Fable: error FABLE: Cannot find replacement for Microsoft.FSharp.Collections.ListModule::Indexed

[nojaf]

Description

Please add a mapping for List.Indexed.
I need to filter an item based on the index but F# doesn't have a filteri method.
So I use Indexed but it is not supported.

Repro code

...
  | RemoveLineItem index ->
    let items = 
      List.indexed model.Items 
      |> List.filter (fun (ix, item) -> not(ix = index))
      |> List.map snd

Expected and actual results

I'm guessing the JS should be something like

const myList = ["foo", "bar"];
const indexed = myList.map((s,i) => [i, s]);

Related information

• Fable version (dotnet fable --version): 1.1.17
• Operating system: Windows 10

[Zaid-Ajaj]

If need a workaround for now:

module List = 
  let withIndices xs = 
      let items = xs |> Array.ofList
      let length = items.Length
      [ for i in 0 .. (length - 1) -> (i, items.[i]) ]

[1 .. 3]
|> List.withIndices
|> List.filter (fun (idx, item) -> idx <> 1) // [ (0, 1); (2; 3) ]

[alfonsogarciacaro]

Thanks for the report! I also took the chance to clean up some functions within the collection modules. It will be fixed in the next release. For now, please use @Zaid-Ajaj polyfill :+1:

Note also that using many List. or Array. intermediate steps can decrease performance specially with big collections as you have to recreate the whole structure every time. You can use Seq. for the intermediate steps, or in your case, you can easily polyfill the function you need:

module List =
    let filteri f (xs: 't list) =
        let mutable i = -1
        List.filter (fun x -> i <- i + 1; f i x)