Is it possible to pass attributes to elements of a Vec?

[nenikitov]

For a format like this:

#[binrw]
pub struct Element(
    #[br(map = |x: u8| x as f32 / u8::MAX as f32)]
    #[bw(map = |x| (x * u8::MAX as f32) as u8)]
    f32,
);
#[binrw]
pub struct Collection(
    #[br(count = 10)]
    Vec<Element>
);

Is it possible to remove the type Element and pass attributes like map or pad_before directly into the Vec (so they affect elements of the vector and not the entire thing)?

[csnover]

Hi, thanks for your question!

There is no way to apply directives to the inner type of a container. Instead, read a whole collection of Vec<u8> and then use the map directive to convert the collection into a Vec<f32> by c.into_iter().map(|v| …).collect(). Alternatively, if you need to avoid the memory overhead of temporarily holding two copies of the data, you can also write custom parse_with/write_with functions (the parser and writer macros help with this).

[nenikitov]

Hi, thank you

I went with a parse_with approach, here is my code if anyone else is interested:

pub fn map_vec_parse<F, Reader, T, O, Arg>(
    map: F,
) -> impl Fn(&mut Reader, Endian, VecArgs<Arg>) -> BinResult<Vec<O>>
where
    F: Fn(T) -> O,
    Reader: Read + Seek,
    T: for<'a> BinRead<Args<'a> = Arg>,
    Arg: Clone,
{
    move |reader, endian, VecArgs { count, inner }| {
        (0..count)
            .into_iter()
            .map(|_| Ok(map(T::read_options(reader, endian, inner.clone())?)))
            .collect()
    }
}
pub fn map_vec_write<F, Writer, T, O>(
    map: F,
) -> impl Copy + Fn(&Vec<T>, &mut Writer, Endian, ()) -> BinResult<()>
where
    F: Copy + Fn(&T) -> O,
    Writer: Write + Seek,
    O: for<'a> BinWrite<Args<'a> = ()>,
{
    move |vec, writer, endian, ()| {
        vec.iter()
            .try_for_each(|e| map(e).write_options(writer, endian, ()))
    }
}

And to use it

#[binrw]
pub struct Collection(
    #[br(count = 10, parse_with = map_vec_parse(|x: u8| x as f32))]
    #[bw(write_with = map_vec_write(|x| *x as u8))]
    Vec<f32>
);