Prelude Control.Lens> [1,2,3] ^? ix 1
Just 2
Prelude Control.Lens> [1,2,3] ^?! ix 1
2
Prelude Control.Lens> [1,2,3] & (ix 1) .~ 20
[1,20,3]Prelude Control.Lens> [[1], [2], [3]] ^. traverse
[1,2,3]
Prelude Control.Lens> [1,2,3] ^.. traverse
[1,2,3]
原文：https://www.cnblogs.com/zwvista/p/9256626.html