Calculating permutations in F#

Question

Inspired by this question and answer, how do I create a generic permutations algorithm in F#? Google doesn\'t give any useful answers to this.

EDIT: I provide my be

Answer 1

If you need permutations with repetitions, this is the "by the book" approach using List.indexed instead of element comparison to filter out elements while constructing a permutation.

let permutations s =
    let rec perm perms carry rem =
        match rem with
            | [] -> carry::perms
            | l ->
                let li = List.indexed l
                let permutations =
                        seq { for ci in li ->
                                let (i, c) = ci
                                (perm
                                        perms
                                        (c::carry)
                                        (li |> List.filter (fun (index, _) -> i <> index) |> List.map (fun (_, char) -> char))) }

                permutations |> Seq.fold List.append []
    perm [] [] s