DOCSLIB.ORG

Squeak by Example

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text

Load more

Portland State University PDXScholar Computer Science Faculty Publications and Presentations Computer Science 2009 Squeak by Example Andrew P. Black Portland State University, [email protected] Stéphane Ducasse Oscar Nierstrasz Damien Pollet Damien Cassou See next page for additional authors Follow this and additional works at: https://pdxscholar.library.pdx.edu/compsci_fac Part of the Programming Languages and Compilers Commons Let us know how access to this document benefits ou.y Citation Details Black, Andrew, et al. Squeak by example. 2009. This Book is brought to you for free and open access. It has been accepted for inclusion in Computer Science Faculty Publications and Presentations by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. Authors Andrew P. Black, Stéphane Ducasse, Oscar Nierstrasz, Damien Pollet, Damien Cassou, and Marcus Denker This book is available at PDXScholar: https://pdxscholar.library.pdx.edu/compsci_fac/110 Squeak by Example Andrew P. Black Stéphane Ducasse Oscar Nierstrasz Damien Pollet with Damien Cassou and Marcus Denker Version of 2009-09-29 ii This book is available as a free download from SqueakByExample.org, hosted by the Institute of Computer Science and Applied Mathematics of the University of Bern, Switzerland. Copyright © 2007, 2008, 2009 by Andrew P. Black, Stéphane Ducasse, Oscar Nierstrasz and Damien Pollet. The contents of this book are protected under Creative Commons Attribution-ShareAlike 3.0 Unported license. You are free: to Share — to copy, distribute and transmit the work to Remix — to adapt the work Under the following conditions: Attribution. You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work). Share Alike. If you alter, transform, or build upon this work, you may distribute the resulting work only under the same, similar or a compatible license. • For any reuse or distribution, you must make clear to others the license terms of this work. The best way to do this is with a link to this web page: creativecommons.org/ licenses/by-sa/3.0/ • Any of the above conditions can be waived if you get permission from the copyright holder. • Nothing in this license impairs or restricts the author’s moral rights. Your fair dealing and other rights are in no way affected by the above. This is a human-readable summary of the Legal Code (the full license): creativecommons.org/licenses/by-sa/3.0/legalcode Published by Square Bracket Associates, Switzerland. SquareBracketAssociates.org ISBN 978-3-9523341-0-2 First Edition, September, 2007. Revised editions with corrections, March 2008, May 2008, September 2009. Contents Preface ix I Getting Started 1 A quick tour of Squeak3 1.1 Getting started.................3 1.2 The World menu................7 1.3 Saving, quitting and restarting a Squeak session.....9 1.4 Workspaces and Transcripts............ 11 1.5 Keyboard shortcuts............... 12 1.6 SqueakMap.................. 15 1.7 The System Browser............... 17 1.8 Finding classes................. 18 1.9 Finding methods................ 20 1.10 Defining a new method.............. 21 1.11 Chapter summary................ 26 2 A first application 29 2.1 The Quinto game................ 29 2.2 Creating a new class Category........... 30 2.3 Defining the class SBECell............. 31 2.4 Adding methods to a class............. 32 2.5 Inspecting an object............... 34 2.6 Defining the class SBEGame............ 36 iv Contents 2.7 Organizing methods into protocols......... 39 2.8 Let’s try our code................ 42 2.9 Saving and sharing Smalltalk code.......... 45 2.10 Chapter summary................ 49 3 Syntax in a nutshell 51 3.1 Syntactic elements............... 51 3.2 Pseudo-variables................ 54 3.3 Message sends................. 55 3.4 Method syntax................. 56 3.5 Block syntax.................. 57 3.6 Conditionals and loops in a nutshell......... 58 3.7 Primitives and pragmas............. 60 3.8 Chapter summary................ 61 4 Understanding message syntax 63 4.1 Identifying messages.............. 63 4.2 Three kinds of messages............. 65 4.3 Message composition.............. 68 4.4 Hints for identifying keyword messages....... 74 4.5 Expression sequences.............. 76 4.6 Cascaded messages............... 76 4.7 Chapter summary................ 76 II Developing in Squeak 5 The Smalltalk object model 81 5.1 The rules of the model.............. 81 5.2 Everything is an Object.............. 81 5.3 Every object is an instance of a class......... 82 5.4 Every class has a superclass............ 89 5.5 Everything happens by message sending....... 93 5.6 Method lookup follows the inheritance chain...... 94 5.7 Shared variables................ 