Practical Programming in Tcl and Tk (3rd Edition)

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9780130220288: Practical Programming in Tcl and Tk (3rd Edition)

Covers user interface development, application integration techniques that leverage Tcl/Tk's cross-platform scripting capabilities, Tcl's compiler, and Tcl/Tk plug-ins for Netscape and Internet Explorer. Discusses Tcl's fundamental mechanisms and operating system interfaces. Softcover. CD-ROM included. DLC: Tcl (Computer prgramming language).

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Review:

Not unlike the hard-to-believe longevity of American Bandstand host Dick Clark, the continuing popularity of Tcl/Tk teaches us an important lesson in software language market adoption: early market share in an area of profound need with an easy-to-use product yields generations of loyal adherents. Long after opinion leaders in the GUI scripting community abandoned Tcl/Tk for more modern scripting languages, Tcl/Tk remains a major player, largely because of John Osterhout's lucid original text Tcl and the Tk Toolkit and more recently because of Brent Welch's encyclopedic Practical Programming in Tcl/Tk, now entering its third edition.

Tcl/Tk survives despite its inelegance ("set x [expr 2 + 2]" rather than "x=2+2") because it was the first practical, well-documented, simple-to-implement solution to the crucial question, "How do I build a GUI for my C program?" Tcl/Tk is available for all major windows and OS environments, including X11 (native Unix-like windows), Microsoft Windows, and Macintosh.

Now with 40 pages of tables of contents, examples, and charts and another 40 pages of newly recompiled indices, Welch's new edition spans 55 chapters divided into seven sections. The sections are divided as follows: two for Tcl, three for Tk, one for the C library, and one for updates in Tcl and Tk distributions and functionality. The third edition appears against a healthy backdrop of new Tcl/Tk development. Perhaps the proudest accomplishment of the Tcl community is TclHttpd (chapter 18), a Web server written largely in Tcl! The new server contributes to efforts to unclog the Web server bottlenecks by providing faster client routes to server-side applications while continuing to support the traditional--albeit slower--common gateway interface. The remainder of the book is not new, but it has been augmented and polished.

Welch has expanded discussions and added examples of Tcl's extensibility and embeddability in C. Tcl/Tk's development scope has grown to include all POSIX internals, such as threads, sockets, TCP/IP interfaces, and secure shells.

Recent Tcl/Tk releases enable meta-level execution through eval() and support for Web client plug-ins, all of which are detailed with examples and careful explanation. Functionally, Tcl/Tk lacks nothing compared to modern scripting languages, except lexical flexibility and object-oriented architecture. Nor does it add anything except familiarity, consistency, and a long history of above-average documentation.

Welch perhaps wisely omits comparison with his competition, just as Dick Clark never mentions Howard Stern. It is beneath the dignity of aging market leaders to look back--or even around. --Peter Leopold

From the Inside Flap:

Preface

Tcl stands for Tool Command Language Tcl is really two things: a scripting language, and an interpreter for that language that is designed to be easy to embed into your application. Tcl and its associated graphical user-interface toolkit, Tk, were designed and crafted by Professor John Ousterhout of the University of California, Berkeley. You can find these packages on the Internet (as explained on page lii) and use them freely in your application, even if it is commercial. The Tcl interpreter has been ported from UNIX to DOS, Windows, OS/2, NT, and Macintosh environments. The Tk toolkit has been ported from the X window system to Windows and Macintosh.

I first heard about Tcl in 1988 while I was Ousterhout's Ph.D. student at Berkeley. We were designing a network operating system, Sprite. While the students hacked on a new kernel, John wrote a new editor and terminal emulator. He used Tcl as the command language for both tools so that users could define menus and otherwise customize those programs. This was in the days of X10, and he had plans for an X toolkit based on Tcl that would help programs cooperate with each other by communicating with Tcl commands. To me, this cooperation among tools was the essence of Tcl.

