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Newsgroups: comp.lang.eiffel,comp.answers,news.answers
From: rogerb@eiffel.demon.co.uk (Roger Browne)
Path: bloom-beacon.mit.edu!hookup!europa.eng.gtefsd.com!howland.reston.ans.net!EU.net!uknet!demon!eiffel.demon.co.uk!rogerb
Organization: Everything Eiffel
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Subject: comp.lang.eiffel Frequently Asked Questions (FAQ)
Followup-To: comp.lang.eiffel
Expires: +1 month
Approved: news-answers-request@MIT.Edu
Summary: Eiffel is a pure object-oriented language designed to promote
software correctness and re-use.
Date: Tue, 26 Apr 1994 08:21:19 +0000
Message-ID: <767348479snz@eiffel.demon.co.uk>
Sender: usenet@demon.co.uk
Xref: bloom-beacon.mit.edu comp.lang.eiffel:2222 comp.answers:5078 news.answers:18657
Archive-name: eiffel-faq
Last-modified: 26 April 1994
EIFFEL: FREQUENTLY ASKED QUESTIONS
----------------------------------
This question-and-answer list is posted monthly to the Usenet newsgroups
comp.lang.eiffel, comp.answers and news.answers.
Please send corrections, additions and comments to Roger Browne
(rogerb@eiffel.demon.co.uk).
This information is abstracted and condensed from the posts of many
contributors to comp.lang.eiffel, supplemented by information from vendors.
No guarantees are made regarding its accuracy.
This compilation is by Roger Browne. Distribution over the Internet or by
electronic mail is unrestricted. Other use requires my permission.
You can receive the latest copy by anonymous file transfer from:
ftp.cm.cf.ac.uk /pub/eiffel/eiffel-faq
rtfm.mit.edu pub/usenet/news.answers/eiffel.faq
or by sending an email message to archive-server@cm.cf.ac.uk with the
following message body:
send Eiffel eiffel-faq
----------
CONTENTS
Changes since the last posting:
Q03: Removed references to vapourware
Q13: Changes to NICE Board
Q16: Various changes to Eiffel reseller list
Q20: Where can I get an Eiffel editor or emacs-mode? (new question)
Thanks to Steve Tynor, Bertrand Meyer, Alan Philips, Mike Davis
and Franck Arnaud for their input.
Frequently Asked Questions:
Q01) What is Eiffel?
Q02) Where did Eiffel come from?
Q03) What Eiffel products are available?
Q04) Are there any school or student discounts?
Q05) Is Eiffel available in the public domain?
Q06) What is Sather? How does it compare to Eiffel?
Q07) What books are available for learning about Eiffel?
Q08) Are any magazines or newsletters available concerning Eiffel?
Q09) Is Eiffel available on PC, Mac, NeXT, Amiga, Atari, ...?
Q10) Is there an archive of the comp.lang.eiffel newsgroup?
Q11) How much memory and disk space does Eiffel development require?
Q12) How large are typical Eiffel executables?
Q13) Are there standards for the Eiffel language?
Q14) How fast do Eiffel applications run?
Q15) Are there any Eiffel user groups?
Q16) Where can I get Eiffel products and services?
Q17) Are there any conferences for Eiffel users?
Q18) Why do Eiffel implementations compile to C?
Q19) What is BON?
Q20) Where can I get an Eiffel editor or emacs-mode?
Language Issues:
L01) What features does Eiffel have?
L02) What changes have been made to the Eiffel language definition?
L03) What libraries come with Eiffel?
L04) What's the big deal about preconditions and postconditions?
L05) Please explain and discuss covariance vs. contravariance.
L06) Is it true that there are "holes" in the Eiffel type system?
L07) Is there support for concurrency in Eiffel?
L08) Why doesn't Eiffel allow function overloading?
L09) Why are there no procedural types in Eiffel?
----------
Q01) What is Eiffel?
Eiffel is an advanced object-oriented programming language that emphasizes
the design and construction of high-quality and reusable software.
Eiffel is not a superset or extension of any other language. Eiffel
strongly encourages object-oriented programming and does not allow
dangerous practices from previous generation languages although it does
interface to other languages such as C and C++. Eiffel supports the concept
of "Design by Contract" to improve software correctness.
Beyond the language aspect Eiffel may be viewed as a method of software
construction. Eiffel is an excellent vehicle for software education,
including for a first programming course.
Eiffel is typically implemented by compilation to C, ensuring wide
portability.
----------
Q02) Where did Eiffel come from?
Eiffel was created by Bertrand Meyer and developed by his company,
Interactive Software Engineering (ISE) of Goleta, CA.
Dr. Meyer borrowed on his extensive experience with OOP, particularly with
Simula. He also added in important concepts from his academic work on
software verification and computer language definition.
Eiffel's design addresses many practical concerns that software engineers
face when creating complex software. Eiffel has evolved continually since
its conception on September 14, 1985 and its first introduction in 1986.
----------
Q03) What Eiffel products are available?
ISE Eiffel 3 is a commercially supported product available from Interactive
Software Engineering.
ISE Eiffel 3 is a complete graphical development environment meant for the
production of quality software, with particular attention being given to
the development of large systems. The environment itself is written in
Eiffel, and is an example of non-trivial system - about 3000 classes.
A version of Eiffel called Eiffel/S is produced by SIG Computer GmbH of
Germany. It is based on the Eiffel 3 language definition.
Eiffel/S Version 3, release 1.3 includes:
- command-line compiler with automatic configuration management
- libraries
- manual
Tower Technology Corporation of Austin, TX produce TowerEiffel. It
includes:
- compiler
- many programming tools
- an emacs-based integrated programming environment
- Eiffel 3 kernel and support libraries
----------
Q04) Are their any school or student discounts?
