Best way to parse an Option<T> based on the contents of the field itself

[lfrancke]

I have a field like this: pub retainer_id: Option<i32> which I'd like to convert to None if the parsed value is -2147483647. Currently it's parsed as Some(-2147483647)

I have a feeling that some combination of skip, cond and default will do the trick but I'm not sure which.

Thank you for this library and your work on it!

[sharksforarms]

In this case, map is how I would approach it since you're still reading the value, but transforming the result based on the value. skip, cond and default can be used to completely bypass reading. Here we always want to read.

use deku::prelude::*;
use std::convert::{TryFrom, TryInto};

#[derive(Debug, PartialEq, DekuRead, DekuWrite)]
struct DekuTest {
    #[deku(map = "|field: i32| -> Result<_, DekuError> { if field != -2147483647 { Ok(Some(field)) } else { Ok(None) } }")]
    retainer_id: Option<i32>
}

fn main() {
    {
        let test_data = (-2147483647i32).to_le_bytes();

        let test_deku = DekuTest::try_from(test_data.as_slice()).unwrap();

        assert_eq!(
            DekuTest {
                retainer_id: None,
            },
            test_deku
            );

        let test_deku: Vec<u8> = test_deku.try_into().unwrap();
        assert!(test_deku.is_empty());
    }

    {
        let test_data = (-12345i32).to_le_bytes();

        let test_deku = DekuTest::try_from(test_data.as_slice()).unwrap();

        assert_eq!(
            DekuTest {
                retainer_id: Some(-12345),
            },
            test_deku
            );

        let test_deku: Vec<u8> = test_deku.try_into().unwrap();
        assert_eq!(test_data.to_vec(), test_deku);
    }
}

[lfrancke]

Doh! Thanks again, I did read through the attribute list, I promise. Somehow I didn't make the connection, again.

Thanks for your help!

[sharksforarms]

Doh! Thanks again, I did read through the attribute list, I promise. Somehow I didn't make the connection, again.

Thanks for your help!

I realize there is a learning curve. I think more examples could help with this. (Actually, maybe these GitHub discussions are a good way of collecting relevant examples!) Thanks for the question!