101 v 5.8 Chapter summary................ 106 6 The Squeak programming environment 109 6.1 Overview................... 110 6.2 The System Browser............... 111 6.3 Monticello.................. 124 6.4 The Inspector and the Explorer........... 132 6.5 The Debugger................. 135 6.6 The Process Browser............... 144 6.7 Finding methods................ 145 6.8 Change sets and the Change Sorter......... 146 6.9 The File List Browser.............. 148 6.10 In Smalltalk, you can’t lose code.......... 151 6.11 Chapter summary................ 152 7 SUnit 155 7.1 Introduction.................. 155 7.2 Why testing is important............. 156 7.3 What makes a good test?............. 157 7.4 SUnit by example................ 158 7.5 The SUnit cook book............... 162 7.6 The SUnit framework.............. 164 7.7 Advanced features of SUnit............ 166 7.8 The implementation of SUnit............ 168 7.9 Some advice on testing.............. 171 7.10 Chapter summary................ 172 8 Basic Classes 175 8.1 Object.................... 175 8.2 Numbers................... 184 8.3 Characters.................. 188 8.4 Strings.................... 189 8.5 Booleans................... 190 8.6 Chapter summary................ 192 vi Contents 9 Collections 193 9.1 Introduction.................. 193 9.2 The varieties of collections............. 195 9.3 Implementations of collections........... 198 9.4 Examples of key classes.............. 199 9.5 Collection iterators............... 209 9.6 Some hints for using collections........... 213 9.7 Chapter summary................ 214 10 Streams 217 10.1 Two sequences of elements............ 217 10.2 Streams vs. collections.............. 218 10.3 Streaming over collections............. 219 10.4 Using streams for file access............ 227 10.5 Chapter summary................ 230 11 Morphic 231 11.1 The history of Morphic.............. 231 11.2 Manipulating morphs.............. 233 11.3 Composing morphs............... 233 11.4 Creating and drawing your own morphs....... 235 11.5 Interaction and animation............. 239 11.6 Interactors.................. 242 11.7 Drag-and-drop................. 243 11.8 A complete example............... 245 11.9 More about the canvas.............. 249 11.10 Chapter summary................ 250 III Advanced Squeak 12 Classes and metaclasses 255 12.1 Rules for classes and metaclasses.......... 255 12.2 Revisiting the Smalltalk object model......... 256 12.3 Every class is an instance of a metaclass........ 258 vii 12.4 The metaclass hierarchy parallels the class hierarchy... 259 12.5 Every metaclass Inherits from Class and Behavior ..... 261 12.6 Every metaclass is an instance of Metaclass ....... 264 12.7 The metaclass of Metaclass is an Instance of Metaclass ... 265 12.8 Chapter summary................ 266 IV Appendices A Frequently Asked Questions 271 A.1 Getting started................. 271 A.2 Collections.................. 271 A.3 Browsing the system............... 272 A.4 Using Monticello and SqueakSource......... 274 A.5 Tools.................... 275 A.6 Regular expressions and parsing.......... 275 Bibliography 277 Index 278 Preface What is Squeak? Squeak is a modern, open source, fully-featured implementation of the Smalltalk programming language and environment. Squeak is highly portable — even its virtual machine is written en- tirely in Smalltalk, making it easy to debug, analyze, and change. Squeak is the vehicle for a wide range of innovative projects from multimedia applications and educational platforms to commercial web development environments. Who should read this book? This book presents the various aspects of Squeak, starting with the basics, and proceeding to more advanced topics. This book will not teach you how to program. The reader should have some familiarity with programming languages. Some background with object-oriented programming would be helpful. This book will introduce the Squeak programming environment, the language and the associated tools. You will be exposed to common idioms and practices, but the focus is on the technology, not on object-oriented design. Wherever possible, we will show you lots of examples. (We have been inspired by Alec Sharp’s excellent book on Smalltalk1.) There are numerous other books on Smalltalk freely available on the web but none of these focuses specifically on Squeak. See for example: stephane.ducasse.free.fr/FreeBooks.html 1Alec Sharp, Smalltalk by Example. McGraw-Hill, 1997 hURL: http://stephane.ducasse.free.fr/ FreeBooks/ByExample/i. x Preface A word of advice Do not be frustrated by parts of Smalltalk that you do not immediately understand. You do not have to know everything! Alan Knight expresses this principle
Recommended publications
  • C Strings and Pointers

