ZoKrates/zokrates_cli/examples/arrays/update.zok

def foo(field[3] mut a) -> field {
    a[1] = 2;
    return a[1];
}

def main() -> (field, field) {
    field[3] a = [0, 0, 0];
    field res = foo(a);
    assert(a[1] == 0);
    return (res, a[1]);
}