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

[Li-yao Xia]

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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