    C Strings and Pointers

    Software Design Lecture Notes Prof. Stewart Weiss C Strings and Pointers C Strings and Pointers Motivation The C++ string class makes it easy to create and manipulate string data, and is a good thing to learn when rst starting to program in C++ because it allows you to work with string data without understanding much about why it works or what goes on behind the scenes. You can declare and initialize strings, read data into them, append to them, get their size, and do other kinds of useful things with them. However, it is at least as important to know how to work with another type of string, the C string. The C string has its detractors, some of whom have well-founded criticism of it. But much of the negative image of the maligned C string comes from its abuse by lazy programmers and hackers. Because C strings are found in so much legacy code, you cannot call yourself a C++ programmer unless you understand them. Even more important, C++'s input/output library is harder to use when it comes to input validation, whereas the C input/output library, which works entirely with C strings, is easy to use, robust, and powerful. In addition, the C++ main() function has, in addition to the prototype int main() the more important prototype int main ( int argc, char* argv[] ) and this latter form is in fact, a prototype whose second argument is an array of C strings. If you ever want to write a program that obtains the command line arguments entered by its user, you need to know how to use C strings.
  • Traits: Experience with a Language Feature

    Traits: Experience with a Language Feature

    7UDLWV([SHULHQFHZLWKD/DQJXDJH)HDWXUH (PHUVRQ50XUSK\+LOO $QGUHZ3%ODFN 7KH(YHUJUHHQ6WDWH&ROOHJH 2*,6FKRRORI6FLHQFH1(QJLQHHULQJ$ (YHUJUHHQ3DUNZD\1: 2UHJRQ+HDOWKDQG6FLHQFH8QLYHUVLW\ 2O\PSLD$:$ 1::DONHU5G PXUHPH#HYHUJUHHQHGX %HDYHUWRQ$25 EODFN#FVHRJLHGX ABSTRACT the desired semantics of that method changes, or if a bug is This paper reports our experiences using traits, collections of found, the programmer must track down and fix every copy. By pure methods designed to promote reuse and understandability reusing a method, behavior can be defined and maintained in in object-oriented programs. Traits had previously been used to one place. refactor the Smalltalk collection hierarchy, but only by the crea- tors of traits themselves. This experience report represents the In object-oriented programming, inheritance is the normal way first independent test of these language features. Murphy-Hill of reusing methods—classes inherit methods from other classes. implemented a substantial multi-class data structure called ropes Single inheritance is the most basic and most widespread type of that makes significant use of traits. We found that traits im- inheritance. It allows methods to be shared among classes in an proved understandability and reduced the number of methods elegant and efficient way, but does not always allow for maxi- that needed to be written by 46%. mum reuse. Consider a small example. In Squeak [7], a dialect of Smalltalk, Categories and Subject Descriptors the class &ROOHFWLRQ is the superclass of all the classes that $UUD\ +HDS D.2.3 [Programming Languages]: Coding Tools and Tech- implement collection data structures, including , , 6HW niques - object-oriented programming and . The property of being empty is common to many ob- jects—it simply requires that the object have a size method, and D.3.3 [Programming Languages]: Language Constructs and that the method returns zero.
  • Basic Structures: Sets, Functions, Sequences, and Sums 2-2

    Basic Structures: Sets, Functions, Sequences, and Sums 2-2

    CHAPTER Basic Structures: Sets, Functions, 2 Sequences, and Sums 2.1 Sets uch of discrete mathematics is devoted to the study of discrete structures, used to represent discrete objects. Many important discrete structures are built using sets, which 2.2 Set Operations M are collections of objects. Among the discrete structures built from sets are combinations, 2.3 Functions unordered collections of objects used extensively in counting; relations, sets of ordered pairs that represent relationships between objects; graphs, sets of vertices and edges that connect 2.4 Sequences and vertices; and finite state machines, used to model computing machines. These are some of the Summations topics we will study in later chapters. The concept of a function is extremely important in discrete mathematics. A function assigns to each element of a set exactly one element of a set. Functions play important roles throughout discrete mathematics. They are used to represent the computational complexity of algorithms, to study the size of sets, to count objects, and in a myriad of other ways. Useful structures such as sequences and strings are special types of functions. In this chapter, we will introduce the notion of a sequence, which represents ordered lists of elements. We will introduce some important types of sequences, and we will address the problem of identifying a pattern for the terms of a sequence from its first few terms. Using the notion of a sequence, we will define what it means for a set to be countable, namely, that we can list all the elements of the set in a sequence.
  • The Future: the Story of Squeak, a Practical Smalltalk Written in Itself

