foo.ml

open Core.Std

let values_of_line line =
  let fields = String.split ~on:',' line in
  List.map ~f:Float.of_string fields

let add_to totals values =
  match totals with
  | [] -> values
  | _  ->
     if List.length totals = List.length values then
       List.map2_exn ~f:(+.) totals values
     else failwith "Inconsistent-length rows"

let sum_channel chan =
  let folder tot line = add_to tot (values_of_line line) in
  In_channel.fold_lines ~f:folder ~init:[] chan

let sum_and_print_file filename =
  In_channel.with_file filename
    ~f:(fun chan ->
      let result = sum_channel chan in
      print_string
        ((String.concat ~sep:"," (List.map ~f:Float.to_string result)) ^ "\n"))

let main () =
  match Sys.argv with
  | [| _; filename |] -> sum_and_print_file filename
  | _ -> failwith "Exactly 1 filename must be given"

let () = main ()

I tried to use Core as well and independently got a very similar solution:

open Core_kernel.Std
let (>>) f g x = g (f x)

module Row = struct
  let parse_exn = String.split ~on:',' >> List.map ~f:Float.of_string
  let to_string = List.map ~f:Float.to_string >> String.concat ~sep:","
  let print = to_string >> print_endline
end

let folder sum line =
  let row = Row.parse_exn line in
  match sum with
  | [] -> row
  | _ when List.length sum = List.length row -> List.map2_exn ~f:(+.) sum row
  | _ -> failwith "Inconsistent-length rows"

let () = match Sys.argv with
  | [|_; filename|] ->
      In_channel.(with_file filename ~f:(fold_lines ~init:[] ~f:folder))
      |> Row.print
  | _ -> failwith "Exactly 1 filename must be given"