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
Path: 
 border1.nntp.dca1.giganews.com!nntp.giganews.com!usenet.blueworldhosting.com!feeder01.blueworldhosting.com!feeder.erje.net!eu.feeder.erje.net!newsfeed.xs4all.nl!newsfeed2.news.xs4all.nl!xs4all!news.stack.nl!aioe.org!.POSTED!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Lazy evaluation
Date: Fri, 19 Sep 2014 16:47:17 +0200
Organization: cbb software GmbH
Message-ID: <11up8h3pwcwf$.1tmy4g7sb6csv$.dlg@40tude.net>
References: <lvfb0p$tsc$1@speranza.aioe.org>
 <85idneIsZeZHqYHJRVn_vwA@giganews.com>
Reply-To: mailbox@dmitry-kazakov.de
NNTP-Posting-Host: ZB2Fb2q1fa4xpMpNKFqV6Q.user.speranza.aioe.org
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
X-Complaints-To: abuse@aioe.org
User-Agent: 40tude_Dialog/2.0.15.1
X-Notice: Filtered by postfilter v. 0.8.2
Xref: number.nntp.dca.giganews.com comp.lang.ada:189058
Date: 2014-09-19T16:47:17+02:00
List-Id: <comp.lang.ada>

On Fri, 19 Sep 2014 09:51:21 -0400, Peter Chapin wrote:

> On 2014-09-18 15:15, Victor Porton wrote:
> 
>> Can we incorporate into Ada some kind of lazy evaluation (as in Haskell), 
>> without breaking backward compatibility?
>> 
>> If yes, would this be much useful?
> 
> Speaking in generalities lazy evaluation tends to make it difficult to
> reason about the performance characteristics of a program. It is my
> understand this is sometimes a problem for Haskell programs that have
> significant performance requirements. Thus it appears that lazy
> evaluation would be an anathema as far as Ada is concerned.

Consider the problem of evaluation order:

   X : String := String'Input (File) & String'Input (File); -- Ill defined!

Now, let "&" be lazy in the right argument:

   function "&" (Left : String; Right : lazy String) return String;

In my opinion, it is not laziness which causes problems, but more the
closures. If closures were limited in some reasonable (from Ada's POV) way,
then laziness would be not an issue.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de