    The Future: the Story of Squeak, a Practical Smalltalk Written in Itself

    Back to the future: the story of Squeak, a practical Smalltalk written in itself Dan Ingalls, Ted Kaehler, John Maloney, Scott Wallace, and Alan Kay [Also published in OOPSLA ’97: Proc. of the 12th ACM SIGPLAN Conference on Object-oriented Programming, 1997, pp. 318-326.] VPRI Technical Report TR-1997-001 Viewpoints Research Institute, 1209 Grand Central Avenue, Glendale, CA 91201 t: (818) 332-3001 f: (818) 244-9761 Back to the Future The Story of Squeak, A Practical Smalltalk Written in Itself by Dan Ingalls Ted Kaehler John Maloney Scott Wallace Alan Kay at Apple Computer while doing this work, now at Walt Disney Imagineering 1401 Flower Street P.O. Box 25020 Glendale, CA 91221 [email protected] Abstract Squeak is an open, highly-portable Smalltalk implementation whose virtual machine is written entirely in Smalltalk, making it easy to debug, analyze, and change. To achieve practical performance, a translator produces an equivalent C program whose performance is comparable to commercial Smalltalks. Other noteworthy aspects of Squeak include: a compact object format that typically requires only a single word of overhead per object; a simple yet efficient incremental garbage collector for 32-bit direct pointers; efficient bulk- mutation of objects; extensions of BitBlt to handle color of any depth and anti-aliased image rotation and scaling; and real-time sound and music synthesis written entirely in Smalltalk. Overview Squeak is a modern implementation of Smalltalk-80 that is available for free via the Internet, at http://www.research.apple.com/research/proj/learning_concepts/squeak/ and other sites. It includes platform-independent support for color, sound, and image processing.
  • Introduction

    Introduction

    Introduction A need shared by many applications is the ability to authenticate a user and then bind a set of permissions to the user which indicate what actions the user is permitted to perform (i.e. authorization). A LocalSystem may have implemented it's own authentication and authorization and now wishes to utilize a federated Identity Provider (IdP). Typically the IdP provides an assertion with information describing the authenticated user. The goal is to transform the IdP assertion into a LocalSystem token. In it's simplest terms this is a data transformation which might include: • renaming of data items • conversion to a different format • deletion of data • addition of data • reorganization of data There are many ways such a transformation could be implemented: 1. Custom site specific code 2. Scripts written in a scripting language 3. XSLT 4. Rule based transforms We also desire these goals for the transformation. 1. Site administrator configurable 2. Secure 3. Simple 4. Extensible 5. Efficient Implementation choice 1, custom written code fails goals 1, 3 and 4, an admin cannot adapt it, it's not simple, and it's likely to be difficult to extend. Implementation choice 2, script based transformations have a lot of appeal. Because one has at their disposal the full power of an actual programming language there are virtually no limitations. If it's a popular scripting language an administrator is likely to already know the language and might be able to program a new transformation or at a minimum tweak an existing script. Forking out to a script interpreter is inefficient, but it's now possible to embed script interpreters in the existing application.
  • Smalltalk's Influence on Modern Programming

    Smalltalk's Influence on Modern Programming

    1 Smalltalk’s Influence on Modern Programming Smalltalk’s Influence on Modern Programming Matt Savona. February 1 st , 2008. In September 1972 at Xerox PARC, Alan Kay, Ted Kaeher and Dan Ingalls were discussing programming languages in the hallway of their office building. Ted and Dan had begun to consider how large a language had to be to have “great power.” Alan took a different approach, and “asserted that you could define the "most powerful language in the world" in "a page of code."” Ted and Dan replied, “Put up or shut up” (Kay). And with that, the bet to develop the most powerful language was on. Alan arrived at PARC every morning at 4am for two weeks, and devoted his time from 4am to 8am to the development of the language. That language was Smalltalk. Kay had “originally made the boast because McCarthy's self-describing LISP interpreter was written in itself. It was about "a page", and as far as power goes, LISP was the whole nine- yards for functional languages.” He was sure that he could “do the same for object-oriented languages” and still have a reasonable syntax. By the eighth morning, Kay had a version of Smalltalk developed where “symbols were byte-coded and the receiving of return-values from a send was symmetric” (Kay). Several days later, Dan Ingalls had coded Kay’s scheme in BASIC, added a “token scanner”, “list maker” and many other features. “Over the next ten years he made at least 80 major releases of various flavors of Smalltalk” (Kay).
  • Type Feedback for Bytecode Interpreters Position Paper ICOOOLPS 2007

