Graphical Applications with Tcl and Tk

Graphical Applications with
Tcl and Tk, Second Edition

Tcl, pronounced tickle, stands for the Tool Command Language. With its associated user interface toolkit, Tk, pronounced tee-kay, you can quickly create cross-platform applications with graphical user interfaces, all without having to learn GTK, Qt, the Windows API, or Mac OS X.

Tcl

Created by John Ousterhout, Tcl is more like a scripting language than a programming language, so it shares a greater similarity to the Bash shell or Perl than it does to C++ or C. For example, the following is the Hello World program in Tcl/Tk:

   # This is a comment.
   button .b -text "Hello World" -command exit
   pack .b

My book, Graphical Applications with Tcl and Tk, covers writing desktop applications with Tcl and Tk.

One Minute Introduction

Tcl has a simple structure. Each line starts out with a command, such as button and a number of arguments. Each command is implemented as a C function (or Java method). This function is responsible for handling all the arguments.

Syntax	Means
command arg1 arg2	Execute the command, with the given arguments.
"Text in quotes"	Pass the text between the quotes as one argument, performing command and variable substitution.
{Text in braces}	Pass the text between the quotes as one argument, defer substitution until later.
$variable	Substitute the value of the given variable.
[command arg]	Execute the command, then substitute the return value in place of the entire command between the square brackets.
command arg1\arg2	Extend command over one line.

Tcl commands can span multiple lines through the use of a line continuation marker, \, or curly braces, { and }.

You can create your own commands in C, Java, or you can use the Tcl proc command to create procedures written in Tcl.

Key Features

Tcl is a high-level scripting language.: You'll find you need to write a lot less code to get your job done, especially when compared to GTK or Windows applications.
Tcl is interpreted.: You can execute your code directly, without compiling and linking (though Tcl compilers are available).
Tcl is extensible.: It's very easy to add your own commands to extend the Tcl language. You can write your commands in C or Tcl (or Java and Tcl with the Jacl interpreter).
Tcl is embeddable in your applications.: The Tcl interpreter is merely a set of C functions that you can call from your code. This means you can use Tcl as an application language, much like a macro language for a spreadsheet application.
Tcl runs on many platforms.: Versions exist for UNIX, Linux, Windows, and Mac OS X platforms. Except for a few platform differences, your Tcl scripts run the same on all systems.
Tcl's auto-loading facility makes for smaller applications.: Tcl will automatically load in your libraries of Tcl procedures (with one line of code to set up the path to your library). Tcl will also, on systems that support it, automatically load dynamic (shared, DLL) libraries when needed. This is very handy.
Tcl is free.: Yep. You can get the sources for free over the Internet. (See "Tcl Downloads" below.)

Running Tcl Programs

Since Tcl is an interpreted language, to run a Tcl program (also called a script), you normally pass the script file to the Tcl interpreter, wish, for example:

wish hello.tcl

You can also use wish in interactive mode and type in commands at the command line.

There's another standard Tcl interpreter, tclsh, which only understands the Tcl language. tclsh does not have any of the Tk user interface commands, so you cannot create graphical programs in tclsh.

Some Tcl freeware applications extend the Tcl language by adding new commands written as C functions. If such is the case, you need to compile the application instead of just passing its Tcl code to the wish interpreter. This application program, from a Tcl perspective, is really a new version of the wish interpreter, which the new C commands linked in. Of course, the application program may be a lot more than merely a Tcl interpreter. (Note: you can also use Tcl's auto-loading capability on systems that support it.)

Extensions

Since Tcl is so easy to extend, many try to share extensions, including the popular itcl, [incr Tcl], ObjectTcl, TclX, Tix and BLT.

These extensions, of course, require an extended Tcl interpreter. In addition, many Tcl freeware applications require a particular Tcl extension to run.

One very popular extension is called Expect, which allows you to place a friendly front-end onto most command-line based UNIX applications, such as telnet, ftp, passwd, fsck, rlogin, tip and so on.

Starkits and Starpacks

One of the most exciting developments in Tcl help you package your applications into simple bundles. A tclkit is a one-file executable Tcl package for a given system. The normal Tcl distribution includes a large number of files that must be installed on each system. With a tclkit, though, you have just one file, containing everything.

A starkit acts similarly, providing a single-file bundle of all of your code for an entire Tcl application. In most cases, starkits work on any platform Tcl does. (Starkits are similar to Jar files for Java applications.)

You can combine a tclkit (the executable engine) and a starkit (your application code) into a starpack, a single-file executable of an entire application. With a starpack, you have an easy-to-install, easy-to-transport Tcl application. Furthermore, because a starpack is an executable program, your users don't even have to know this is a Tcl application.

Eric Foster-Johnson