Is there a way to get int-index that corresponds to a given Index?

[ArtemyB]

Hi, in the task I'm solving currently I need to get an int-index for alist<'T>'s element (or for the Index of the element). I see there is tryAt: int -> Index option function in AList module, however couldn't find anything like a reverse one (i.e. Index -> int option). Is there an "easy" way to do that at all, or the only option is to create some kind of separate Index-int cache (maybe by folding the list to amap<Index, int> -- not sure whether it's a good solution)?

[krauthaufen]

Hey, the main problem here is, that the int may actually change when something is inserted into the list.
That is why we can't really give you an int for an element.

It would however be possible to implement something like AList.mapIntIndex : (aval<int> -> 'a -> 'b) -> alist<'a> -> alist<'b> where the aval<int> will change appropriately. Of course for small lists you can always do something like list |> AList.toAVal |> AVal.map (Seq.mapi mapping) |> AList.ofAVal but that looses adaptivity on its elements making alist more or less pointless.

Would such a combinator help in your scenario?

[ArtemyB]

Wow, thanks for such a quick reaction 👍

the main problem here is, that the int may actually change when something is inserted into the list.

Got it, that's what I suspected.

It would however be possible to implement something like AList.mapIntIndex : (aval -> 'a -> 'b) -> alist<'a> -> alist<'b> where the aval will change appropriately.

Would it be possible via that function to construct AMap<Index, int>, which can be further used to retrieve corresponding int-index?

[ArtemyB]

Of course for small lists you can always do something like list |> AList.toAVal |> AVal.map (Seq.mapi mapping) |> AList.ofAVal but that looses adaptivity on its elements making alist more or less pointless.

Hmm, this is a good option just to make the thing done -- luckily I don't have big lists currently, so this should work fine.

I think I don't require adaptivity in this case (to retrieve an int-index). For clarification, here is what I'm solving:

• There is a spreadsheet-like UI component that is used to output a set of data. The data can be changed (by the component itself and from the outside). Before output to the component the data could also be preprocessed (e.g. filtered).
• For rendering the component uses a callback function of kind columnIndex: int -> rowIndex: int -> CellView. The callback is passed to the component on its initialization as a parameter.
• When some cell should be re-rendered, I have to notify the component to do so by specifying the int-coordinates of the updated cell -- that's when I have to know the int (so seems like it doesn't need to be adaptive). And the notification call itself is handled via AddCallback on the output adaptive list.

[krauthaufen]

So in essence you just need the int in a callback to communicate with the outside world? This should be fairly simple to achieve with reasonable runtime (e.g. less than linear in the number of elements). I can put something together next week if i don't forget it....

[ArtemyB]

So in essence you just need the int in a callback to communicate with the outside world

Actually right, my current solution could be reduced to that 🤔 (I'm just using intermediate events for the sake of API intuitiveness, but it doesn't change the essence)

[ArtemyB]

And big thanks for the help and for the great library itself -- it seems perfect for the task 👍

[ArtemyB]

Btw I can create an issue based on this discussion if that could be useful somehow 🤷‍♂️