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: 103376,330ec86e1824a689
X-Google-Attributes: gid103376,public
X-Google-ArrivalTime: 2002-08-28 07:32:51 PST
Newsgroups: comp.lang.ada
Path: 
 archiver1.google.com!news1.google.com!newsfeed.stanford.edu!news-spur1.maxwell.syr.edu!news.maxwell.syr.edu!news-out.visi.com!hermes.visi.com!uunet!ash.uu.net!world!news
From: Robert A Duff <bobduff@shell01.TheWorld.com>
Subject: Re: Run-Time Type Assignment
Sender: news@world.std.com (Mr Usenet Himself)
Message-ID: <wccn0r7hx5y.fsf@shell01.TheWorld.com>
Date: Wed, 28 Aug 2002 14:31:53 GMT
References: <mailman.1030517642.16877.comp.lang.ada@ada.eu.org>
 <5ee5b646.0208280304.614d11fc@posting.google.com>
 <wccvg5vhzsp.fsf@shell01.TheWorld.com>
 <VsGQTT8PXH3v@eisner.encompasserve.org>
NNTP-Posting-Host: shell01.theworld.com
Organization: The World Public Access UNIX, Brookline, MA
X-Newsreader: Gnus v5.7/Emacs 20.7
Xref: archiver1.google.com comp.lang.ada:28496
Date: 2002-08-28T14:31:53+00:00
List-Id: <comp.lang.ada>

Kilgallen@SpamCop.net (Larry Kilgallen) writes:

> What is a downward closure ?

The ability to pass a procedure as a parameter to a procedure.
The "downward" means the closure is passed "down" from caller to
callee.  This is more restrictive than the "full closures" which are
found in Lisp and many other languages of that ilk; a full closure may
be *returned* from a procedure, and may be assigned into a variable.

GNAT supports downward closures via the 'Unrestricted_Access attribute,
but that's not Ada (unfortunately).

In Pascal, downward closures are very useful for making iterators and
other user-defined control structures.

- Bob