Reverse a list in place

reverse() mutates a list through a mut binding or &mut reference and returns (). It allocates nothing and moves no elements out of the list:

def flip(items: &mut list[i64]) -> ():
    items.reverse()

def main() -> Result[(), AllocError | IoError]:
    mut items = [10, 20, 30]?
    flip(&mut items)
    print(items)?
    items.reverse()
    print(items)?
    Ok(())

[30, 20, 10]
[10, 20, 30]

Owned elements such as classes and nested lists work too. Reversing does not copy or destroy them; eventual list cleanup follows their new order. To keep the original order separately, explicitly copy the list first.