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haskelllenses

Use Lens as `map`

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?

Solution

  • 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