[Haskell-cafe] Append after index and update items after inserted item with lens

[Cody Goodman]

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" <lysxia at gmail.com> 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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