How to Parse a Number Enum

[Pi-Cla]

Oftentimes when we want input from the user in the form of a number we may only want to accept a subset of all possible values.

For example only integers from 0-2.

Now we can already sort of do this in Clap via this:

use clap::Parser;

#[derive(Parser)]
#[command(version, about, long_about = None)]
struct Cli {
    #[arg(value_parser = clap::value_parser!(u8).range(0..3))]
    thing: u8,
}

fn main() {
    let cli = Cli::parse();

This will reject the user if they attempt to put in an integer that is not from 0-2 but Rust's type system does not know this. So for this match statement I need to then add an unsatisfying unreachable!() macro.

    match cli.thing {
        0 => doThing(),
        1 => doOtherThing(),
        2 => doADifferentThing(),
        _ => unreachable!(),
    }
}

and if I need to match against the value multiple times this can quickly get tedious...

One alternative is to instead create a specialized enum that ONLY allows for a value between 0-2 to occur such as:

pub enum ThingNum {
    Zero,
    One,
    Two,
}

but then if I try to get clap to parse this enum it will default to the values being [zero, one, two] rather than the integer values of [0,1,2]. So clap would force the user to type in "two" rather than "2".

Is there any (hopefully concise?) way to allow the user to enter a decimal but then immediately parse the value into an enum?
(so the cli struct can end up looking something like below)

struct Cli {
    thing: ThingNum,
}

[epage]

Sorry for the delay

#[derive(ValueEnum)]
pub enum ThingNum {
    #[value(name = "0")]
    Zero,
    #[value(name = "1")]
    One,
    #[value(name = "2")]
    Two,
}

Should work. See https://docs.rs/clap/latest/clap/_derive/index.html#possible-value-attributes

You can also use value_parser = clap::value_parser!(u8).range(0..3).map(|num| match { ... })