This early vision imagined that applications would be large bodies of compiled code and a small amount of Tcl used for configuration and high-level commands. John's editor, mx, and the terminal emulator, tx, followed this model. While this model remains valid, it has also turned out to be possible to write entire applications in Tcl. This is because the Tcl/Tk shell, wish, provides access to other programs, the file system, network sockets, plus the ability to create a graphical user interface. For better or worse, it is now common to find applications that contain thousands of lines of Tcl script.

This book was written because, while I found it enjoyable and productive to use Tcl and Tk, there were times when I was frustrated. In addition, working at Xerox PARC, with many experts in languages and systems, I was compelled to understand both the strengths and weaknesses of Tcl and Tk. Although many of my colleagues adopted Tcl and Tk for their projects, they were also just as quick to point out its flaws. In response, I have built up a set of programming techniques that exploit the power of Tcl and Tk while avoiding troublesome areas. This book is meant as a practical guide to help you get the most out of Tcl and Tk and avoid some of the frustrations I experienced.

It has been about 10 years since I was introduced to Tcl, and about five years since the first edition of this book. During the last several years I have been working under John Ousterhout, first at Sun Microsystems and now at Scriptics Corporation. I have managed to remain mostly a Tcl programmer while others in our group have delved into the C implementation of Tcl itself. I've been building applications like HTML editors, e-mail user interfaces, Web servers, and the customer database we run our business on. This experience is reflected in this book. The bulk of the book is about Tcl scripting, and the aspects of C programming to create Tcl extensions is given a lighter treatment. I have been lucky to remain involved in the core Tcl development, and I hope I can pass along the insights I have gained by working with Tcl.

Why Tcl?

As a scripting language, Tcl is similar to other UNIX shell languages such as the Bourne Shell (sh), the C Shell (csh), the Korn Shell (ksh), and Perl. Shell programs let you execute other programs. They provide enough programmability (variables, control flow, and procedures) to let you build complex scripts that assemble existing programs into a new tool tailored for your needs. Shells are wonderful for automating routine chores.

It is the ability to easily add a Tcl interpreter to your application that sets it apart from other shells. Tcl fills the role of an extension language that is used to configure and customize applications. There is no need to invent a command language for your new application, or struggle to provide some sort of user-programmability for your tool. Instead, by adding a Tcl interpreter, you structure your application as a set of primitive operations that can be composed by a script to best suit the needs of your users. It also allows other programs to have programmatic control over your application, leading to suites of applications that work well together.

The Tcl C library has clean interfaces and is simple to use. The library implements the basic interpreter and a set of core scripting commands that implement variables, flow control, and procedures (see page 22). There is also a set of commands that access operating system services to run other programs, access the file system, and use network sockets. Tk adds commands to create graphical user interfaces. Tcl and Tk provide a "virtual machine" that is portable across UNIX, Windows, and Macintosh environments.

The Tcl virtual machine is extensible because your application can define new Tcl commands. These commands are associated with a C or C++ procedure that your application provides. The result is applications that are split into a set of primitives written in a compiled language and exported as Tcl commands. A Tcl script is used to compose the primitives into the overall application. The script layer has access to shell-like capability to run other programs, has access to the file system, and can call directly into the compiled part of the application through the Tcl commands you define. In addition, from the C programming level, you can call Tcl scripts, set and query Tcl variables, and even trace the execution of the Tcl interpreter.

There are many Tcl extensions freely available on the Internet. Most extensions include a C library that provides some new functionality, and a Tcl interface to the library. Examples include database access, telephone control, MIDI controller access, and expect, which adds Tcl commands to control interactive programs.

The most notable extension is Tk, a toolkit for graphical user interfaces. Tk defines Tcl commands that let you create and manipulate user interface widgets. The script-based approach to user interface programming has three benefits:

Development is fast because of the rapid turnaround; there is no waiting for long compilations.

The Tcl commands provide a higher-level interface than most standard C library user-interface toolkits. Simple user interfaces require just a handful of commands to define them. At the same time, it is possible to refine the user interface in order to get every detail just so. The fast turnaround aids the refinement process.

