Pro Perl Debugging 164
Michael J. Ross writes "The typical computer program has more bugs than there are ants at a picnic -- except ants are usually easier to find. Programs written in Perl are no exception, because the compactness of the language does not make any existent bugs easier to spot; they can simply be packed into fewer lines of code. To help remedy this problem, Richard Foley and Andy Lester, two seasoned Perl programmers, offer a new book, Pro Perl Debugging: From Professional to Expert." Read the rest of Michael's review.
| Pro Perl Debugging: From Professional to Expert | |
| author | Richard Foley with Andy Lester |
| pages | 269 |
| publisher | Apress |
| rating | 8 |
| reviewer | Michael J. Ross |
| ISBN | 1590594541 |
| summary | A comprehensive tutorial and reference for the Perl debugger |
This title was published in hardcover in March 2005 by Apress, a relatively new member of the technical publishing world. The publisher has a Web page for the book that includes links to all of the source code in a Zip file, the table of contents in PDF format, and a form for submitting errata. The book comprises 269 pages, the majority of which are organized into 16 chapters: Introduction (not to be confused with the true Introduction immediately preceding it), Inspecting Variables and Getting Help, Controlling Program Execution, Debugging a Simple Command Line Program, Tracing Execution, Debugging Modules, Debugging Object-Oriented Perl, Using the Debugger As a Shell, Debugging a CGI Program, Perl Threads and Forked Processes, Debugging Regular Expressions, Debugger Customization, Optimization and Performance Hints and Tips, Command Line and GUI Debuggers, Comprehensive Command Reference, Book References and URLs.
For programmers who wish to learn how to fully utilize Perl's debugger, what options are open to them? A terse summary of the debugger's commands are always close by, within the debugger itself. Those Perl coders who have yet to try the built-in Perl debugger, really owe it to themselves to give it a whirl. In most cases, it is superior to embedding lots of "print" statements in your scripts, and then wading through the results. Simply include perl.exe's -d flag on the system command line, and you should be put right into the debugger, and see the debugger's "DB<1>" command prompt -- the "1" meaning that it is ready for your first command. To display the aforementioned command summary, simply enter "h", or "|h" to see the output one screen-ful at a time, which you will probably want to do unless your system window can show all of the dozens of lines at once. The command summary is best used as a quick reference, and naturally cannot be expected to serve as any sort of tutorial. Yet it has its use, and for that, it's fine.
Most Perl books devote at least some space to explaining the basics of firing up and using Perl's debugger. The (in)famous "camel book," Larry Wall's Programming Perl, has a chapter on the debugger. It covers breakpoints, running, stepping, tracing, displaying code, commands, debugger customization, debugger options, unattended execution, creating your own debugger, and performance profiling. Aside from that last topic, the chapter is mostly an expansion of the command summary mentioned earlier. It is sparse on examples, and does not cover any advanced topics, such as using the debugger in the context of forking, threads, and POE, as well as the debugger's special capabilities for regular expressions, CGI programs, and shelling out.
The advanced topics are where Pro Perl Debugging really shines in relation to the coverage that I have seen in any other book, partly because the authors have the space to thoroughly explore those topics in depth, and to provide much more meaty examples, with adequately illustrative sample code. Even for the more complex topics, the writing is clear, and the examples are worthwhile.
The authors clearly intend for the book to serve as both a comprehensive tutorial and a reference for the Perl debugger. In both respects, they succeed admirably. But the practical value of their accomplishment could be called into question by any programmer who has grown tired of the limitations of the Perl debugger, and has switched over to any Perl-capable standalone GUI debugger or integrated development environment (IDE). More specifically, watching a variable change value, while stepping through the lines of a Perl script using the debugger, requires that the programmer manually or programmatically echo that variable's value, by issuing a print command ("p") followed by the variable name, one way or another. This process quickly becomes tedious when multiple variables need to be watched, because each individual variable must be printed, one at a time. Admittedly, previously entered print statements can be recalled by using the up-arrow key, but only if the particular command has not been pushed out of the debugger's limited storage. This usually becomes even more frustrating when trying to print the values of indexed arrays, hashes, and nested arrays and other structures. There are workarounds, but none are pretty, and even the most promising techniques still seem to require excessive focusing on the debugger commands themselves, drawing attention away from the code being debugged.
As a result, some disheartened Perl coders eventually switch back to embedding "print" statements in their code. Fortunately, there is a better alternative, in the form of IDEs, which can automatically report the changing values of a large set of variables, none of which need to be typed in, owing to the drag-and-drop capabilities of most IDEs. There are many IDEs available, including freeware and open source offerings. Most if not all of them support advanced editing, syntax highlighting and verification, visual breakpoints, and other much-appreciated capabilities. Even if they were to lack all of these features, and only have the advantage of easily and dynamically displaying the current values of variables, then they would be much more pleasant to use than the built-in Perl debugger. This is especially true in the case of nested structures, which can be expanded with a mouse click within most IDEs. All of this being said, it should be noted that the authors include a chapter that briefly touches upon the most well-known Perl GUI debuggers -- but at only seven pages in length, the chosen applications get only a cursory treatment, highlighting their major features.
