f64_to_u64.rs

// source: https://gist.github.com/dns2utf8/a1288783cdd387f6d3381f8697d98c45

extern crate rand; // 0.6.1

fn main() {
    let number: f64 = rand::random();
    let int = maybe_round(number);
    println!("Hello, {} => {:?}!", number, int);
}

fn maybe_round(number: f64) -> Result<u64, ()> {
    if number.is_normal() {
        Ok(number.round() as u64)
    } else {
        Err(())
    }
}

Run on play.rust-lang.org