Exploring Expect

Chapter 18. Debugging Scripts

A description of debugging techniques could easily fill an entire book or more—and rightfully so. In any software development, debugging often takes more time than the programming (not to mention design^[59]).

The need for debugging is even more exaggerated in Tcl where many people code while thinking. The interpretive nature of the language makes this seductively easy.

Good design principles can help avoid getting stuck in a quagmire. Careful thought can prevent many common traps. In the same way, careful (and imaginative) thought is helpful in solving problems. Some people claim never to use debuggers. I am not one of them, but sometimes it helps to walk away from the keyboard and simply think through what could possibly be happening—or what cannot possibly be happening but is anyway.

Tracing

In this section, I will briefly recap some useful techniques and mention a few more that are very useful for tracking down problems. All of these have to do with tracing control. I will go into other debugging techniques later.

Simple output commands (using puts and send) can go a long way towards finding problems. However, putting commands in and taking them out is a hassle. You can conditionalize their execution with a variable.

if (verbose) {
    puts ". . ."
}

Rather than having raw if/puts commands throughout your code, it is cleaner to isolate them in one place, such as a procedure called vprint (for “verbose print”).

proc vprint {msg} { global verbose if {$verbose} ...