Abstracting over iterators

nested_iterators.rs

#![feature(box_syntax)]

fn go(outer: impl Iterator<Item = impl Iterator<Item = usize>>) {
    print!("[ ");
    for inner in outer {
        print!("[");
        for x in inner {
            print!(" {} ", x)
        }
        print!("]");
    }
    print!(" ]\n");
}

fn main() {
    let grid = vec![vec![1, 2, 3], vec![4, 5]];
    
    // Unrolling works
    go(grid.iter().map(|v| v.iter().cloned()));
    go(grid.iter().map(|v| v.iter().cloned().rev()));
    go(grid.iter().rev().map(|v| v.iter().cloned()));
    go(grid.iter().rev().map(|v| v.iter().cloned().rev()));
    
    // Abstracting works like this
    type It<'a, T> = Box<dyn Iterator<Item = Box<dyn Iterator<Item = T> + 'a>> + 'a>;
    let its: [It<usize>; 4] = [
    box grid.iter().map(|v| box v.iter().cloned() as Box<dyn Iterator<Item = usize> + '_>),
    box grid.iter().map(|v| box v.iter().cloned().rev() as Box<dyn Iterator<Item = usize> + '_>),
    box grid.iter().rev().map(|v| box v.iter().cloned() as Box<dyn Iterator<Item = usize> + '_>),
    box grid.iter().rev().map(|v| box v.iter().cloned().rev() as Box<dyn Iterator<Item = usize> + '_>),
    ];
    for it in its {
        go(it);
    }
}