PartitionByEq.fs

//tail recursive

let part xs =
  let rec f acc =
    function
    | [] -> List.rev acc
    | x :: xs -> f ((x, 1 + List.length (List.takeWhile ((=) x) xs)) :: acc) (List.skipWhile ((=) x) xs)

  f [] <| List.ofSeq xs

//non-tail recursive

let part' xs =
  let rec f =
    function
    | [] -> []
    | x :: xs -> (x, 1 + List.length (List.takeWhile ((=) x) xs)) :: f  (List.skipWhile ((=) x) xs)

  f <| List.ofSeq xs