use existent::When;
let x = 100u32;

assert_eq!(None, x.when(x > 200));
assert_eq!(Some(x), x.when(x < 200));

use existent::Unless;
let xs = vec!["", "Three", "", "Two", "One"];

let filtered = xs.into_iter()
                 .filter_map(|s| s.unless(s.is_empty()))
                 .collect::<Vec<&str>>();

assert_eq!(filtered, vec!["Three", "Two", "One"])

fn expensive_computation(bar: usize) -> usize {
    42 * bar
}

let mut bar = 1;
assert_eq!(Some(42), (|| expensive_computation(bar)).do_unless(false));
assert_eq!(None, (|| expensive_computation(bar)).do_unless(true));

bar = 2;
assert_eq!(Some(84), (|| expensive_computation(bar)).do_when(true));
assert_eq!(None, (|| expensive_computation(bar)).do_when(false));