    Type Feedback for Bytecode Interpreters Position Paper ICOOOLPS 2007

    Type Feedback for Bytecode Interpreters Position Paper ICOOOLPS 2007 Michael Haupt1, Robert Hirschfeld1, and Marcus Denker2 1 Software Architecture Group Hasso-Plattner-Institut University of Potsdam, Germany 2 Software Composition Group Institute of Computer Science and Applied Mathematics University of Berne, Switzerland michael.haupt,hirschfeld @hpi.uni-potsdam.de, [email protected] { } Abstract. This position paper proposes the exploitation of type feed- back mechanisms, or more precisely, polymorphic inline caches, for purely interpreting implementations of object-oriented programming languages. Using Squeak’s virtual machine as an example, polymorphic inline caches are discussed as an alternative to global caching. An implementation proposal for polymorphic inline caches in the Squeak virtual machine is presented, and possible future applications for online optimization are outlined. 1 Introduction Bytecode interpreters are small in size and comparatively easy to implement, but generally execute programs much less efficiently than just-in-time (JIT) compilers. Techniques like threaded interpretation [9, 11, 2] focus on speeding up bytecode interpretation itself, and caching [4, 5, 1] improves the performance of message sends—the most common operation in object-oriented software [7]. It is interesting to observe that, while threading mechanisms are used natu- rally to a varying degree in bytecode interpreter implementations, such systems usually employ only global caching to speed up dynamic method dispatch. A global cache is clearly beneficial with respect to overall performance. Still, it does not provide optimal support for polymorphic message send sites, and it does not allow for exploiting type information (we provide details on these issues in the following section). In our opinion, the employment of polymorphic inline caches (PICs) [5] instead can provide means for achieving significant speedups in bytecode interpreters while exhibiting only a moderate increase in memory footprint and implementation complexity.
  • Graphical Interface

    Graphical Interface

    Appendix: How to program in Croquet Author: S.M. Wolff Project: Cell-Forum Project Table of Contents Table of Contents .............................................................................................................. 2 Graphical Interface ........................................................................................................... 6 Control structures .............................................................................................................. 9 Testing variables ............................................................................................................... 9 If then else ........................................................................................................................ 10 Switch ............................................................................................................................ 10 While .............................................................................................................................. 10 For .................................................................................................................................. 11 Data structures ............................................................................................................ 12 Ordered Collection ...................................................................................................... 12 Classes, Methods and Shared Variables .................................................................... 13 Packages .....................................................................................................................
  • Interface Evolution Via Virtual Extension Methods Brian Goetz Fourth Draft, June 2011

    Interface Evolution Via Virtual Extension Methods Brian Goetz Fourth Draft, June 2011

    Interface evolution via virtual extension methods Brian Goetz Fourth draft, June 2011 1. Problem statement Once published, it is impossible to add methods to an interface without breaking existing implementations. (Specifically, adding a method to an interface is not a source- compatible change.) The longer the time since a library has been published, the more likely it is that this restriction will cause grief for its maintainers. The addition of closures to the Java language in JDK 7 place additional stress on the aging Collection interfaces; one of the most significant benefits of closures is that it enables the development of more powerful libraries. It would be disappointing to add a language feature that enables better libraries while at the same time not extending the core libraries to take advantage of that feature1. V1 of the Lambda Strawman proposed C#-style static extension methods as a means of creating the illusion of adding methods to existing classes and interfaces, but they have significant limitations – for example, they cannot be overridden by classes that implement the interface being extended, so implementations are stuck with the “one size fits all” implementation provided as an extension2, and they are not reflectively discoverable. 2. Virtual extension methods3 In this document, we outline a mechanism for adding new methods to existing interfaces, which we call virtual extension methods. Existing interfaces can be augmented without compromising backward compatibility4 by adding extension methods to the interface, whose declaration would contain instructions for finding the default implementation in the event that implementers do not provide a method body.
  • Cross-Language Interop Poster

