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[Caml-list] kprintf with user formatters
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Date: -- (:)
From: Pierre Weis <pierre.weis@i...>
Subject: Re: [Caml-list] kprintf with user formatters
Hi,

> I am writing a multi-threaded program, so that I need some
> synchronization when pretty-printing things :
[...]
> module Debug = struct
>   let m = Mutex.create()
>   let printf x = 
>   Format.kprintf 
>     (fun s -> Mutex.lock m; print_endline s; Mutex.unlock m) 
>    x
> end;;
> (*
> module Debug :  sig 
>   val m: Mutex.t
>   val printf : ('a, unit, string, unit) format4 -> 'a 
> end
> *)
> 
> but now, I can't _nicely_ use my previously defined printers :
> type t
> val format_t: Format.formatter -> t -> unit
> val x: t
> 
> I would like to write :
> <<
> Debug.printf "val t = %a" format_t x
> >>
> 
> but I cannot since the user printer is expected to be "unit->t->string"
> (while it is "formatter->t->unit" for the standard Format.printf "%a")

Your problem is thus that the function Debug.printf is not the right
one (since it has not the right type :)

Hence, you should use another definition for Debug.printf

 let printf fmt =
   Format.kfprintf (fun ppf -> ()) Format.err_formatter fmt

This way, you get:

printf "%a";;
- : (Format.formatter -> '_a -> unit) -> '_a -> unit = <fun>

as expected.

However, your problem is simpler than that: you just have to define a
custom formatter devoted to your debugging purposes. For instance:

module Debug = struct
   let m = Mutex.create ()
   let out s idx len =
     Mutex.lock m;
     prerr_string (String.sub s idx len); flush stderr;
     Mutex.unlock m
   let flush () = flush stderr
   let dppf = Format.make_formatter out flush
   let printf fmt = Format.fprintf dppf fmt
 end;;

Now you can substitute Debug.printf to any well-typed occurrence of
Format.printf, since we have:

#  Debug.printf;;
- : ('a, Format.formatter, unit) format -> 'a = <fun>
# Format.printf;;
- : ('a, Format.formatter, unit) format -> 'a = <fun>

> currently I use :
>
> let (!!) f = (fun () x -> 
>   f Format.str_formatter x;
>   Format.flush_str_formatter());;
> (* val (!!): (Format.formatter -> 'a -> 'b) -> unit -> 'a -> string *)
> 
> Debug.printf "val t = %a" !!format_t x
>
> this seems to be working, but I wonder if :
>  - there is a better solution

See above: a custom formatter is probably simpler and more elegant.

>  - this use of the global value "Format.str_formatter" is really
> 	thread-safe (I would say no...)

I'm afraid I could not answer for sure to this one: let's say that
using Format.str_formatter is as thread safe as using a Buffer.t value
and the Buffer.add_* functions.

Also, can we consider in the first place that using any printf variant
could be thread safe ? If the answer is no, you would need to stick to
pp_* versions of the basic primitives and this would be a major
modification of your code.

>  - I missed something

May be you missed the fact that Format was designed from the beginning
to support formatting in parallel to user's defined pretty-printing
engines.

>  - printf could not understand a conversion character 'A' that would
> 	always need a _general_ user-formatter (of type formatter->t->unit)

Once more, I cannot answer to this question: I need some time to
figure out what it mean in term of typing (and semantics of printf)...

Tell me if the suggested solution solves your problem.

Best regards,

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/


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