Find first even number in a list (generic version)

find_even_generic.rs

extern mod std;
use num::Num;

// Based on some example code by Armin Ronacher

fn find_even<T:Copy Num cmp::Eq>(vec: &[T]) -> Option<T> {
    let zero: T = Num::from_int(0);
    let two: T = Num::from_int(2);
    for vec.each |x| {
        if *x % two == zero {
            return Some(*x);
        }
    }
    None
}

fn main() {
    io::println(match find_even(@[1,2,3,5,7,9,10]) {
        Some(x) => int::str(x),
        None    => ~"No even numbers"
    });
}

In other words you just have to check every pointer, always?

But what if you check the pointer against nil on line 37, and then assume it's non-nil and dereference it on line 52, and a later pull request accidentally deletes the check on line 37? Now your assumption is wrong and stuff blows up.

In Rust it is, as I understand it, impossible to have a pointer that will blow up if dereferenced. If you have a &T you know that it isn't nil/null/undefined, the check (if necessary) has already been done. That's encoded in the type system. It's a big safety feature.