Parse a number of a certain size in nom

Question

I can parse a number like this fine:

map_res(digit1, |s: &str| s.parse::<u16>())

but how can I parse a number only if it is within a certain range?

Answer 1

You could check that the parsed number fits in the range and return an error if not:

map_res(digit1, |s: &str| {
    // uses std::io::Error for brevity, you'd define your own error
    match s.parse::<u16>() {
        Ok(n) if n < MIN || n > MAX => Err(io::Error::new(io::ErrorKind::Other, "out of range")),
        Ok(n) => Ok(n),
        Err(e) => Err(io::Error::new(io::ErrorKind::Other, e.to_string())),
    }
})

The match can also be expressed with the and_then and map_err combinators:

map_res(digit1, |s: &str| {
    s.parse::<u16>()
        .map_err(|e| io::Error::new(io::ErrorKind::Other, e.to_string()))
        .and_then(|n| {
            if n < MIN || n > MAX {
                Err(io::Error::new(io::ErrorKind::Other, "out of range"))
            } else {
                Ok(n)
            }
        })
})