Both ISE Eiffel and SIG Eiffel/S include aggressive site-licensing and
discount licenses for schools and universities.
Eiffel/S offers an inexpensive student or trial license. This license is
limited to building systems with up to 75 classes. You do not have to be a
student to buy it, and you get a discount if you subsequently upgrade to
the full version.
ISE is also selling student licenses on their lower cost platforms.
TowerEiffel offers a much reduced price for student, university or non-
commercial licenses.
----------
Q05) Is Eiffel available in the public domain?
There is not currently a public domain Eiffel compiler. ISE has expressed
willingness to support the serious efforts of those who wish to create a PD
Eiffel, but so far no such effort has succeeded.
There is, however, a somewhat limited Eiffel 2.3 interpreter for the Atari
ST which is shareware (DM50). The documentation is in German, and the
example files seem quite interesting. Inheritance does not seem to be
supported (!), but there is an interesting extension to allow "for all" and
"for some" in assertions.
The following Eiffel archive sites allow anonymous file transfer:
ftp.tu-clausthal.de
pub/atari/languages/eiffel/vici_102.lzh
The Atari ST interpreter referred to above.
ftp.cm.cf.ac.uk
/pub/eiffel
University of Wales. Contains the latest version of this FAQ, plus the
front-end parser (ep) and various public domain classes. To contribute,
contact Ted Lawson (ted@cm.cf.ac.uk).
ftp.fu-berlin.de
/pub/heron/ep.tar.Z
There is an Eiffel front-end parser (HERON) in the public domain,
created by Burghardt Groeber and Olaf Langmack of the Freie Universitat
in Berlin. Olaf has announced that the Freie Universitat has agreed to
join the NICE consortium and keep the front-end parser in sync with the
Eiffel language definition as it evolves.
ftp.informatik.uni-stuttgart.de
/pub/eiffel
Falkultaet Informatik der Universitaet Stuttgart, Germany. Contains a
compiler generator, several encapsulations, a pretty-printer for
Eiffel/S, and some utility classes. To contribute, contact Joerg Schulz
(schulz@adam.informatik.uni-stuttgart.de).
utarlg.uta.edu
CSE/EIFFEL
UT-Arlington, USA. Contains some code from Eiffel Outlook back issues.
wuarchive.wustl.edu
/graphics/gif/e/eiffel_tower
Contains a GIF graphic of the eiffel tower
(also on plaza.aarnet.edu.au from Australia only).
----------
Q06) What is Sather? How does it compare to Eiffel?
Sather is an object-oriented language, originally patterned after Eiffel,
created by Stephen Omohundro and others at ICSI of Berkeley, CA.
Sather simplifies some Eiffel constructs, eliminates others, and adds some
powerful constructs of its own such as iteration abstraction, built-in
arrays, overloading and object constants.
Sather is available for free, under a very unrestrictive license. The
documentation for Sather, and the ICSI implementation of it, are available
by anonymous file transfer from the following sites:
ftp.ICSI.Berkeley.edu /pub/sather
ftp.gmd.de /pub/Sather
sra.co.jp /pub/lang/sather
lynx.csis.dit.csiro.au /pub/sather
See the usenet newsgroup comp.lang.sather for more details.
----------
Q07) What books are available for learning about Eiffel?
The classic text for learning about Eiffel (as well as Object-Oriented
programming in general) is Dr. Meyer's "Object Oriented Software
Construction". Although the language has evolved significantly since the
book's date of publication, the presentation of the basic problems and
solutions which motivate the object-oriented mind set are still quite
compelling. This is the book to get if you are new to the object-oriented
world as well as to Eiffel. (Prentice Hall, ISBN 13-629031-0)
Also by Dr. Meyer, "Eiffel: The Language", combines an introduction to
Eiffel, the language reference, and a good deal of philosophy into its 600
pages. This is the state of the art in OO thinking, but is a rigorous and
comprehensive book which some readers may find heavy going despite Dr.
Meyer's clarity of expression. It is, however, the definitive language
reference, and essential reading for all serious Eiffel users. This book is
now in its second _printing_ (same ISBN), with some minor corrections and
clarifications (this is not a second _edition_, and none is currently
underway). (Prentice Hall, ISBN 13-247925-7)
Dr. Meyer and Jean-Marc Nerson have edited a new book about Eiffel called
"Object-Oriented Applications". It includes an introduction to Eiffel
technology followed by seven in-depth descriptions of large applications
written in Eiffel. (Prentice Hall, ISBN 13-013798-7)
Robert Switzer, from Gottingen University in Germany, has written "Eiffel:
An Introduction". This is a very clear and concise primer for those wishing
to learn Eiffel, with many code fragments, and two substantial Eiffel
applications. (Prentice Hall, ISBN 13-105909-2)
ISE distributes a set of 6 video lectures (about one hour each) entitled
"Object-Oriented Software Construction", taught by Bertrand Meyer. These
provide an overall introduction to the method and use ISE Eiffel 3 to
illustrate the concepts.
Frieder Monninger's book "Eiffel: Objektorientiertes Programmieren in der
Praxis" is a very down-to-earth Eiffel handbook for German speakers.
(Heise, ISBN 3-88229-028-5).
Bertrand Meyer's "Reusable Software: The Base Object-Oriented Component
Libraries" (Prentice Hall, ISBN 0-13-245499-8, about 530 pages) describes
principles of library design and the taxonomy of fundamental computing
structures that led to ISE's EiffelBase libraries. Also serves as a manual
for the EiffelBase libraries.
