How to print filenames and line numbers with the Logs library

Learning
1

Hey,

I’m using the Logs library. I’d like log entries to contain the filename and the line number where it was created. This is a bit tricky though - I could use __FILE__ and __LINE__ but they’re hard to use in abstractions.

In Rust I’d either create a macro, or better I’d use #[track_caller] which makes available std::panic::Location::caller().line() to get the line number of the caller. Are either of these approaches viable in OCaml?

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2

That’s not really a satisfactory answer but if already having the file improves things for you can define such a line at the top of your modules:

let[@inline] file m ?header ?tags fmt =
  m ?header ?tags ("%s: " ^^ fmt) __FILE__

(or add it in a tag if you want more structured access). Then:

let main () =
  Logs.(set_reporter (Logs.format_reporter ()));
  Logs.err (fun m -> (file m) "Hey %s!" "ho");
  ()

let () = main ()
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3

Oh and I forgot to mention. I tend to play with these things with various degrees of success in my ad-hoc testing frameworks, so that when a test fails I can directly jump to the failing test in compilation mode. See for example here and the printing stuff here. Maybe you can try to play with that too.

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4

You could use something like ppx_here, which gives a Lexing.position wherever you write [%here]. But you would have to write [%here] at each call site of a logging function in order to get that callsite’s position.

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5

Note that the __POS__ value/macro also returns a source-code position 4-tuple that is roughly equivalent to Lexing.position, although it has the same problem w.r.t. abstraction as ppx_here and the other macros that have been mentioned.

@dbuenzli and @bcc32’s answers get as far as I think it’s possible to go in stock OCaml without PPX. With PPX – and some familiarity with Ppxlib – this feature is relatively straightforward to implement in ~20 LoC. (And while we’re at it, we can also generate the (fun f -> f ...) CPS wrapper that Logs requires.)

let () =
  Logs.(set_reporter (format_reporter ()));
  [%log err "Important numbers: %d, %f" 42 3.14];
  [%log warn "Lorem ipsum dolor sit amet"]

(* prints: *)

; dune exec ./main.exe
main.exe: [ERROR] [Line 3] Important numbers: 42, 3.140000
main.exe: [WARNING] [Line 4] Lorem ipsum dolor sit amet

This works as follows:

open Ppxlib

(* Input: [%log <log_fn> <fmt_string> <args...>]

   Output: Logs.(<log_fn>) (fun f -> 
             f ("[Line %d] " ^^ fmt_string) __LINE__ <args...>)
 *)
let expansion_function ~loc ~path:_ payload =
  let open Ast_builder.Default in
  let log_fn, fmt_string, args =
    match payload.pexp_desc with
    | Pexp_apply (fn, (Nolabel, fmt) :: args) -> (fn, fmt, List.map snd args)
    | _ ->
        Location.raise_errorf ~loc "ppx_logs: invalid payload %a"
          Pprintast.expression payload
  in
  let args =
    [%expr "[Line %d] " ^^ [%e fmt_string]] :: [%expr __LINE__] :: args
    |> List.map (fun x -> (Nolabel, x))
  in
  [%expr Logs.([%e log_fn]) (fun f -> [%e pexp_apply ~loc [%expr f] args])]

let extension =
  Extension.declare "log" Extension.Context.expression
    Ast_pattern.(single_expr_payload __)
    expansion_function

let rule = Context_free.Rule.extension extension
let () = Driver.register_transformation ~rules:[ rule ] "ppx_logs"

(Just a proof-of-concept. I expect an implementation that uses tags & handles shadowing and error reporting appropriately would be around twice this much code, so not too bad.)

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After sending this, I remembered that Logs recently gained a PPX thanks to @ulrikstrid. This generates the (fun f -> f ...) wrapper but doesn’t attach any other information. Perhaps “add source positions via Logs tags” would be an appropriate feature request over there :slight_smile:

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7

I also should update the ppx to use another name then the global Logs module, maybe have it configurable.

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