More concretely: I want to implement a dprintf function that works like this let dprintf fmt = let flag = true in if flag then Printf.eprintf fmt else ??? What do I write instead of ??? to make dprintf a no-op if flag = false? I tried Printf.ifprintf stderr fmt which sort of works? …

If you really want absolute zero-cost I think the only way is to lift the if flag then .. test outside of the function at each calling site: if flag then dprintf fmt ...; You could write a tiny ppx to make this nicer (and probably this already exists) so that you can write [%dprintf fmt ...] Bes…

[image] nojb: and probably this already exists We are using the following tiny plugin to optimize debug logging in Why3: [image] src/util/debug_optim.ml · master · Why3 / why3 · GitLab Why3 is a software verification platform, featuring a versatile ML-style language and interface…

Isn’t Printf.{ifprintf|ikfprintf} what you are really looking for? Cf. https://caml.inria.fr/pub/docs/manual-ocaml/libref/Printf.html

To be clear, we do use Printf.ifprintf in Why3. But it is still way too slow, because the logging code is called in the critical loop of a bytecode interpreter. Moreover, logging is not decided at compile time; it can be enabled/disabled at runtime. That is why we are also using a compiler plugin, …

I think it might be interesting to go in the opposite direction: making a non-variadic version of fprintf makes it easier to skip all the interpretation of the format string in the debug mode. A simple version of non-variadic fprintf can be made externally by grouping all arguments in a heterogeneou…

If you want a slightly less mystical take on the issue, I suggest the approach used by @dbuenzli ’s logs library ( docs ). It wraps the printing in a function which is only called when necessary: Logs.err (fun m -> m "invalid kv (%a,%a)" pp_key k pp_val v); That approach has been benchmarked very tho…

But syntactically speaking, this solution is not very cute/terse.

Logging is almost the archetypic motivating example for macros. B/c it gets a good bit more complicated than just “check Debug.debug and if it’s true, then log”. What you really want is some sort of set of bits, and each log-line (or at least, different sets of log-lines) are enabled by different…

Yeah, I should just use cppo maybe.

How to write a variadic function like printf

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grayswandyr June 9, 2020, 1:51pm 4

I think this discussion could interest you: Format.kprintf usage (incl. the following link https://github.com/mirage/ocaml-git/pull/130#issuecomment-149326614 ).

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How to write a variadic function like printf

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