    Cross-Language Interop Poster

    Low Cognitive Overhead Bridging "Interoperability isn't just a technical problem, it's also a user interface problem. We have to make it easy to call between languages, so that users can The demo shown at FOSDEM always pick the most appropriate tool for the job at hand." example [ this year showed fully dynamic "This shows how to call a C function from Smalltalk" development spanning C objc_setAssociatedObject: { Objective-C and Smalltalk. The self. 42. developer can inspect the ’fishes’. existing class hierarchy and C enumValue: OBJC_ASSOCIATION_ASSIGN }. the code for any methods "Note how C enumerations can be accessed. When encountering this construct, the Pragmatic Smalltalk compiler will generate exactly the same code as an implemented in Smalltalk. It Objective-C compiler would for this line: is also possible to add, modify, objc_setAssociatedObject(self, 42, @"fishes", OBJC_ASSOCIATION_ASSIGN); or replace methods, add It will get the types for the function by parsing the header and automatically map Smalltalk types to C and Objective-C types. The instance variables and classes Easy Development programmer doesn't need to write any bridging or foreign function interface at run time. Invoking a code." nonexistent class or method (C objc_getAssociatedObject: {self . 42 }) log. ] pops up a dialog asking the user to implement it, all from within a running app. The final version is statically compiled, with no explicit user Cross-Language Interoperability for intervention required. Fast, Easy, and Maintainable Code David Chisnall The World Today: But I wanted to use the But libfoo is C, libbar Frobnicator framework, and it's I have a great idea is C++, and I want to only for Java.
  • Identitymodel Documentation

    Identitymodel Documentation

    IdentityModel Documentation Dominick Baier and Brock Allen May 11, 2021 IdentityModel 1 IdentityModel 3 2 IdentityModel.AspNetCore 5 3 IdentityModel.AspNetCore.OAuth2Introspection7 4 IdentityModel.OidcClient 9 5 oidc-client.js 11 5.1 Overview................................................. 11 5.2 Discovery Endpoint........................................... 12 5.3 Token Endpoint.............................................. 14 5.4 Token Introspection Endpoint...................................... 17 5.5 Token Revocation Endpoint....................................... 17 5.6 UserInfo Endpoint............................................ 18 5.7 Dynamic Client Registration....................................... 18 5.8 Device Authorization Endpoint..................................... 19 5.9 Protocol and Claim Type Constants................................... 19 5.10 Creating Request URLs (e.g. for Authorize and EndSession endpoints)................ 20 5.11 Fluent API for the X.509 Certificate Store................................ 22 5.12 Base64 URL Encoding.......................................... 22 5.13 Epoch Time Conversion......................................... 22 5.14 Time-Constant String Comparison.................................... 22 5.15 Overview................................................. 23 5.16 Worker Applications........................................... 23 5.17 Web Applications............................................ 25 5.18 Extensibility............................................... 27 5.19 Overview................................................
  • Smalltalk Language Mapping Specification

    Smalltalk Language Mapping Specification

    Smalltalk Language Mapping Specification New Edition: June 1999 Copyright 1995, 1996 BNR Europe Ltd. Copyright 1998, Borland International Copyright 1991, 1992, 1995, 1996 Digital Equipment Corporation Copyright 1995, 1996 Expersoft Corporation Copyright 1996, 1997 FUJITSU LIMITED Copyright 1996 Genesis Development Corporation Copyright 1989, 1990, 1991, 1992, 1995, 1996 Hewlett-Packard Company Copyright 1991, 1992, 1995, 1996 HyperDesk Corporation Copyright 1998 Inprise Corporation Copyright 1996, 1997 International Business Machines Corporation Copyright 1995, 1996 ICL, plc Copyright 1995, 1996 IONA Technologies, Ltd. Copyright 1996, 1997 Micro Focus Limited Copyright 1991, 1992, 1995, 1996 NCR Corporation Copyright 1995, 1996 Novell USG Copyright 1991,1992, 1995, 1996 by Object Design, Inc. Copyright 1991, 1992, 1995, 1996 Object Management Group, Inc. Copyright 1996 Siemens Nixdorf Informationssysteme AG Copyright 1991, 1992, 1995, 1996 Sun Microsystems, Inc. Copyright 1995, 1996 SunSoft, Inc. Copyright 1996 Sybase, Inc. Copyright 1998 Telefónica Investigación y Desarrollo S.A. Unipersonal Copyright 1996 Visual Edge Software, Ltd. The companies listed above have granted to the Object Management Group, Inc. (OMG) a nonexclusive, royalty-free, paid up, worldwide license to copy and distribute this document and to modify this document and distribute copies of the modified ver- sion. Each of the copyright holders listed above has agreed that no person shall be deemed to have infringed the copyright in the included material of any such copyright holder by reason of having used the specification set forth herein or having con- formed any computer software to the specification. PATENT The attention of adopters is directed to the possibility that compliance with or adoption of OMG specifications may require use of an invention covered by patent rights.