Spooky control flow with Format.kasprintf & friends

Learning
,
7

Here’s an example that uses kfprintf:

let indirect =
  { Logs.report =
      (fun src level ~over k msgf ->
        msgf
        @@ fun ?header ?tags fmt ->
        let k = Format.(kfprintf (fun _ -> k ()) str_formatter fmt) in
        let msg = Format.flush_str_formatter () in
        (* in real usage, the log messages aren't going to stdout/stderr *)
        print_endline @@ "Log message: " ^ msg;
        over ();
        k)
  }

let () =
  Logs.set_level ~all:true @@ Some Logs.Debug;
  Logs.set_reporter indirect;
  Logs.info (fun m -> m "No format annotations");
  for i = 1 to 5 do
    Logs.info (fun m -> m "message %d" i)
  done;
  Logs.info (fun m -> m "All done")

Outputs:

$ ocamlc -I `ocamlfind query logs` logs.cma  -o loggingtest src/exe/test.ml && ./loggingtest
Log message: No format annotations
Log message: 
Log message: message 1
Log message: message 2
Log message: message 3
Log message: message 4
Log message: message 5All done