----------
Q08) Are any magazines or newsletters available concerning Eiffel?
Eiffel Outlook is a bi-monthly newsletter devoted to Eiffel and Sather. It
is 24 pages long and focuses mainly on practical and technical issues
involved in using these languages. Contact Tower Technology Corporation for
more information. Trial subscriptions and back issues are available.
Eiffel Outlook is distributed by:
Jay-Kell in Canada
SIG Computer in Germany
Everything Eiffel in the United Kingdom and France
Simon Parker in Ireland
IMSL in Japan
Enea Data in Sweden
Class Technology in Australia
Tower Technology in the USA and for all other countries
Eiffel Post is a four page newsletter (in German) for customers of SIG
Computer, the makers of Eiffel/S.
The Eiffel Interest Group of the UK and Ireland publishes an excellent
newsletter for its members.
NICE has produced a four-page glossy flyer "Eiffel Success Stories",
intended to be the first of a series.
ISE produces an 8-page newsletter "Eiffel World" which will appear several
times a year.
----------
Q09) Is Eiffel available on PC, Mac, NeXT, Amiga, Atari, ...?
This is the latest information that I have, but it does change rapidly.
SIG Eiffel/S 1.3 is available for DOS on the PC. As at January 1994, the
following C compilers are supported: Microsoft C 7.0, Borland C++ 3.x, Gnu
2.2.2 (with DJ Expander), Gnu 2.4.5 (with emx-g expander), Symantec C++
6.0, Watcom 9.5. SIG uses Symantec in-house. It works best with a 32-bit
compiler, e.g. Gnu, Symantec, Watcom.
Eiffel/S 1.3 is also available for OS/2 (using GCC 2.4.5 emx-g, Watcom C
9.5 or IBM C/Set) and Windows/NT (Microsoft 32-bit C, Watcom C 9.5 or
Symantec C++ 6.0), and for the following Unix systems: Linux, PC Unix
(Interactive, SCO, and ESIX), SPARC, NeXT, NeXTstep-Intel, HP/9000, DEC
5xxx, Sony News, DG Aviion and RS/6000.
ISE's Eiffel 3 (Release 3.2) is available for DEC Alpha (OSF-1), DEC MIPS
(Ultrix), DG Aviion (and hence other 88Open platforms), Fujitsu, HP UX, IBM
RS/6000, Pyramid, SCO (Open Desktop), Silicon Graphics and Sparc (Solaris
2.3 or SunOS). NeXTSTEP (Intel and Motorola) is also available (for
EiffelBench only, so far). The VMS version is nearing completion.
Development is underway for other platforms.
Tower Corporation's TowerEiffel (Release 1.2) is available on Sun SPARC or
compatible running SunOS 4.1.x with gcc 2.2.2, gcc 2.4.5 or Sun's acc 2.0.1
C compiler; also running Solaris 2.1 with gcc 2.4.5. A preliminary version
is available for OS/2 and Solaris x86. Ports to NeXT 486 and Windows NT are
underway.
Future ports of TowerEiffel are planned for AIX, IRIX, HPUX, DG/UX,
NextStep, Windows NT, OS/2, VMS and various Intel-based Unix systems.
----------
Q10) Is there an archive of the comp.lang.eiffel newsgroup?
An archive of the newsgroup is available on gatekeeper.dec.com. The DEC
Western Research Lab hosts it. This archive does not contain articles after
September 1992.
The files are in /pub/plan/eiffel/usenet/USENET-xx-yyy, where `xx'
represents the last two digits of the year and `yyy' the month of posting,
e.g., /pub/plan/eiffel/usenet/USENET-91-AUG. Compressed versions of the
files are also available.
>By anonymous FTP to gatekeeper.dec.com (16.1.0.2)
cd pub/plan/eiffel/usenet
get USENET-xx-yyy
(or to get the compressed copy, bin, get USENET-xx-yyy.Z)
>From a UUCP neighbour of decwrl:
"uucp decwrl!~/pub/plan/eiffel/usenet/USENET-xx-yyy.Z"
>From the DEC Easynet:
DECWRL::"/pub/plan/eiffel/usenet/USENET-xx-yyy"
USENET-88-DEC and USENET-88-DEC.Z are the oldest entries in the archives.
There is also an archive of comp.lang.eiffel at wuarchive.wustl.edu (login
as anonymous, send e-mail address as password). The files are in
/usenet/comp.lang.eiffel and subdirectories. Each message is in a separate
file, so it's not as convenient as the DEC archive, but it's more up-to-
date.
----------
Q11) How much memory and disk space does Eiffel development require?
To install and run Eiffel/S 1.3 under DOS, you need 10MB of disk space and
at least 4MB RAM (8MB recommended). Other products and platforms require
more.
However, for serious Eiffel work you could easily use 100MB or more.
----------
Q12) How large are typical Eiffel executables?
(How large are typical C executables?)
Seriously, Eiffel does impose a minimum size which is large since even
trivial Eiffel applications bring in a lot of classes. So, a simple program
like "Hello World" will create a relatively large executable.
Interestingly, Eiffel applications seem to grow less rapidly as new
capabilities are added. Reuse does help out tremendously in this regard. A
good Eiffel compiler allows large applications to be smaller than equally
functional applications written in C.
Note that leaving assertion checking in the code increases the size of
applications a lot. Despite this, many of us prefer that they remain
throughout development. Some even deliver a PRECONDITIONS-only version of
their applications to their early customers.
----------
Q13) Are there standards for the Eiffel language?
