use std::fmt::{self, Formatter, Display};

#[derive(Debug)]
enum Error {
    InvalidId,
    Conversion,
}

impl Display for Error {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Error::InvalidId => write!(f, "Missing book ID"),
            Error::Conversion => write!(f, "Could not convert book ID into a number"),
        }
    }
}

fn book_id(id: Option<&str>) -> Result<String, Error> {
    id
        .ok_or(Error::InvalidId)
        .and_then(|v| v.parse::<i16>().map_err(|_| Error::Conversion))
        .map(|n| format!("Your book ID is {}", n))
}

fn response(inp: Result<String, Error>) -> String {
    inp.unwrap_or_else(|err| format!("{}", err))
}

fn main() {
    println!("{}", response(book_id(Some("10"))));
    println!("{}", response(book_id(None)));
    println!("{}", response(book_id(Some("abc"))));
}