The user interface can be factored out from the rest of your application. The developer can concentrate on the implementation of the application core and then fairly painlessly work up a user interface. The core set of Tk widgets is often sufficient for all your user interface needs. However, it is also possible to write custom Tk widgets in C, and again there are many contributed Tk widgets available on the network.

There are other choices for extension languages that include Visual Basic, Scheme, Elisp, Perl, Python, and Javascript. Your choice between them is partly a matter of taste. Tcl has simple constructs and looks somewhat like C. It is easy to add new Tcl primitives by writing C procedures. Tcl is very easy to learn, and I have heard many great stories of users completing impressive projects in a short amount of time (e.g., a few weeks), even though they never used Tcl before.

Java has exploded onto the computer scene since this book was first published. Java is a great systems programming language that in the long run could displace C and C++. This is fine for Tcl, which is designed to glue together building blocks written in any system programming language. Tcl was designed to work with C, but has been adapted to work with the Java Virtual Machine. Where I say "C or C++", you can now say "C, C++, or Java," but the details are a bit different with Java. This book does not describe the Tcl/Java interface, but you can find TclBlend on the CD-ROM. TclBlend loads the Java Virtual Machine into your Tcl application and lets you invoke Java methods. It also lets you implement Tcl commands in Java instead of C or C++.

Javascript is a language from Netscape that is designed to script interactions with Web pages. Javascript is important because Netscape is widely deployed. However, Tcl provides a more general purpose scripting solution that can be used in a wide variety of applications. The Tcl/Tk Web browser plugin provides a way to run Tcl in your browser. It turns out to be more of a Java alternative than a JavaScript alternative. The plugin lets you run Tcl applications inside your browser, while JavaScript gives you fine grain control over the browser and HTML display. The plugin is described in Chapter 20.

Tcl and Tk Versions

Tcl and Tk continue to evolve. See beedub/book/ for updates and news about the latest Tcl releases. Tcl and Tk have had separate version numbers for historical reasons, but they are released in pairs that work together. The original edition of this book was based on Tcl 7.4 and Tk 4.0, and there were a few references to features in Tk 3.6. This third edition has been updated to reflect new features added through Tcl/Tk 8.2:

Tcl 7.5 and Tk 4.1 had their final release in May 1996. These releases feature the port of Tk to the Windows and Macintosh environments. The Safe-Tcl security mechanism was introduced to support safe execution of network applets. There is also network socket support and a new Input/Output (I/O) subsystem to support high-performance event-driven I/O.

Tcl 7.6 and Tk 4.2 had their final release in October 1996. These releases include improvements in Safe-Tcl, and improvements to the grid geometry manager introduced in Tk 4.1. Cross-platform support includes virtual events (e.g., < > as opposed to ), standard dialogs, and more file manipulation commands.

Tcl 7.7 and Tk 4.3 were internal releases used for the development of the Tcl/Tk plug-in for the Netscape Navigator and Microsoft Internet Explorer Web browsers. Their development actually proceeded in parallel to Tcl 7.6 and Tk 4.2. The plug-in has been released for a wide variety of platforms, including Solaris/SPARC, Solaris/INTEL, SunOS, Linux, Digital UNIX, IRIX, HP/UX, Windows 95, Windows NT, and the Macintosh. The browser plug-in supports Tcl applets in Web pages and uses the sophisticated security mechanism of Safe-Tcl to provide safety.

Tcl 8.0 features an on-the-fly compiler for Tcl that provides many-times faster Tcl scripts. Tcl 8.0 supports strings with embedded null characters. The compiler is transparent to Tcl scripts, but extension writers need to learn some new C APIs to take advantage of its potential. The release history of 8.0 spread out over a couple of years as John Ousterhout moved from Sun Microsystems to Scriptics Corporation. The widely used 8.0p2 release was made in the fall of 1997, but the final patch release, 8.0.5, was made in the spring of 1999.

