ocaml printf function: skip formatting entirely if some condition holds

Question

(extracted from ocaml: exposing a printf function in an object's method, so it can be answered independently)

I have the following (simplified) ocaml code, for a logger:

type log_level =
    | Error
    | Warn
    | Info

let ord lvl =
    match lvl with
    | Error -> 50
    | Warn  -> 40
    | Info  -> 30

let current_level = ref (ord Warn)

let logf name lvl =
    let do_log str =
        if (ord lvl) >= !current_level then
            print_endline str
    in
    Printf.ksprintf do_log

The logf function can be used with a printf format, as in:

logf "func" Warn "testing with string: %s and int: %d" "str" 42;

Is there any way to achieve the typical logging behaviour of only formatting arguments when they're actually needed? i.e something like:

let logf name lvl <args> =
    if (ord lvl) >= !current_level then
        Printf.printf <args>

I'm thinking it's because only the compiler knows how many arguments there will be in a format expression, and I guess there's no such thing as varargs in ocaml? So you can't ever define the full body of a printf function, you can only use currying and let the compiler magic figure it out. Is there any way to achieve what I want? Perhaps with mkprintf?

Answer 1

The continuation functions such as Printf.kfprintf are provided specifically to allow format wrappers such as this. It looks something like this:

open Printf

type log_level = Error | Warn | Info

let ord = function Error -> 50 | Warn  -> 40 | Info  -> 30

let string_of_lvl = function
  | Error -> "error"
  | Warn -> "warn"
  | Info -> "info"

let current_level = ref (ord Warn)

let printf_with_info name lvl =
  kfprintf fprintf stdout "[<%s>] <%s>: " name (string_of_lvl lvl)

let logf name lvl =
  if ord lvl >= !current_level then match lvl with
   | Error | Warn -> printf
   | Info -> printf_with_info name lvl
  else
    ifprintf stdout

Answer 2

This seems to get me some of the way:

let logf name lvl =
    if (ord lvl) >= !current_level then
        Printf.fprintf stdout
    else
        Printf.ifprintf stdout

I'm surprised, as I thought this might mean !current_level would be evaluated too early (at module load time or something). But changing the current_level at runtime does seem to work correctly. I guess currying isn't pure like in haskell (which makes sense, given the impurity above ;)).

But it restricts me to using the fprintf family. I'd prefer to use ksprintf, because I also want a formatter, which (depending on log level) may add information like:

`[<name>] <level>: <message>`

Perhaps I could achieve this by concatenating formats (using ^^), but I don't see how.

Answer 3

Whatever you do, calling any Printf.*printf funciton will parse the format string at runtime. This may change in 4.02. Or you can use some syntax extension for conditional printing.