The Cold, Thin Edge

While it is potentially dangerous, people went so wild over this feature of Tcl that an extension to Tcl called Expect, a programming environment in its own right, was invented and has soared to new heights of popularity among certain users.

For example, ftp is a fairly simple program. You interact via a command line with a local program which then executes your commands. Because this uses the simple Unix STDIN/STDOUT method of interaction, you can write shell scripts to ftp files; I use such a script to retreive RFCs from the Internet automatically. However, a program like telnet is virtually impossible to script because you are not sending data to the program itself—you are sending data through a network connection to be interpreted on a remote machine. So, if you need to maintain a large number of routers, and if the only way to configure or check on these routers is via telnet, you are in trouble.

Expect solves this problem by using Unix's pseudo-tty mechanism. With Expect, you can script dialogues between your program and another one in which your program responds intelligently to the other. Think of a dialer program like dip or chat, except you can script dialogues with other programs instead of modems.

Expect is the height of inter-program communication, short of socket-based or sysV-ipc. (If you don't know, don't ask.) While it originally started as a Tcl extension, it has also been rendered into a C library; you can access its features from within C programs or from other environments which can use C libs, such as Perl.

In the introduction to his book Tcl and the Tk Toolkit, John Ousterhout mentions that even though Tcl was originally designed to be a simple scripting language where all programs would have at least “some new C code”, the simplicity of the environment which they gave the programmer proved too enticing. “Most Tcl/Tk users never write any C code at all,” Ousterhout writes, “and most Tcl/Tk applications consist solely of Tcl scripts.”

This is either a good or a bad thing, depending on whether your criteria are ease-of-use or efficiency/power. Responding to the rise of Tcl in his typically understated manner, GNU Luminary and urban legend Richard Stallman posted a USENET article entitled “Why you should not use Tcl”:,

Tcl was not designed to be a serious programming language. It was designed to be a “scripting language”, on the assumption that a “scripting language” need not try to be a real programming language. So Tcl doesn't have the capabilities of one.

The ability to interact with other programs in new, unorthodox and some would say dangerous ways is what makes Tcl so appealing to some and so appalling to others. This is typical of the dilemma in using Unix tools from within non-shell programs.

It usually comes down to a matter of time. If you're trying to enter your code in the country fair, these techniques aren't going to win you a blue ribbon. If, however, you want to get it done by 7 PM so you can go to the fair, these might do the trick.

In an age of near-gigaflops-speed chips in home computers, a few wasted cycles here and there aren't going to kill anyone, especially for a program that will be run once or twice and then thrown away. Extending the shell philosophy to development work is also an attractive option—the speed with which you can hack together workable programs makes these techniques alluring to programmers on a tight deadline. Tcl/Tk is a perfect example of extending the shell philosophy to speed up development cycles. Of course, the inefficiencies of this approach are the cause of nearly all of the intense debate over the merits of Tcl/Tk.

Whether it be Tcl, shell, Perl, or C, no matter what your programming technique of choice might be, there is usually an option whereby tools from other programming environments can be imported for your use. If Richard Stallman writes you a nasty letter criticizing you for it, though, don't say you weren't warned.