde]

I'm using parse_display to handle both Display and FromStr for an enum. One of the enum variants, F(u8), represents function keys and I want to ensure the value is between 1 and 12 inclusive. I'm using #[display("f{0}")] for display formatting, but when parsing, I want to validate that the integer following "f" is in the correct range. Here's a snippet of my enum for context:

#[derive(Hash, Eq, PartialEq, From, Clone, Debug, Display, FromStr)]
#[display(style = "lowercase")]
enum KeyCode {
    // ... other variants ...
    #[display("f{0}")]
    F(u8),
    // ... other variants ...
}

How can I do this? It would be cool if I could just pass a lambda/closure to the macro that returns true or false and that would be evaluated when parsing, and if it returns false then the parsing would fail.

Thanks for the help!

[frozenlib]

By attaching #[from_str(new = ...)] to the variant as shown below, you can check the value and cause the parsing to fail.

use parse_display::{Display, FromStr};

#[derive(Hash, Eq, PartialEq, Clone, Debug, Display, FromStr)]
#[display(style = "lowercase")]
enum KeyCode {
    // ... other variants ...
    #[display("f{0}")]
    #[from_str(new = Self::new_f(_0))]
    F(u8),
    // ... other variants ...
}
impl KeyCode {
    fn new_f(value: u8) -> Option<Self> {
        if (1..=12).contains(&value) {
            Some(KeyCode::F(value))
        } else {
            None
        }
    }
}

fn main() {
    use std::str::FromStr;
    println!("{:?}", KeyCode::from_str("f13")); // Err(ParseError("parse failed."))
    println!("{:?}", KeyCode::from_str("f12")); // Ok(F(12))
}