A small OCaml program using Async

sort.ml

open Core
open Async

let parse contents = String.split_lines contents
                  |> String.concat
                  |> String.split_on_chars ~on:[',']
                  |> List.map ~f:Int.of_string

let read_then_sort fs =
  let ds = List.map ~f:Reader.file_contents fs
  in Deferred.all ds
 >>| List.map ~f:parse
 >>| List.concat
 >>| List.sort ~compare:Int.compare
 >>| List.to_string ~f:Int.to_string
 >>= fun ns -> printf "%s\n" ns; return ()

let () =
  Command.async_spec
    ~summary:"Read and then sort!"
    Command.Spec.(empty +> anon (sequence ("filename" %: string)))
    (fun fs () -> read_then_sort fs)
  |> Command.run

Try is with let%bind instead of >>| and >>=