Hi Li-Yao, I made a mistake with my example. In my real world problem there are many elements after which need updated. On Mar 13, 2018 5:50 AM, "Li-yao Xia" wrote:

The only item of index satisfying (> 2) in the list things is (A 3).

Li-yao

On 03/13/2018 02:44 AM, Cody Goodman wrote:

...

Hello,

I'm trying to insert a new elemnt into a list after a given index and increment the orderId accordingly with the lens library.

I'm a bit confused at why this code isn't working:

``` import Control.Lens

data A = A { _customOrderId :: Int } deriving Show

things = [ A {_customOrderId = 0} , A {_customOrderId = 1} , A {_customOrderId = 2} , A {_customOrderId = 3} ]

main = print $ things & ( (_drop 2 %~ ((A 22)<|)) . ((traversed . indices (> 2) . customerOrderIdLens) %~ (+ 1)) ) ``` The first portion works to insert where desired, but the second portion `((traversed . indices (> 2) . customerOrderIdLens) %~ (+ 1))` only increments the order id in indices 4.

Any idea why this is happening?

Thanks,

Cody Goodman

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