takewhile.rs

fn main() {
    println!("{:?}", take_while(&[1, 2, 3, 4, 5], |&x| x < 3));
}

fn take_while<T, F>(xs: &[T], f: F) -> &[T]
    where F: Fn(&T) -> bool
{
    let mut i = 0;
    while f(&xs[i]) {
        i += 1;
    }
    &xs[0..i]
}