How to delete an element from an array using lenses

Question

I'm trying to remove an element from an Array a using the index of the element but I can't find a way to do it with Lenses. I'm also wondering why Array does not provide an instance for the At lens. Any pointer on it?

Answer 1

I'm also wondering why Array does not provide an instance for the At lens.

You can look at the Control.Lens.At module docs for a hint:

... not Array-like ones, where you cannot satisfy the Lens laws

One of the laws which breaks is set l v' (set l v s) ≡ set l v' s. The reason being that deleting an index moves the other indices. So the following can't make sense:

myArray
  & at 6 .~ Nothing
  & at 6 .~ Just 10

I don't know of a way to have deleting array elements be Lens like.