Function arg_combinators::opt_by

pub fn opt_by<F, T, E>(
    short: &str, 
    long: &str, 
    doc: &str, 
    hint: &str, 
    f: F
) -> ArgExt<impl Arg<Item = Option<T>>> where
    F: Fn(String) -> Result<T, E>,
    E: Clone + Debug + Display,