mapv.ml

module ListExtra = struct
  open Core.Std
    
  (*- Maps a function to each "column" in a list of "rows".
    
    mapv [[1;2;3]; [3;4;5]] ~f:(List.fold ~init:0 ~f:(+));;
    - : int list = [4; 6; 8]
  *)
  let mapv ls ~f = let open List in
    let cmp x y = if length x = length y then 0
      else if length x > length y then 1
      else -1 in
    let min = min_elt ls ~cmp:cmp in
    let fn' i _ = map ls ~f:(fun x -> nth_exn x i) |> f
    in match min with
    | Some m' -> mapi m' ~f:fn'
    | None -> []

end