The definition of the Eiffel language is in the public domain. This
definition is controlled by NICE, the Non-profit International Consortium
for Eiffel. This means that anyone or any company may create a compiler,
interpreter, or whatever having to do with Eiffel. NICE reserves the right
to validate that any such tool conforms to the current definition of the
Eiffel language before it can be distributed with the Eiffel trademark.
(i.e. advertised as an "Eiffel" compiler.)
The Eiffel trademark is owned and controlled by NICE. NICE is using Dr.
Meyer's book, "Eiffel: The Language" (2nd Printing), as the initial
definition of the language.
The NICE board of directors consists of Bertrand Meyer, Simon Parker and
Roger Browne (chairman).
NICE has formed the Eiffel Standards Group (ESG) to resolve standards
questions and control the evolution of the language. The three current
Eiffel Compiler Vendors (ISE, SIG and Tower) are represented in the ESG as
well as many important and influential users of Eiffel.
There are three committees -- Language, Library, and Future Directions.
The Language Committee will address the ambiguities in the Eiffel Version 3
language specification as well as the differences that appear between the
current Eiffel 3 implementations.
The Library Committee will standardize the Kernel library, then consider
interface standards for the data structures cluster and other key Eiffel
clusters.
The Future Requirements Committee will prioritize the long range direction
of the standards work performed by the other two committees.
The NICE Interoperability Program (NIP) tracks the reporting and resolution
of interoperability problems. If you are aware of a problem whereby correct
Eiffel code will not run under a particular implementation, or where
correct Eiffel code produces different results under different
implementations, you are invited to report this by email to
nice-nip@atlanta.twr.com
NICE (Nonprofit International Consortium for Eiffel)
2701 Stratford Drive
Austin, TX 78746
TEL: (512) 328 6406
FAX: (512) 328 0466
email: nice@twr.com
----------
Q14) How fast do Eiffel applications run?
Early versions of Eiffel were slow. Recent implementations have improved
dramatically. However, to achieve maximum performance under any Eiffel
implementation, run-time assertion monitoring must be switched off.
It's hard to generalise, but compared to C++, simple computation-intensive
applications will run perhaps 15% slower. Large applications are often
dominated by memory management rather than computation. ISE recently
demonstrated that by simply adding a call to the garbage collector's "full-
collect" routine at a time when there were known to be few live objects,
performance became dramatically faster than a corresponding C++ version.
----------
Q15) Are there any Eiffel user groups?
International Eiffel UK & Ireland Eiffel Interest Group
User Group (IEUG) Caroline Browne
Darcy Harrison - Attention: IEUG 9 Princeton Court
ISE Inc. 55 Felsham Road
270 Storke Road, Suite 7 London SW15 1AZ
Goleta, CA 93117, USA TEL 081 780 1088
TEL (805) 685-1006 FAX 081 780 1941
FAX (805) 685-6869 (publishes a newsletter and holds
email darcyh@eiffel.com a quarterly meeting and seminar)
GUE, Groupe des Utilisateurs Eiffel (France)
Jean-Marc Nerson
104 rue Castagnary, 75015 Paris
TEL +33 1 45 32 58 80
FAX +33 1 44 32 58 81
email marc@eiffel.fr
(meets every two months or so)
----------
Q16) Where can I get Eiffel products and services?
Interactive Software Engineering, Inc. Jay-Kell Technologies, Inc.
270 Storke Road, Suite 7 48 Lakeshore Road, Suite #1
Goleta, CA 93117 Pointe Claire, Quebec
TEL 805-685-1006 Canada H9S 4H4
FAX 805-685-6869 TEL +51 4 630 1005
email info@eiffel.com FAX +51 4 630 1456
SIG Computer GmbH Tower Technology Corporation
zu den Bettern 4 1501 Koenig Lane
D 35619 Braunfels, Germany Austin, TX 78756 USA
TEL +49 6472 2096, FAX +49 6472 7213 TEL 512-452-9455
email eiffel@sigcomp.de FAX 512-452-1721
(cyrillic email eiffel@sigcomp.msk.su) email: tower@twr.com
Class Technology Pty. Ltd.
6 Pound Road
Hornsby NSW 2077, Australia
TEL +61 2 477 6188
FAX +61 2 476 4378
email class@peg.pegasus.oz.au
Everything Eiffel Simon Parker
6 Bambers Walk 45 Hazelwood
Wesham PR4 3DG Shankill
England Co Dublin, Ireland
TEL +44 772 687525 TEL +353 1 282 3487
email rogerb@eiffel.demon.co.uk email sparker@eiffel.ie
EtnoTeam SOL
Via Adelaide Bono Cairoli 34 104 rue Castagnary
20217 Milano 75015 Paris
Italy France
TEL +39 2 261621 TEL +33 1 45 32 58 80
FAX +39 2 26110755 FAX +33 1 44 32 58 81
Enea Data Sritech Information Technology
Box 232, Nytorpsvagen 5 744/51 2nd Floor
S-183 23 Taby, Sweden 10 Mian Road, 4th Block
TEL +46 8 792 25 00 Jayanagar, Bangalore, India 560011
FAX +46 8 768 43 88 TEL +91 812 640661
email eiffel@enea.se FAX +91 812 643608
Cromasoft, SA de CV Objective Methods Ltd
Mazatlan 161 PO Box 17356 (77 Chamberlain Rd)
Col Condesa, 06140 Mexico Karori, Wellington, New Zealand
TEL +52 5 286 82 13 TEL +64 4 476 9499
FAX +52 5 286 80 57 FAX +64 4 476 9237 or 8772
email claudio@croma.sunmexico.sun.com email dkenny@swell.actrix.gen.nz
Cybertech Forefront Computer Services
Systens Integration for CIM Pty. Ltd.
