Use Lens as `map`

Question

I want to convert this line of code map (^?! ix 0) [[0, 1], [4, 5], [9, 1]] to entirely use lenses, so something like [[0, 1], [4, 5], [9, 1]] & each . ix 0. However, the types don't match up. What is the correct way to do this?

Answer 1

Use folded:

Prelude Control.Lens> [[0, 1], [4, 5], [9, 1]] ^.. folded . ix 0
[0,4,9]

Works on any Foldable.

Also, if you plan to always extract the first element, perhaps it would be clearer to use the _head traversal from Control.Lens.Cons instead of ix.

[[0, 1], [4, 5], [9, 1]] ^.. folded . _head