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From: Pierre Weis <pierre.weis@inria.fr>
Message-Id: <200211220941.KAA07554@pauillac.inria.fr>
Subject: Re: [Caml-list] selective printf-like function
In-Reply-To: <4.3.2.7.2.20021121091643.033aad28@localhost> from Chris Hecker at "Nov 21, 102 09:17:25 am"
To: checker@d6.com (Chris Hecker)
Date: Fri, 22 Nov 2002 10:41:14 +0100 (MET)
Cc: acc@CS.Stanford.EDU, caml-list@inria.fr
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> >I'd like to write a function that will optionally print something.  It
> >seems simple, but I also want it to be able to take format strings and act
> >like printf.  So for example,
> 
> There was a thread on a related topic just last week.  Search the archives.
> 
> Chris

Right, but the answer was using Obj.magic, which is not only useless
but also harmful to recommand to a beginner (once more, you must
consider that you need a correctness proof to use Obj.magic!).

So, here is a simpler solution only using regular functional
programming:

open Printf;;

let debug = ref false;;

let dprintf fmt =
 if !debug then kprintf (fun s -> print_string s; "") fmt
 else kprintf (fun s -> "") fmt;;
val dprintf : ('a, unit, string) format -> 'a = <fun>

The only problem is that dprintf returns a string instead of returning
a unit value as we would like it to do. This is due to the type of the
first argument to kprintf (string -> string). It will be improved in
the next release of Caml, thanks to the better typing of kprintf (the
first argument of kprintf now can have the more general type string ->
'a).

In the next release, you thus will prefer to write simply:

let dprintf fmt =
 if !debug then kprintf print_string fmt else kprintf ignore fmt;;

Anyhow, the preceding version will still be accepted by the new
compiler, so that your code will not be broken.

Hope this helps,

Pierre Weis

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


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