Suarez 1281, Third Floor,Apt.A 115 Seaford Road
CP-1288 Buenos Aires Seaford, Victoria 3198
Argentina Australia
TEL +54 1 28 1950 TEL +61 3 785 1122
FAX +54 1 322 1071 or +54 1 963 0070 FAX +61 3 770 0961
SOOPS Software Research Associates
Sarphatistraat 133 1-1-1 Hirakawo-Cho
NL-1018 GC Amsterdam, The Netherlands Chiyoda-ku Tokyo 102, Japan
TEL +31 20 525 6644 TEL +81 3 3234 8789
FAX +31 20 624 6392 FAX +81 3 3262 9719
email A731CISK@HASARA11.BITNET email sugita@sra.co.jp
Information and Math Science Lab Inc. ZumaSoft
2-43-1, Ikebukuro, Toshima-ku 6235 Paseo Canyon Drive
Tokyo 171, Japan Malibu, California 90265, USA
email fushimi@idas.imslab.co.jp TEL & FAX +1 310 457-6263
TEL +81 3 3590 5211 email 72674.3161@compuserve.com
FAX +81 3 3590 5353
Objectif Concept Abstraction (Jacques Silberstein)
Passage Cour-Robert 5 18 Faubourg de l'Hopital
CH 1700 Fribourg, Switzerland 2000 Neuchatel, Switzerland
TEL (41) 37-232977 phone +41.38.25.04.93
FAX (41) 37-464889 fax +41.38.259.857
email 100015.304@compuserve.com
----------
Q17) Are there any conferences for Eiffel users?
The conferences listed here are not just for Eiffel. Eiffel shares the
spotlight with other object-oriented languages including C++ and Smalltalk.
Aug 1 - 5, 1994
TOOLS USA '94, Santa Barbara, California
Deadline for papers: 18 March
Nov 29 - Dec 1, 1994
TOOLS PACIFIC '94, Melbourne, Australia
Deadline for papers: 22 July
TOOLS is the major international conference devoted to the applications of
Object-Oriented technology. Other events, such as Eiffel User Group
meetings or NICE meetings are often held in conjunction with TOOLS.
TOOLS Conferences
PO Box 6863, Santa Barbara, CA 93160, USA
TEL (805) 685 1006, FAX (805) 685 6869
email tools@tools.com
----------
Q18) Why do Eiffel implementations compile to C?
(Although current Eiffel implementations compile to C or C++, native code
compilers may become available in the future.)
By using C as a target language, an Eiffel implementor can:
- bring Eiffel to the marketplace faster and at lower cost
- port their implementation more easily to other platforms
- take advantage of optimisation provided by the C compiler
Much of the technology that makes Eiffel relatively simple to use also
makes it more difficult to implement (an Eiffel-to-C compiler is perhaps 4
to 5 times more difficult to create than a native Pascal compiler).
Compiling Eiffel to C seems to work well under Unix. C is sometimes thought
of as the native code of Unix.
On the other hand, C is not universal on other platforms, and the Eiffel
purchaser may need to buy a C compiler as well, and possibly replace it if
the supported C compilers change with new versions of the Eiffel compiler.
With a native-code compiler, you'd get somewhat better throughput and the
potential for smaller executables and slightly better performance. You'd
also get a higher price and an even longer wait for Eiffel to show up on
other than the leading market share machines.
----------
Q19) What is BON?
BON ("Business Object Notation") is a method for high-level analysis and
design, offering a seamless transition to an Eiffel implementation. The
method emphasizes Design by Contract and systematic development. For more
information on BON, see the Communications of the ACM, September 1992.
----------
Q20) Where can I get an Eiffel editor or emacs-mode?
Franck Arnaud's Eiffel extension to the Windows/WindowsNT programmers
editor Codewright from Premia allows you to see Eiffel code in colour, has
smart indenting and a few templates. Available by anonymous FTP from
ftp://ftp.cm.cf.ac.uk/pub/eiffel/tools/cweiffel.zip
The WINEDIT shareware programmer's editor offers colour syntax
highlighting, works with Eiffel/S under MS-Windows, and is available from:
ftp://src.doc.ic.ac.uk/computing/systems/ibmpc/windows3/programr/we-30d.zip
and ftp://gatekeeper.dec.com/.f/micro/msdos/win3/programr/we-30d.zip
Alan Philips' free Programmers File Editor also works with Eiffel/S under
MS-Windows, has templates but not syntax highlighting, available from:
ftp://src.doc.ic.ac.uk/computing/systems/ibmpc/simtel/windows3/pfe0506.zip
and ftp://gatekeeper.dec.com/.f/micro/msdos/win3/programr/pfe0506.zip
Tower supplies an emacs-mode to anyone who requests it. Send mail to
elisp@atlanta.twr.com to get the latest version.
----------
L01) What features does Eiffel have?
Eiffel is a pure object-oriented language. Its modularity is based on
classes. It stresses reliability, and facilitates design by contract. It
brings design and programming closer together. It encourages the re-use of
software components.
Eiffel offers classes, multiple inheritance, polymorphism, static typing
and dynamic binding, genericity (constrained and unconstrained), a
disciplined exception mechanism, systematic use of assertions to promote
programming by contract, and deferred classes for high-level design and
analysis.
Eiffel has an elegant design and programming style, and is easy to learn.
----------
L02) What changes have been made to the Eiffel language definition?
