Fetch Primes Attempt 1

PrimeFetcher.fs

module PrimeFetcherFSharp

let FetchPrimesUpTo (max : int) : int[] = 
    match max with
    | x when x < 2 -> [||]
    | x when x = 2 -> [|2|]
    | x when x < 5 -> [|2; 3|]
    | _ ->
        let candidates = [|for x in 0 .. max -> if x < 2 then 0 else x|]
        let RemoveMultiples (coll : int[]) (x : int) =
            Array.iteri (fun index num -> 
                if (num % x) = 0 && num <> x then 
                   coll.[index] <- 0) coll
            coll
        let input = Array.concat [ [|2|]; [| 3 .. 2 .. max/2 |] ]
        Array.fold RemoveMultiples candidates input
        |> Array.filter (fun num -> num <> 0)