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 The Lighter Side of CPAN

or All I Ever Needed to Know About Perl I Learnt from a Pun

Programming can be stressful game. We sit at our machines, day in, day out. Endlessly staring at a monitor fathoming where that last devious bug has buried itself. It's not surprising then that sometimes it all gets too much and someone, somewhere snaps. The strangest things become hilarious and valuable hours will be wasted developing a strained play on words into a fully tested and highly featured module.

Portions of this creative outpouring of humour often find their way onto the CPAN. There is even the special Acme::* name space set aside for these bizarre freaks so they do not interfere with the normal smooth running of that ever so special service. It may seem a little pointless for someone to release these into the wild, but you may be surprised what you could learn from them. A good joke is usually marked out by being both funny and simple and the same applies to jokes told in Perl. All unnecessary detail is stripped away leaving a short piece of code which makes a perfect example of how to use, abuse or create a language feature.

There now follows a brief tour of some (but not all) of the more amusing extensions to Perl, along with some hints on how you might improve your own programs by taking inspiration from the tricks they use to implement their punch lines. (Ba-boom tching!)

Getting it Wrong the Wright Way

Perl is a lazy language. It appeals to the discerning man of leisure who wants nothing more than a chance to get things done quickly. There are some who would have Perl be more lazy but, until rescued by Dave Cross and his band of visionaries, they were forced to work hard making sure that every single keystroke they made was correct. Dismayed by errors like:

 Undefined subroutine &main::find_nexT called at line 39.

many turned away from Perl. Now they return in droves, using Symbol::Approx::Sub their lives are made an order of magnitude easier, leaving them more time to sip cocktails as they lounge in deck-chairs. This module can even save you money, observe the simple calculation of your fiscal dues after the careful application of a couple of typos:

 use Symbol::Approx::Sub;
 sub milage  { 40_000   };
 sub taxipay {     10   };
 sub tax2pay {$_[0]*0.4 };
 sub myage   {     25   };

 # Sell car
 print "For Sale: Very Good Car, only @{[ miage()]} on the clock\n";

 # Cheque for tax man
 my $income = 40_000;
 print "Must pay taxes, cheque for: @{[ taxpay($income) ]}\n";

A calculation which could not be faulted by any government, but which will leave you with a brand new car and, half the time, a whopping rebate:

 For Sale: Very Good Car, only 25 miles on the clock
 Must pay taxes, cheque for: 10

How does this all work? There are two major bits of magic going on: installing a handler which intercepts calls to undefined subroutines and a tool for divining the names of those routines which it can call instead.

The handler is implemented by creating an AUTOLOAD function for the module which uses Symbol::Approx::Sub. When Perl is asked to run a subroutine which it cannot locate it will invoke AUTOLOAD in the package where it was first told to look for the subroutine. This is handed the same arguments as the original function call and is told which subroutine Perl was looking for in the global $AUTOLOAD variable. AUTOLOAD is mainly used to write lazy accessors for object data, this example:

    my ($self, $value) = @_;
    my ($field) = $AUTOLOAD =~ /.*::(.*)$/;
    return if $field eq 'DESTROY';
    return @_==1 ? $self->{$field} : $self->{$field} = $value;

provides simple get/set methods. So that:

 print $object->name, "\n";

prints Harry even though you haven't had to explicitly write a name method for your object.

Perl stores all information about non-lexical variables, filehandles and subroutine names in a massive hash. You can inspect this yourself but doing so requires code which is so close to spaghetti you could plate it up and serve it to an Italian. The easy way out, wisely taken by the Symbol::Approx::Sub module, is to use the Devel::Symdump module which provides a friendly and clean interface to Perl's symbol table. Devel::Symdump provides various useful tools: if you are scratching your head trying to resolve an inheritance tree the isa_tree method will help, if you want to find exactly what a module exports into your namespace you'll find the diff method a constant friend.

Presently living in the past

Ever since the ancients erected the huge lumps of stone that paved the way for the digital watches which we hold so dear mankind has needed to know when he is. Perl is no different and now has many -- although some might say too many -- date and time related modules, around eighty of them in fact. Simple statistics tell us that at least a few of those should be so useless we couldn't possibly resist trying to find something to do with them.

Date::Discordian, although lacking in chicken references. Gobble.

This could, in a limited set of circumstances, be helpful though. Imagine the scene: a trusted client is on the 'phone demanding a completion date for the project doomed to persist. You reach for the keyboard and, in a moment of divine inspiration, type:

 perl -MDate::Discordian -le'print discordian(time+rand(3e7))'

you then soberly relate the result to your tormentor, ``Prickle Prickle, Discord 16 YOLD 3168'' and, suddenly, everything is alright. Well, they've put the 'phone down and left you in peace. If you prefer to provide a useful service, then you might be better off investigating the Time::Human module by Simon Cozens. This creates person-friendly descriptions of a time, transforming the excessively precise 00:23:12.00 into a positively laid-back ``coming up to twenty-five past midnight''. The module is internationalised and could be used in conjunction with a text-to-speech system, such as festival, to build an aural interface to something like a ticket booking system.

Moving swiftly on, we come to Date::Tolkien::Shire, a king amongst date modules. Most newspapers carry an ``on this day in history'' column -- where you find, for instance, that you were born on the same day as the man who invented chilli-paste -- but no broadsheet will tell you what happened to Frodo and his valiant companions as they fought to free Middle Earth from the scourge of the Dark Lord. The undeceptively simple:

 use Date::Tolkien::Shire;
 print Date::Tolkien::Shire->new(time)->on_date, "\n";

outputs (well, output a few days ago):

 Highday Winterfilth 30 7465
 The four Hobbits arrive at the Brandywine Bridge in the dark, 1419.