Eiffel is still a relatively new language, and there have been a number of
changes to its definition. Here is a summary of the major changes:
1. Changes between the publication of "Object-Oriented Software
Construction" in 1988, and the release of Eiffel 2.3:
- Constrained genericity enables a generic class to restrict its
generic parameters to the descendants of a given class
- The indexing clause allows information about a class to be
recorded for extraction by archival, browsing and query tools
- The assignment attempt operator "?=" provides a way to make
type-safe assignments going against the inheritance hierarchy
- User-defined infix and prefix operator features
- Expanded types support composite objects without dynamic
allocation, and with value semantics
- The obsolete clause for smooth library evolution
- The unique keyword for implicitly-assigned integer codes
- The multi-branch instruction (similar to a case statement)
- The boolean operator for implication ("implies")
2. Changes with the introduction of Eiffel Version 3:
- The feature adaptation subclause must now be terminated with "end"
- Semicolons as instruction separators are optional
- Groups of features are bracketed by a feature clause. All features
are exported unless the feature clause specifies a restriction.
The repeat subclause is no longer needed, because inherited
features keep the original export status they had in the parent
unless they are redefined, or are the subject of an export
subclause in the feature adaptation clause.
- Preconditions can only be replaced by weaker ones, postconditions
can only be replaced by stronger ones. This is now enforced by the
language through the use of "require else" in preconditions and
"ensure then" in postconditions.
- Ambiguities in repeated inheritance are resolved by a select
clause.
- A feature can no longer be replicated and redefined in the same
feature adaptation clause, however the same effect can be achieved
through repeated inheritance
- Two or more features may be defined at the same time (e.g. "f1, f2
is...").
- The keyword "frozen" before a feature name prohibits redefinition
of the feature in descendants
- In an anchored declaration, the anchor may now also be a formal
argument of the enclosing routine
- A class may have zero, one or more creation procedures, designated
with the "creation" keyword. A new creation syntax using the "!!"
symbol allows the appropriate creation procedure to be specified.
It is also possible to directly create an object of any type which
conforms to the entity to which it is being attached.
- The meaning of dot notation has been made more uniform, and
alternative constructs have been provided for the special
language-defined features that previously used dot notation:
x.Create is now !! x
y.Clone(x) is now y := clone(x)
x.Forget is now x := Void
x.Void is now x = Void
x.Equal(y) is now equal(x, y)
- Manifest arrays can be specified, for example
<<"Jan", "Feb", "Mar">>
which also provides a way to pass a variable number of arguments
to a routine.
- The command-line parameters are made available to the creation
procedure of the root class as an array of strings.
- A default rescue procedure called default_rescue may be defined
and inherited.
- A class may be declared to be an expanded class, in which case any
type based on that class will be expanded.
- An object may no longer contain a reference to an expanded object
that is a sub-object of another object. Instead, upon assignment
of an expanded object to a non-expanded object, the expanded
object will be cloned, and a reference to the newly-cloned object
will be stored in the non-expanded object.
- The operator "div" has been replaced by "//", and the operator
"mod" has been replaced by "\\".
3. Changes between first and second printings of "Eiffel: The Language"
- New basic types INTEGER_REF, REAL_REF, CHARACTER_REF and
BOOLEAN_REF etc have been introduced to provide non-expanded basic
types.
- Introduction of the POINTER type to enable external references to
be passed around in Eiffel programs.
- Calls from Eiffel to external routines no longer implicitly pass
the current object as the first parameter.
----------
L03) What libraries come with Eiffel?
ISE Eiffel 3:
EiffelBase (the basic library) is a library of reusable components covering
data structures and algorithms. It is the result of long-going systematic
effort at classifying the fundamental patterns and structures of computer
science in a Linnaean manner. It relies heavily on the Eiffel method, in
particular assertions (preconditions, postconditions, class invariants),
Design by Contract, constrained genericity, and repeated inheritance.
EiffelVision (the GUI library) is an encapsulation of essential graphical
abstractions. It makes it possible to build graphical Eiffel applications
without having to learn and use the internal details of X, Motif, OpenLook
or other window systems, as they are all encapsulated in EiffelVision's
classes in a form that is closer to application-related concepts.
EiffelLex provides a set of lexical analysis facilities.
EiffelParse (still in a somewhat provisional state) is an object-oriented
approach to parsing.
Other libraries are under development; in particular, third-party
products are being integrated into the EiffelShelf distribution.
(If you are interested in submitting components to the EiffelShelf,
for profit or just for fame, please contact shelf@eiffel.com)
SIG Eiffel/S:
The universal classes -- GENERAL, PLATFORM, ANY and NONE
The special classes, some of which are treated specially by the compiler
-- PART_COMPARABLE, COMPARABLE, NUMERIC, HASHABLE, BOOLEAN, CHARACTER,
INTEGER, REAL, DOUBLE, ARRAY, BITS n, OBJECT_STRUCTURE and STRING
ENVIRONMENT -- provides access to command line arguments and environment
variables
BASIC_IO -- access to standard input, standard output and error output
serial I/O
FORMAT -- conversion of other data types to and from strings
EXCEPTION -- fine grained access to exception handling
SYSTEM_TIME -- system time interface
INTERNAL -- control of the garbage collector
The FILES cluster: FILE, FILE_SYSTEM, FSYS_DAT -- files are modelled as
persistent dynamic arrays
TEXTFILE -- treats an ASCII text file as an array of strings
The SORTER class -- a sorting 'expert' that knows how to sort arrays
optimally
The MATH class -- trig, log, truncation, and a few constants
The basic container classes -- classified according to uniqueness (can
the same object occur more than once in the container?), ordering (are
objects in the container kept in sorted order?) and search access (does
one search for a key, or for the object itself?), as well as by
efficiency (is speed or size more important?): LIST, SORTED_LIST,
SIMPLE_TABLE, HASH_TABLE, SORTED_TABLE, SHORT_LIST, SHORT_TABLE,
SHORT_SORTED_LIST and SHORT_SORTED_TABLE
Other container classes -- associative arrays accessed by a hashable
key: DICTIONARY (with unique keys) and CATALOG (with multiple items per
key)
Specialised container classes -- STACK, QUEUE, PRIORITY_QUEUE and
KEY_PRIORITY_QUEUE
Abstract container classes -- define much of the interface of
containers: COLLECTION, TABLE, SORTED_COLLECTION and SORT_TABLE.
