How to provide a default for .many() parsers

[xitep]

I was wondering how I could achieve providing a calculated default for a .many() parser? The following doesn't work :/

#[test]
    fn default_positional() {
        use bpaf;

        let roots = bpaf::positional::<String>("ROOTS")
            .fallback_with::<_, String>(|| Ok(String::from(".")))
            .many();

        struct Roots { roots: Vec<String>, }

        let r = construct!(Roots {roots}).to_options().run_inner(Args::from(&[]));
        assert_eq!(vec![String::from(".")], r.unwrap().roots);
    }

[xitep]

one way to achieve a default value would be this:

let roots = bpaf::positional::<String>("ROOTS").many().map(|items| {
        if items.is_empty() {
            vec![String::from(".")]
        } else {
            items
        }
    });

[pacak]

Since 0.8.0 you can use fallback in combination with some

    let a = short('a')
        .argument::<u32>("ARG")
        .some("potatoes")
        .fallback(vec![1, 2, 3]);