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=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,a00006d3c4735d70
X-Google-Attributes: gid103376,public
X-Google-ArrivalTime: 2004-02-26 00:45:08 PST
Path: 
 archiver1.google.com!news2.google.com!news.maxwell.syr.edu!newsfeed.icl.net!newsfeed.fjserv.net!colt.net!news.tele.dk!news.tele.dk!small.news.tele.dk!uninett.no!ntnu.no!not-for-mail
From: Preben Randhol <randhol+valid_for_reply_from_news@pvv.org>
Newsgroups: comp.lang.ada
Subject: Re: In-Out Parameters for functions
Date: Thu, 26 Feb 2004 08:45:07 +0000 (UTC)
Organization: PVV
Message-ID: 
 <slrnc3rcgh.1pg.randhol+valid_for_reply_from_news@k-083152.nt.ntnu.no>
References: <pan.2004.02.24.01.44.30.311621@email.ro>
 <1077634311.254581@master.nyc.kbcfp.com>
 <pan.2004.02.24.23.23.33.578111@email.ro>
 <mailman.20.1077669870.327.comp.lang.ada@ada-france.org>
 <pan.2004.02.25.09.01.17.858502@email.ro>
 <1077718871.47635@master.nyc.kbcfp.com>
 <54cp3095jmv8s17h63d4bjdus0tec7l7pt@jellix.jlfencey.com>
 <1077721343.481619@master.nyc.kbcfp.com>
 <ljgp301lgj3r0totpo3hlkmt3dbnbe0i33@jellix.jlfencey.com>
 <1077724400.221032@master.nyc.kbcfp.com>
 <lqhp30lbl0o3fu7nk9mj88satb6mmg8u8s@jellix.jlfencey.com>
 <1077727496.631820@master.nyc.kbcfp.com> <103q2cf890pus79@corp.supernews.com>
 <1077744815.17965@master.nyc.kbcfp.com>
NNTP-Posting-Host: k-083152.nt.ntnu.no
X-Trace: tyfon.itea.ntnu.no 1077785107 13687 129.241.83.152 (26 Feb 2004
 08:45:07 GMT)
X-Complaints-To: usenet@itea.ntnu.no
NNTP-Posting-Date: Thu, 26 Feb 2004 08:45:07 +0000 (UTC)
User-Agent: slrn/0.9.8.0 (Linux)
Xref: archiver1.google.com comp.lang.ada:5825
Date: 2004-02-26T08:45:07+00:00
List-Id: <comp.lang.ada>

On 2004-02-25, Hyman Rosen <hyrosen@mail.com> wrote:
> Randy Brukardt wrote:
>> "Hyman Rosen" <hyrosen@mail.com> wrote
>>>Where do you find permission for Ada to not evaluate one
>>>operand of an "and" or "or" expression?
>> 
>> 11.6(5), of course.
>
> For the particular case of sqrt(x), where we're talking
> about Ada's built-in sqrt, and the only possible side-effect
> is for it to raise an exception. But if instead we have
> (gnurble(x) > 0.5 and b), and the compiler knows that b is
> false but has no information about what gnurble does, it
> must call gnurble(x) even though the result is irrelevant.

Or you have to do:

   value := gnuble(x);

   if value > 0.5 and b then 

to be sure that any side effects are performed.


   with Ada.Text_IO;  use Ada.Text_IO; 

   procedure eval is
      flag : Boolean := false;

      function gnurble(value : Float) return Float is
      begin
         Put_Line ("I was called");
         return value + 99.0;
      end gnurble;

   begin
      if flag and gnurble (1.0) > 2.0 then
   --   if gnurble (1.0) > 2.0 and flag then   -- or this case
   --   if false and gnurble (1.0) > 2.0 then  -- or this
   --   if gnurble (1.0) > 2.0 and false then  -- or this
         Put_Line ("True");
      else
         Put_Line ("False");
      end if;

   end eval;


For all these cases I get that the gnurble function was called and the
if statment is of course false. Even with optimisation.

However if you do this:


   with Ada.Text_IO;  use Ada.Text_IO; 

   procedure eval is
      flag : Boolean := false;

      function gnurble(value : Float) return Float is
      begin
         flag := true;                        -- Added a side effect
         Put_Line ("I was called");
         return value + 99.0;
      end gnurble;

   begin
      if flag and gnurble (1.0) > 2.0 then    -- Case 1 
    --  if gnurble (1.0) > 2.0 and flag then  -- Case 2
         Put_Line ("True");
      else
         Put_Line ("False");
      end if;

   end eval;

The result is:

Case 1:

% ./eval 
I was called
True

Case 2:

% ./eval 
I was called
True

Wheras if you do:

   if flag and then gnurble (1.0) > 2.0 then

then gnurble is never called of course.

Compiler GNAT 3.15p
-- 
"Saving keystrokes is the job of the text editor, not the programming
 language."