Iterator classes -- objects stored within containers can be visited
sequentially with iterators. More than one iterator can be active on a
container at one time: TRAVERSABLE, TWOWAY_TRAVERSABLE, ITERATOR and
TWOWAY_ITER.
The GRAPH Cluster -- a graph is defined by the classes VERTEX and EDGE.
It may have weighted edges (WT_GRAPH) or unweighted edges (GRAPH).
Iterators are provided to visit the edges emanating from a vertex
(EDGE_ITER); or all the vertices of a graph in breadth-first order
(BREADTH_ITER), depth-first order (DEPTH_ITER) or topological order
(TOP_ITER).
The MATCHER Cluster -- the MATCHER class is a pattern matcher that can
build and activate an automaton to search for patterns in text.
Effective descendants search for text using the Rabin-Karp algorithm
(RK_MATCHER), the Knuth-Morris-Pratt algorithm (KMP_MATCHER) and the
Boyer-Moore algorithm (BM_MATCHER). Others search for Regular
Expressions (RE_MATCHER) and lists of keywords (KEYWORD_MATCHER).
TXT_MATCHER is an iterator that searches for multiple occurrences of a
pattern in an array of strings, using any of the matcher classes.
The documentation is brief but readable, including examples and hints
for adding new containers or matchers. All in all, a smaller but
possibly tighter set of libraries.
(This response may give the appearance that Eiffel/S libraries are much
more extensive than ISE's, but the converse is true.)
The Eiffel Booch Components are available for use with TowerEiffel. Most of
them can be made safe in the presence of multiple threads of control. They
come with testing classes which double as training aids. They include:
Data Structures
Bags, collections, deques, dictionaries, graphs, lists, maps, queues,
rings, sets, stacks and trees.
Tools
Pattern-matching, search, sort.
Support Classes
Node, hash table, dictionary, synchronisation, date and time.
----------
L04) What's the big deal about preconditions and postconditions?
The big deal is that it supports programming by contract. For example,
preconditions (require clauses) are simple boolean statements that are used
to check that the input arguments are valid and that the object is in a
reasonable state to do the requested operation. If not, an exception is
generated. Similarly, postconditions (ensure clauses) make sure that a
method has successfully performed its duties, thus "fulfilling its
contract" with the caller. Invariants are boolean expressions that are
checked every time an object method returns back to a separate object.
You can use these ideas in any object-oriented programming language, but
usually must supply your own assertion mechanisms or rely on programmer
discipline. In Eiffel, the ideas are integrated into the whole fabric of
the environment. We find them used by:
-- the exception handling mechanism.
(Tracebacks almost always identify the correct culprit code since
preconditions almost always denote an error in the calling method, while
postconditions denote an error in the called method.)
-- the automatic compilation system.
(Assertions can be disabled entirely or selectively by type on a per
class basis.)
-- the Eiffel compiler
(Invariants, preconditions and postconditions are all inherited in a
manner that makes logical sense.)
(Assertion expressions are not allowed to produce side effects so they
can be omitted without effect.)
-- the automatic documentation tools
(Preconditions and postconditions are important statements about what a
method does, often effectively describing the "contract" between the
caller and callee. Invariants can yield information about legal states
an object can have.)
In the future we expect to see formal methods technology work its way into
the assertion capability. This will allow progressively more powerful
constraints to be put into place. In addition, if a conjecture by Dr. Meyer
bears fruit, the notion of preconditions may be extended into an important
mechanism for the development of parallel programming.
----------
L05) Please explain and discuss covariance vs. contravariance.
Consider the following situation: we have two classes PARENT and CHILD.
CHILD inherits from PARENT, and redefines PARENT's feature 'foo'.
class PARENT
feature
foo (arg: A) is ...
end; -- PARENT
class CHILD
inherit
PARENT redefine foo
feature
foo (arg: B) is ...
end; -- CHILD
The question is: what restrictions are placed on the type of argument to
'foo', that is 'A' and 'B'? (If they are the same, there is no problem.)
Here are two possibilities:
(1) B must be a child of A (the covariant rule - so named because in
the child class the types of arguments in redefined routines are
children of types in the parent's routine, so the inheritance
"varies" for both in the same direction)
(2) B must be a parent of A (the contravariant rule)
Eiffel uses the covariant rule.
At first, the contravariant rule seems theoretically appealing. Recall that
polymorphism means that an attribute can hold not only objects of its
declared type, but also of any descendant (child) type. Dynamic binding
means that a feature call on an attribute will trigger the corresponding
feature call for the *actual* type of the object, which may be a descendant
of the declared type of the attribute. With contravariance, we can assign
an object of descendant type to an attribute, and all feature calls will
still work because the descendant can cope with feature arguments at least
as general as those of the ancestor. In fact, the descendant object is in
every way also a fully-valid instance of the ancestor object: we are using
inheritance to implement subtyping.
