# Flatten an array

If your array contains `Int` as well as `[Int]`, there is no easy way to flatten it.

For example: `[1, 2, 3, 4, [5, 6]]` is not easy to flatten. You need to write a custom recursive function. Not good. Avoid if you can.

But `[[1, 2, 3], [4, 5, 6]]` is easy to flatten, as it's has a type of `[[Int]]`.