F# Combine List of Results

combine.fs

open Microsoft.FSharp.Collections

/// Combines collection of Results into List of Values or Errors.
/// List<Result<'a, 'b>> -> Result<List<'a>, List<'b>> 
let combine (results: List<Result<'a, 'b>>): Result<List<'a>, List<'b>> =
    let rec _combine (ok: List<'a>) (err: List<'b>) (res: List<Result<'a, 'b>>) =
        res |> List.tryHead
        |> function
        | None -> (ok, err)
        | Some curr ->
            match curr with
            | Ok x -> _combine (List.append [x] ok) err (List.tail res)
            | Error e -> _combine ok (List.append [e] err) (List.tail res)
    // Invoke recursive call
    _combine [] [] results
    |> function
       | (values, []) -> Ok values
       | (_, errors) -> Error errors
       
       
// All OKs
let results: List<Result<int, string>> =  [ Ok 42; Ok 84 ]    
let combination = combine results
printfn "%A" combination

       
// All Errors
let results1: List<Result<int, string>> =  [ Error "Err1"; Error "Err2" ]    
let combination1 = combine results1
printfn "%A" combination1

// Oks and Errors, should get Errors
let results2: List<Result<int, string>> =  [ Ok 42; Ok 84; Error "Err1"; Error "Err2" ]    
let combination2 = combine results2
printfn "%A" combination2