Doing a bunch of sequential I/O with Lwt



Hi, I wrote this simple program, and it works as expected:

open Base
open Lwt_io

let get_random_hanzi () =
  let buf = Buffer.create 1 in
  let chr = Random.int_incl 0x4e00 0x9fff |> Uchar.of_scalar_exn in
  Caml.Buffer.add_utf_8_uchar buf chr;
  Buffer.contents buf

let () = begin
    with_file ~mode:Lwt_io.output "output.txt" begin fun channel ->
      let writeln content = write_line channel content in
      let%lwt () = writeln "Do you know these random Chinese characters?" in
      let%lwt () = writeln "Test your knowledge!\n" in
      let%lwt () = begin
        for%lwt _i = 1 to 10 do
          writeln (get_random_hanzi ())
      end in
      let%lwt () = writeln "\nHow many did you get right?" in
      Lwt_io.printf "Wrote to file output.txt\n"

However, I ended up using a lot of the let%lwt syntax sugar. Is there a nicer way to write this sort of asynchronous code?


I’m not sure it’s nicer, but you can use expr;%lwt in place of let%lwt () = expr in. For example:

writeln "Do you know these random Chinese characters?";%lwt
writeln "Test your knowledge!\n";%lwt
for%lwt _i = 1 to 10 do
  (* Code goes here *)

which is a little closer to what you’d have with non-Lwt code.


I personally think it looks less noisy that way, so thank you!