Great CPAN Modules

Given the proliferation of SAX filters, I expect this to become a popular system for 

XML processing in Perl.

The Perl XML community is interesting aside from any particular module. 

Using the Perl XML mailing list as a hub, the community is unusual for its cohesion. 

Module ideas are vigorously discussed and the community even shares a Source 

Forge project and Web space. It may be that this cohesion has come as a response 

to the largely uphill battle facing the Perl XML developers. As Matt Sergeant put it, 

 Right now our biggest battle seems to be for acceptance within the Perl com 

munity as a whole. People completely accept things like the DBI, Tk/Gtk, and LWP 

as a vital part of Perl, but are quick to dismiss XML as something they just don't 

need and really don't like. We're kind of the bastard child of the Perl community. 

But we're winning small battles quite often now. 

Parse::RecDescent

No discussion of great CPAN modules would be complete without at least one module 

by the  Mad Scientist of Perl,  Dr. Damian Conway.

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 His Parse::RecDescent module 

generates recursive descent parsers from a vaguely yacc like grammar. If you've 

never used yacc, the concept is simple: you write a description of the grammar you 

want to parse and the parser compiler, yacc, generates an executable parser that 

will parse your grammar. Parse::RecDescent extends this concept in a number of 

directions that allow the grammar writer far more freedom of expression than yacc.

Parse::RecDescent's interface is superficially object oriented parser objects are 

instantiated by passing the grammar to Parse::RecDescent >new(). The returned 

parser object has methods to parse input text. However, the real interface to 

Parse::RecDescent is in the grammar. The syntax of Parse::RecDescent grammars 

is essentially an entirely new programming language designed specifically for 

building parsers that interact with Perl. The advantage of this approach over an 

entirely object oriented interface are brevity and flexibility.

The success of Parse::RecDescent is largely the usual mix of a common problem 

and an efficient solution. However, like many of Damian's modules, it is also an 

enjoyable technical challenge for the user. Learning to use Parse::RecDescent is 

fun in and of itself; the fact that it helps you solve parsing problems too can seem 

almost secondary!

The most common complaint about Parse::RecDescent is that it's too slow and 

uses too much memory. My opinion is that for what Parse::RecDescent  accomplishes, 

it's amazing that it uses as few resources as it does. However, to answer these critiques, 

Damian is hard at work on a completely new module, Parse::FastDescent, which may 

25. See Dr. Damian Conway's sites at http://www.cs.monash.edu.au/~damian and 

http://www.yetanother.org/damian/.

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