let comb =

  let rec impl = function

    _, 0       -> [[]]

  | [], _      -> []

  | x :: xs, m -> [ for xs in impl (xs, m - 1) -> x :: xs ] @ impl (xs, m) 

​

  fun xs m -> impl (xs, m)

let combTailRec =

  let rec impl k = function

    _, 0 -> k [[]]

  | [], _ -> k []

  | x :: xs, m -> impl (fun acc -> impl (fun xxs -> k ([ for xs in xxs -> x :: xs ] @ acc)) (xs, m - 1)) (xs, m)

​

  fun xs m -> impl id (xs, m)

​

let combTailRecNoAppend =

  let cons x xs = x :: xs

  let flip f y x = f x y

  let rec impl k = function

    _, 0 -> k [[]]

  | [], _ -> k []

  | x :: xs, m -> impl (fun acc -> impl (List.fold (fun acc -> cons x >> flip cons acc) acc >> k) (xs, m - 1)) (xs, m)

​

  fun xs m -> impl id (xs, m)

​

let foo args =

  [ comb; combTailRec; combTailRecNoAppend ]

  |> List.map ((||>) args)

  |> printfn "%A"

​

foo ([ 50 .. 10 .. 90 ], 3)