From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-0.3 required=5.0 tests=BAYES_00,
	REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 1094ba,ddafdc033e838b1d
X-Google-Thread: 103376,cd51fe79965e9795
X-Google-Attributes: gid1094ba,gid103376,public
Path: 
 g2news1.google.com!news2.google.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.fortran,comp.lang.ada
Subject: Re: Order of logical operations
Followup-To: comp.lang.fortran
Date: Sat, 05 Jun 2004 12:44:06 +0200
Organization: At home
Message-ID: <2idme4Fm2e5nU1@uni-berlin.de>
References: <slrncbtfkn.247.tconnors@tellurium.ssi.swin.edu.au>
 <m11xkwd6ct.fsf@macfortran.local>
 <slrn-0.9.7.4-31741-29330-200406040800-tc@hexane.ssi.swin.edu.au>
 <40C0208A.80DBE53A@wldelft.nl> <Um3wc.43236$3x.40166@attbi_s54>
Reply-To: mailbox@dmitry-kazakov.de
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7Bit
X-Trace: news.uni-berlin.de bbhKFENske/nroJ2DMFEbgeRCGvZBN+cW/ibYQwDpF12/ANdI=
User-Agent: KNode/0.7.2
Xref: g2news1.google.com comp.lang.fortran:1705 comp.lang.ada:1120
Date: 2004-06-05T12:44:06+02:00
List-Id: <comp.lang.ada>

James Van Buskirk wrote:

> "Arjen Markus" <arjen.markus@wldelft.nl> wrote in message
> news:40C0208A.80DBE53A@wldelft.nl...
> 
>> Short-circuiting does not necessarily lead to quicker programs.
>> And if you really need it, you might invent a new operator,
>> like ".cand." (the implementation ought to be very simple).
> 
> As others have pointed out such operators have to be defined
> at the language level.

Not necessarily.

The language might support a more general concept: lazy arguments. Consider:

-- This is not Ada!
function "and then" (Left : Boolean; Right : lazy Boolean)
   return Boolean is
begin
   if Left then
      return True;
   else
      return Right;
         -- Right is evaluated only at this point on the
         -- caller's context.
   end if;
end "and then";

It is a very powerful mechanism that allows to enforce the evaluation order
of subroutine arguments:

-- This is not Ada!
function "&" (A : String, B : lazy String) return String is
   Value_A : String renames A; -- This evaluates A
begin
   return Value_A & B; -- B is evaluated after A
end "&";

Now it is safe to write:

... := Read_String & Read_String & Read_String;

Though, that could be tricky to implement, especially if we consider lazy
arguments of recursive calls, protected operations, side effects of lazy
evaluations on alien contexts and all that stuff.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de