Lwt: Lwt_log: how can we use it?

[dannywillems]

Are there simple examples to use Lwt_log to write logs? Do we need to initialize something before using Lwt_log.info "Hello, world!"?

Is it recommended to use it to log lots of information (to stdout and in files)?

[aantron]

@dannywillems, try the code in this post: https://discuss.ocaml.org/t/about-lwt-concurrency-library/150/6:

let () =
  Lwt_log_core.default :=
    Lwt_log.channel
      ~template:"$(date).$(milliseconds) [$(level)] $(message)"
      ~close_mode:`Keep
      ~channel:Lwt_io.stdout
      ();

  Lwt_log_core.add_rule "*" Lwt_log_core.Info;

  Lwt_main.run begin
    Lwt_log_core.info "blah"
  end

(* ocamlfind opt -linkpkg -package lwt.unix foo.ml && ./a.out *)

As per that post, you would normally use module Lwt_log, not Lwt_log_core everywhere. I was just being explicit about what actual underlying module everything was coming from.

[aantron]

Also, even if this answers your question, let's leave the issue open to remind about improving the docs.

[rgrinberg]

@dannywillems Have a look at https://github.com/dbuenzli/logs for a logging solution. It has a better API for logging and isn't Lwt specific. Lwt_log should be deprecated IMO.

[dannywillems]

Thanks @rgrinberg and @aantron. Sorry to answer now.

[aantron]

No worries, most open source takes place at the rate at which people happen to by luck be available :)

[aantron]

I'm in favor of deprecating Lwt_log in principle, as we evidently don't have the labor resources to properly maintain it. We would suggest people use logs, in particular Logs_lwt`.

I found the docs of Logs_lwt also a bit nebulous last December, so maybe they ought to be improved as well. Not sure what their current status is, and I don't remember the specific difficulties I was having. I did end up using it successfully, though.

@rgrinberg, any thoughts on what to do about Lwt_log in terms of packaging? Perhaps we could factor it out into its own package, and then constrain that package with an upper bound when Lwt_log becomes incompatible with Lwt?

[aantron]

Ok, I added the example to the module description, stated the relation to Lwt_log_core (telling the reader that most of Lwt_log is documented in Lwt_log_core), and recommended checking out Logs_lwt (linking to it and the home page of logs). Given the theme is we don't have enough labor resources to work on Lwt_log, including factoring it, I think this should be good for now. Please let me know any thoughts.

[rgrinberg]

any thoughts on what to do about Lwt_log in terms of packaging? Perhaps we could factor it out into its own package, and then constrain that package with an upper bound when Lwt_log becomes incompatible with Lwt?

Seems like it would be easier to just fix it rather than mess around with upper bounds?

I agree with splitting the package for the purpose of getting rid of more dotted findlib names.

[aantron]

Factoring out Lwt_log is beginning in #484 by @hannesm, in particular see https://github.com/ocsigen/lwt/pull/484#issuecomment-338173757 for a plan.