What better task could there be for crontab but to run this in the wee hours and update /etc/motd for our later enjoyment. Implementing this is, as ever, left as an exercise for the interested reader.

There is a more useful side to Date::Tolkien::Shire or, at the very least, it does light the way for other modules. As well as the on_date() method it provides an overloaded interface to the dates it returns. This allows you to compare dates and times as if they were normal numbers, so that:

 $date1 = Date::Tolkien::Shire->new(time);
 $date2 = Date::tolkien::Shire->new(time - 1e6);

 print 'time is '.( $date1 > $date2  ? 'later':'earlier' ).
     "than time -1e6\n";

prints time is later than time -1e6, the more prosaic Date::Simple module provides a similar interface for real dates and ensures they stringify with ISO formatting.

From One Date to Another

It is often said that computers and relationships don't mix well but this isn't entirely true. If you feel alone in the world and need to find that special person Perl is there to help you. Your first task is to meet someone. Perhaps by putting an advert on a dating service. Of course, you want to find the very best match and, being fond of concise notation, decide you will search for your companion with the help of the geek code. But how is your prospective mate to know what all those funny sigils mean? With the help of the Convert::GeekCode module of course:

 use Convert::GeekCode;

 print join("\n",geek_decode(<<'ENDCODE'),"\n");
 GS d+ s:- a--a? C++> UB++ P++++ !L E+ W+++ N++ K? w--() !M PS++ PE+
 Y PGP+ t+(-) 5++ !X R+ !tv b+++  DI++ D+++ G e* h y?
 ------END GEEK CODE BLOCK------

Will tell you, amongst other things, that ``I don't write Perl, I speak it. Perl has superseded all other programming languages. I firmly believe that all programs can be reduced to a Perl one-liner.''

So, you've got a reply and someone wants to meet you. This is a worrying prospect though as you feel you'll need to brush up on your conversation skills a little before meeting your date. Again, Perl comes to your aid with Chatbot::Eliza, especially useful if you want to meet a simple minded psychologist. Fire her up with:

 perl -MChatbot::Eliza -e'Chatbot::Eliza->new->command_interface'

and enjoy hours of elegant conversation:

 you:    I like pie
 Eliza:  That's quite interesting.

If your wit and repartee fail to impress, you may want to convince your partner that you have a deep and lasting interest in some obscure branch of mystical poetry. Doing this requires some mastery of ZenPAN combined with a careful study of Lee Goddard's Poetry::Aum. More than any other module, this teaches you that true understanding comes from within: by inspecting the source of all your powers. The source code, that is.

If none of this works or you find you've arranged a date with a total bore don't despair. There are ways to move the encounter towards an interesting conclusion. Simply let Michael Schwern's Bone::Easy take the pain out of dumping your burden.

 perl -MBone::Easy -le'print pickup()'

 When are you going to drain that?

How could all this be useful though? Convert::GeekCode hints at Perl's greatest strength, data transformation. The remaining twenty or so Convert::* modules can sometimes be a Godsend. If you are having trouble with EBCDIC encoded text or need to make your product catalogue acceptable to people who need whichever of metric and Imperial units you haven't provided you'll find something to furnish the answer.

Chatbot::Eliza on the other hand is a shining example of code whose behaviour you can change easily. Because it was written using Perl's OO features and a bit of thought was applied while deconstructing the problem it addresses it is full of hooks from which you can dangle your own bits of code, perhaps to use a different interface or a text to speech system. Can Bone::Easy teach you anything? Who knows...

A Day at the Races

Having foolishly followed my dating advice above you will have a great deal of time to yourself but do not fear, you can still keep yourself amused. If you have a sporting bent Jos Boumans's ACME::Poe::Knee uses the wonderful POE framework to race ponies across your screen, you could even make bets with yourself sure in the knowledge that you'll end the day even. One day POE may fulfill its original purpose and morph into a multi user dungeon (MUD) although at the moment, alas, it is far too busy being useful.

If you get tired of watching ACME::Poe::Knee you can instead follow Sean M Burke's Games::Worms, in combination with the cross-platform Tk Perl bindings, as it draws pretty patterns on your screen. Tk is only one of many graphical toolkits for Perl which can be used to quickly prototype an interface design or glue together a range of command line applications with a common frontend.

When Bugs Attack

Every now and then, despite all your best efforts to program extremely and extract the maximum of laziness from Perl, you will come across a deeply buried, complicated and fatal bug in your code. Your spirits sink when you discover one, the next two days of your precious time will be filled with cryptic error messages flashing all over your terminal:

 Something is very wrong at line 22.

It needn't be like this though, there is a better way. You'll still have to fight this bug for days but you can keep your blood pressure at bay with a little application of Damian Conway's Coy. Simply add:


to your environment (ideally somewhere global like /etc/.cshrc) so that any time Perl explodes all over your hard disc you'll be greeted by a soothing Haiku to take the edge off your pain:

        A woodpecker nesting 
        in a lemon tree. Ten 
        trout swim in a stream.
                Tor Kin Tun's commentary...

                Something is very wrong
                        (Analects of line 22.)

Setting PERL5OPT can help you in normal circumstances. Should you be developing an existing library you will often want to switch from the new to the old version, saying export PERL5OPT=-Ipath/to/new is less hassle than fiddling with use lib 'path/to/new' within your code.

These, along with a much larger host of useful modules, are available from the CPAN.

 ^Back to Top^ | ©Alex Gough Mon Mar 14 2005 ...more than you might imagine.