(* recursively generate all the permutations of a list *)
let rec perms L =
  if L <> [] then
    let nextPerm e =
      let tl = List.filter (fun x -> x <> e) L
      List.map (fun perm -> e::perm) (perms tl)
    List.concat <| List.map (nextPerm) L
  else [[]]

perms [1;2;3;4]