F#: Filter items found in one list from another list

Question

Say I have two lists:

let a = [1 .. 1000]
let b = [250 .. 500]

How do I get a new list that contains the values {1-249, 501-1000}?

Answer 1

If you want a pure F# solution, your options will vary based on your requirements. If your lists don't contain duplicated items and you don't care about the order of your output, you can just do:

(Set.of_list a) - (Set.of_list b) |> Set.to_list

If you know that your items are sorted, this should work and be efficient:

let exclude =
  let rec exclude = function
    | [],_ -> [] 
    | a,[] -> a
    | (x::xs as l),(y::ys as r) -> 
        if x < y then x :: (exclude (xs, r))
        elif x > y then exclude (l, ys)
        else exclude (xs, ys)
  fun a b -> exclude (a,b)

If you have two lists which may contain duplicates, aren't necessarily sorted, you want results in the order they occured in a, and you don't care about performance, you can do:

a |> List.filter (fun x -> not (List.contains x b))