Tk changed its version to match Tcl at 8.0. Tk 8.0 includes a new platform-independent font mechanism, native menus and menu bars, and more native widgets for better native look and feel on Windows and Macintosh.

Tcl/Tk 8.1 features full Unicode support, a new regular expression engine that provides all the features found in Perl 5, and thread safety so that you can embed Tcl into multithreaded applications. Tk does a heroic job of finding the correct font to display your Unicode characters, and it adds a message catalog facility so that you can write internationalized applications. The release history of Tcl/Tk 8.1 also straddled the Sun to Scriptics transition. The first alpha release was made in the fall of 1997, and the final patch release, 8.1.1, was made in May 1999.

Tcl/Tk 8.2 is primarily a bug fix and stabilization release. There are a few minor additions to the Tcl C library APIs to support more extensions without requiring core patches. Tcl/Tk 8.2 went rapidly into final release in the summer of 1999.

Who Should Read This Book

This book is meant to be useful to the beginner in Tcl as well as the expert. For the beginner and expert alike, I recommend careful study of Chapter 1, Tcl Fundamentals. The programming model of Tcl is designed to be simple, but it is different from many programming languages. The model is based on string substitutions, and it is important that you understand it properly to avoid trouble in complex cases. The remainder of the book consists of examples that demonstrate how to use Tcl and Tk productively. For your reference, each chapter has tables that summarize the Tcl commands and Tk widgets they describe.

This book assumes that you have some programming experience, although you should be able to get by even if you are a complete novice. Knowledge of UNIX shell programming will help, but it is not required. Where aspects of window systems are relevant, I provide some background information. Chapter 2 describes the details of using Tcl and Tk on UNIX, Windows, and Macintosh.

How to Read This Book

This book is best used in a hands-on manner, trying the examples at the computer. The book tries to fill the gap between the terse Tcl and Tk manual pages, which are complete but lack context and examples, and existing Tcl programs that may or may not be documented or well written.

I recommend the on-line manual pages for the Tcl and Tk commands. They provide a detailed reference guide to each command. This book summarizes much of the information from the manual pages, but it does not provide the complete details, which can vary from release to release. HTML versions of the on-line manual pages can be found on the CD-ROM that comes with this book.

Other Tcl Books

This book was the second Tcl book after the original book by John Ousterhout, the creator of Tcl. Since then, the number of Tcl books has increased remarkably. The following are just some of the books currently available.

Tcl and the Tk Toolkit (Addison-Wesley, 1994) by John Ousterhout provides a broad overview of all aspects of Tcl and Tk, even though it covers only Tcl 7.3 and Tk 3.6. The book provides a more detailed treatment of C programming for Tcl extensions.

Exploring Expect (O'Reilly & Associates, Inc., 1995) by Don Libes is a great book about an extremely useful Tcl extension. Expect lets you automate the use of interactive programs like ftp and telnet that expect to interact with a user. By combining expect and Tk, you can create graphical user interfaces for old applications that you cannot modify directly.

Graphical Applications with Tcl & Tk (M&T Press, 1996) by Eric Johnson is oriented toward Windows users. The second edition is up-to-date with Tcl/Tk 8.0.

Tcl/Tk Tools (O'Reilly & Associates, Inc., 1997) by Mark Harrison describes many useful Tcl extensions. These include Oracle and Sybase interfaces, object-oriented language enhancements, additional Tk widgets, and much more. The chapters were contributed by the authors of the extensions, so they provide authoritative information on some excellent additions to the Tcl toolbox.

CGI Developers Resource, Web Programming with Tcl and Perl (Prentice Hall, 1997) by John Ivler presents Tcl-based solutions to programming Web sites.

Effective Tcl/Tk Programming (Addison Wesley, 1997) by Michael McLennan and Mark Harrison illustrate Tcl and Tk with examples and application design guidelines.

Interactive Web Applications with ...

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