use std::ops::Index;

struct Example<T: 'static>(&'static [T]);

impl<F, I, T> Index<F> for Example<T> where F: FnOnce(usize) -> I, [T]: Index<I> {
    type Output = <[T] as Index<I>>::Output;

    fn index(&self, f: F) -> &Self::Output {
        &self.0[f(self.0.len())]
    }
}

fn main() {
    let my_slice = Example(&[1, 2, 3, 4]);
    println!("Prefix: {:?}", &my_slice[|n| ..(n/2)]);
    println!("Midpoint: {}", my_slice[|n| n/2]);
    println!("Suffix: {:?}", &my_slice[|n| (n/2)..]);
}