Fallback for Option?

[failable]

Why can't I set a fallback value for Option<String> type in this case?

use bpaf::Bpaf;

#[allow(dead_code)]
#[derive(Debug, Clone, Bpaf)]
#[bpaf(options)]
pub struct Options {
    #[bpaf(long, fallback(Some(String::from("main"))))]
    revision: Option<String>,

    #[bpaf(long, fallback(String::from("main")))]
    revision2: String,
}

fn main() {
    println!("{:?}", options().run())
}

[pacak]

First let's take a look at the code that gets generated in both cases:

let revision = long("revision").argument::<String>("ARG").optional().fallback(Some(String::from("main")));
let revision2 = long("revision2").argument::<String>("ARG").fallback(String::from("main"));

Both fallback and optional work by catching "item not found" error and converting it into a value - specified value in case of fallback and None in case of optional. Later additionally wraps present value with Some.

• https://docs.rs/bpaf/0.9.5/bpaf/trait.Parser.html#method.optional
• https://docs.rs/bpaf/0.9.5/bpaf/trait.Parser.html#method.fallback

If revision is always will be present either from user input or from the fallback - does it make sense to have it Option? If you are planning to use this datatype somewhere else and it needs to be Option because of that - you can do something like this:

use bpaf::Bpaf;

#[allow(dead_code)]
#[derive(Debug, Clone, Bpaf)]
#[bpaf(options)]
pub struct Options {
    #[bpaf(long, argument, fallback(String::from("main")), map(Some))]
    revision: String,
}

fn main() {
    println!("{:?}", options().run())
}