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 (or a specific error) and that would be evaluated when parsing, and if it returns false then the parsing would fail.

I thought about the concept of "refinement types" from something like F*, that would work great here, but I'm not sure if refinement types are supported in Rust (through some crate).

Thanks for the help!

[H2CO3]

Having only skimmed the docs, it looks like the crate offers two solutions:

1. #[from_str(regex = "1[12]|[1-9]")]
2. #from_str(new = "...")], and specify a constructor function that returns Result.

[fritzrehde]

Nice solutions!
For the regex, do you mean 1[0-2]|[1-9], or how does your solution match 10? (Sorry if this is a dumb question, not that experienced with regex)

[H2CO3]

Yeah, my mistake, of course 0 needs to be there.