Nonetheless, given the intended purpose of Pro Perl Debugging, and its target audience, the book cannot be faulted for its contents nor its approach to presenting the material. Anyone looking for a detailed and competent explication of the native Perl debugger, would likely not be able to find a more thorough treatment anywhere else.
Michael J. Ross is a freelance writer, computer consultant, and the editor of PristinePlanet.com's free newsletter."
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Obsolete before printing? (Score:4, Interesting)
I suppose this might be handy a bit the same way knowing something like ed or vi is handy for anybody using UNIX (or similar) systems -- even the absolute strangest distribution (or installation) will almost inevitably include at least that much and it's pretty consistent. OTOH, debugging Perl on a machine that hasn't really been equipped for it doesn't sound to me like a lot better idea than using ed to write most of my code...
Re:existent? (Score:2, Interesting)
extant [bartleby.com] and existent [bartleby.com].
Print statements (Score:3, Interesting)
Actually, after ~25 years of writing code, I've found that 'print' statements are the best alternative. I've used every kind of debugger and IDE in the past, but in the last couple of years I've found that I rarely use them.
When you're in a debugger, you're down in the trees and it's hard to see the forest. With skillfully placed print statements that pick out the relevant data, stack traces for exceptions and a decent language, you can get a commanding overview of what's going on in your app. With a good editor, you can manipulate print statements just about as fast as breakpoints in an IDE, too. Just map a function key in your editor to "save file; make go".
debugging loops (Score:2, Interesting)
This method will debug loops of all kinds, and if each 'warn' is labeled well, it's very easy to see what happened.
(Note: the above may or may not depend on the use of '#!/usr/bin/perl -w' (warnings) as your interpreter.)
Re:In defense of print statements (Score:2, Interesting)
Admittedly, there is a time and place for everything. I especially favor print statements when you need to consider non-localized program behavior: compare values at many different points of the program or across many iterations of some loop. In such cases, using the debugger is usually annoying and far too slow. A really useful technique for this kind of debugging is to format the printf output so that it can be piped into a database like postgres or mysql. Once you have the debugging output in a database table, you have a really powerful way to pore through the data trying to figure out what's going on.
[B/v/L]
Re:I luv Perl, but... (Score:3, Interesting)
Hey, just cuz you can't write a decent regex...
Re:use strict and Data::Dumper! (Score:3, Interesting)
use Coy; [cpan.org]
Re:I luv Perl, but... (Score:2, Interesting)
> (a page generator for 195 content categories in2 languages from
> source data files) than it would have to write it in perl and
> debug it.
*shrug*. Maybe you just think Guido's Way. Some of us don't. I tried to learn Python, but every time I tried to do anything in it, I ran into frustrating scenarios wherein I'd need to write eighty lines of code just to do something really simple. Maybe it's because I didn't know Python well enough yet to write it compactly, but whatever the reason, I just couldn't make myself finish learning the language.
In any case, a page generator for 195 content categories in two langauges from source data files sounds more like a round of golf; if it were an actual programming task, 80% of the time would be spent deciding which three modules to use (for parsing the source data, for generating the content's form, and for the language issue), and writing the code to put it all together would be 5%. (Allow 2-3% for debugging, and the rest of the time would go into writing the documentation.) Sounds *much* easier than learning a new language. (The *fastest* I ever learned a programming language to my satisfaction (discounting data languages such as SQL) was two weeks, and I spent twice that long attempting to learn Python... there's absolutely no way writing the page generator you describe could take that long.)
> The whole "TMTOWTDI" is just wrong, from a maintainability point of view.
Why? Why does it make the code less maintainable to solve each problem in the way that is natural to solve that problem, rather than trying to coerce every problem into a particular paradigm that may or may not be a natural fit for the problem? The best feature of Perl is its multiparadigmatic nature, that allows the programmer to select the best type of solution for any given problem. (Well, okay, maybe the *best* feature is the CPAN, but the multiparadigmaticity is a close second.)
I find that well-written Perl code is much more maintainable than code in most other languages. (Poorly-written Perl code is, of course, completely unmaintainable, but it's possible to write unmaintainable code in any language.)
> And perl's greediness in matching strings
Umm, did you read the documentation? If you want non-greedy matching, that's easy enough to do. The really handy things, though, are lookahead and lookbehind assertions. I just wish we could get variable-width lookbehind; presumably Perl6 will make this possible.
> Perl - because when it all looks like line noise, nobody can
> criticize the "quality" of your code.
If it all looks like line noise, the code doesn't *have* any quality.