Tcl Basics

By IIT Delhi

Part I introduces the basics of Tcl. Everyone should read Chapter 1, which describes the fundamental properties of the language. Tcl is really quite simple, so beginners can pick it up quickly. The experienced programmer should review Chapter 1 to eliminate any misconceptions that come from using other languages. Chapter 2 is a short introduction to running Tcl and Tk on UNIX, Windows, and Macintosh systems. You may want to look at this chapter first so you can try out the examples as you read Chapter 1.

Chapter 3 presents a sample application, a CGI script, that implements a guestbook for a Web site. The example uses several facilities that are described in detail in later chapters. The goal is to provide a working example that illustrates the power of Tcl.

The rest of Part I covers basic programming with Tcl. Simple string processing is covered in Chapter 4. Tcl lists, which share the syntax rules of Tcl commands, are explained in Chapter 5. Control structure like loops and if statements are described in Chapter 6. Chapter 7 describes Tcl procedures, which are new commands that you write in Tcl. Chapter 8 discusses Tcl arrays. Arrays are the most flexible and useful data structure in Tcl. Chapter 9 describes file I/O and running other programs. These facilities let you build Tcl scripts that glue together other programs and process data in files. After reading Part I you will know enough Tcl to read and understand other Tcl programs, and to write simple programs yourself.

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Tcl Style Guide

By Ray Johnson

This is a manual for people who are developing Tcl code for Wish or any other Tcl application. It describes a set of conventions for writing code and the associated test scripts. There are three reasons for the conventions. First, the conventions ensure that certain important things get done; for example, every procedure must have documentation that describes each of its arguments and its result, and there must exist test scripts that exercise every line of code. Second, the conventions guarantee that all of the Tcl and Tk code has a uniform style. This makes it easier for us to use, read, and maintain each other’s code. Third, the conventions help to avoid some common mistakes by prohibiting error-prone constructs such as building lists by hand instead of using the list building procedures.

This document is based heavily on the Tcl/Tk Engineering Manual written by John Ousterhout. John’s engineering manual specified the style of the C code used in the implementation of Tcl/Tk and many of its extensions. The manual is very valuable to the development of Tcl/Tk and is an important reason why Tcl is a relatively easy system to maintain. Deciding any style standard involves making trade-offs that are usually subjective. This standard was created in an iterative process involving the Tcl/Tk group at Sun Laboratories. I don’t claim that these conventions are the best possible ones, but the exact conventions don’t really make that much difference. The most important thing is that we all do things the same way.

Please write your code so that it conforms to the conventions from the very start. For example, don’t write comment-free code on the assumption that you’ll go back and put the comments in later once the code is working. This simply won’t happen. Regardless of how good your intentions are, when it comes time to go back and put in the comments you’ll find that you have a dozen more important things to do; as the body of uncommented code builds up, it will be harder and harder to work up the energy to go back and fix it all. One of the fundamental rules of software is that its structure only gets worse over time; if you don’t build it right to begin with, it will never get that way later.

The rest of this document consists of 8 major parts. We start with Section 2 which discusses executable files. Section 3 discusses the overall structure of packages and namespaces. Section 4 describes the structure of a Tcl code file and how to write procedure headers. Section 5 desribes the Tcl naming conventions. Section 6 presents low-level coding conventions, such as how to indent and where to put curly braces. Section 7 contains a collection of rules and suggestions for writing comments. Section 8 describes how to write and maintain test suites. Section 9 contains a few miscellaneous topics, such as keeping a change log.

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Practical Programming in Tcl and Tk

By Brent Welch, Ken Jones, and Jeff Hobbs

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.

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TCL Wise : Guide to the Tcl Programming Language

By Salvatore Sanfilippo

Dear Reader, this book is an introduction to the main ideas of the Tcl programming language: If you wish to learn a simple and powerful programming language, this book is for you. To read this book, the only prior knowledge needed is some basic understanding of programming in any language: from C, to Python, Perl, Lisp, if you understand the meaning of function and variable and other basic concepts you should not experience problems continuing to read.

Why Tcl? Because it's a simple and general programming language, that can be used with success to develop applications in short time. Tcl is a programmable programming language, as you will discover later. There are few ideas inside, that it's possible to combine to create programs, and to extend the language in itself in order to attack programming problems in a very direct way.

Tcl was created by John Ousterhout around 1988 as an embeddable command language. Today Ousterhout is no longer actively developing the language, and the evolution of Tcl is in the hands of the Tcl Core Team, elected to direct the development of the Tcl Core in the summer of 2000, and the rest of the Tcl comunity. The Tcl Core Team is working towards the Tcl 8.5 version, that should be released in the fall of 2005.

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