However, in programming real-world applications we frequently need to
specialize related classes jointly.
Here is an example, where PLOT_3D inherits from PLOT, and DATA_SAMPLE_3D
inherits from DATA_SAMPLE.
class PLOT
feature
add(arg: DATA_SAMPLE) is ...
class PLOT_3D
inherit
PLOT redefine add
feature
add(arg: DATA_SAMPLE_3D) is ...
This requires the covariant rule, and works well in Eiffel.
It would fail if we were to put a PLOT_3D object into a PLOT attribute and
try to add a DATA_SAMPLE to it. It fails because we have used inheritance
to implement code re-use rather than subtyping, but have called a feature
of the ancestor class on an object of the descendant class as if the
descendant object were a true subtype. It is the compiler's job to detect
and reject this error, to avoid the possibility of a run-time type error.
Here's another example where a real-world situation suggests a covariant
solution. Herbivores eat plants. Cows are herbivores. Grass is a plant.
Cows eat grass but not other plants.
class HERBIVORE class PLANT
feature
eat(food: PLANT) is ...
diet: LIST[PLANT]
class COW class GRASS
inherit inherit
HERBIVORE PLANT
redefine eat
end
feature eat(food: GRASS) is ...
This does what we want. The compiler must stop us from putting a COW object
into a HERBIVORE attribute and trying to feed it a PLANT, but we shouldn't
be trying to do this anyway.
Also consider the container 'diet'. We are not forced to redefine this
feature in descendant classes, because with covariant redefinition of the
argument to 'eat', the feature 'diet' can always contain any object that
can be eaten (e.g. grass for a cow). (With contravariant redefinition of
the argument to 'eat', it would be necessary to re-open the parent class to
make the type of the container 'diet' more general).
To summarise: Real-world problems often lend themselves to covariant
solutions. Eiffel handles these well. Incorrect programs in the presence of
covariant argument redefinition can cause run-time type errors unless the
compiler catches these.
Sather uses the contravariant rule, but uses separate mechanisms for
subtyping and code reuse and only allows dynamic binding on true subtypes.
This seems to make contravariance work well, but it can force the Sather
programmer to use concrete types when modelling covariant problems.
Concrete types cannot be further subtyped in Sather, so this can reduce the
potential for re-use (in Eiffel, any type can be further subtyped, but the
compiler must check that it is used validly).
----------
L06) Is it true that there are "holes" in the Eiffel type system?
No. The design of Eiffel makes it possible to catch all type errors at
compile time, so that an Eiffel program cannot abort with a run time type
error.
However, to catch the more obscure type errors at compile time, the
compiler must analyse the way that classes interact within the entire
system, rather than just looking at each class one by one. This type of
system-wide checking is also necessary for many compiler optimisations.
Because system-wide compile-time validity checking can be complex, some
compilers insert run-time traps for these errors instead, and some may fail
to correctly trap these errors. Ask your Eiffel compiler vendor how they
handle these type problems.
----------
L07) Is there support for concurrency in Eiffel?
Eiffel 3 does not support concurrency; neither do current commercial
compilers. However, work on concurrency is one of the hottest Eiffel-
related research topics.
For four articles on concurrency facilities for Eiffel, including Bertrand
Meyer's article "Systematic Concurrent Object-Oriented Programming", see
the September 1993 "Communications of the ACM" (Vol. 36, Number 9).
----------
L08) Why doesn't Eiffel allow function overloading?
In Eiffel, no two features of a class may have the same identifier,
regardless of their respective signatures. The prevents the use of
function overloading ("multiple polymorphism"), a common programming
technique in languages like C++.
Eiffel is designed to be minimal: it includes exactly the features that its
designer considered necessary, and nothing else.
Because Eiffel already supports (single) polymorphism through its
inheritance system, the only positive thing that function overloading buys
you is reducing the number of feature names you have to learn. This is at
the expense of reducing the ability of the compiler to trap mistakes (often
type errors).
Readability is also enhanced when overloading is not possible. With
overloading you would need to consider the type of the arguments as well as
the type of the target before you can work out which feature is called.
With multiple inheritance and dynamic binding this is awkward for a
compiler and error-prone for a human. There is no intuitive rule which
could be used to disambiguate routine calls where there is no "nearest"
routine.
However, in Eiffel it's easy to write one routine with arguments of the
most general applicable type, then use the assignment attempt operator to
carry out the appropriate operation according to the run-time type of the
arguments (thereby explicitly programming the disambiguation "rules").
Having said that, the lack of multiple polymorphism does force us to write
some common mathematical operations (e.g. matrix math) in an awkward way,
and forces arithmetic expressions to be treated specially (the "arithmetic
balancing rule", ETL p385). But no-one has come up with a solution which is
so simple, elegant and useful that it improves the quality of Eiffel as a
whole.
----------
L09) Why are there no procedural types in Eiffel?
The notion of allowing a routine to be passed as an argument to a routine
is in many people's view incompatible with the O-O method. The definition
of object-orientation implies that every operation belongs to an object
type, so one does not manipulate routines just by themselves.
A possible technique when one feels the need to use a routine argument is
to write a class and include the routine in it. Then (rather than passing a
routine argument) pass an object - an instance of this class - to which the
routine can then be applied. This is a more flexible approach in the long
term. For example, you may later add an "undo" routine to your routine-
containing class, or an attribute such as "time of last execution".
--
-- Roger Browne, 6 Bambers Walk, Wesham, PR4 3DG, UK | Ph 0772-687525
-- Everything Eiffel: compilers/libraries/publications | +44-772-687525