Just a Theory

By David E. Wheeler

Generating XML in Perl

I’ve been working on a big Bricolage project recently, and one of the requirements is to parse an incoming NewsML feed, turn individual stories into Bricolage SOAP XML, and import them into Bricolage. I’m using the amazing–if hideously documented–XML::LibXML to do the parsing of the incoming NewsML, taking advantage of the power of XPath to pull out the bits I need. But then came the question: what should I use to generate the XML for Bricolage?

Based on feedback from various Tweeps, I tried a few different approaches: XML::LibXML, XML::Genx, and of course the venerable XML::Writer. In truth, they all suck for one reason: interface. As a test, I wanted to generate this dead simple XML:

<?xml version="1.0" encoding="utf-8"?>
<assets xmlns="http://bricolage.sourceforge.net/assets.xsd">
    <story id="1234" type="story">
    <name>Catch as Catch Can</name>
    </story>
</assets>

Just get a load of how you create this XML in XML::LibXML:

use XML::LibXML;

my $dom = XML::LibXML::Document->new( '1.0', 'utf-8' );
my $assets = $dom->createElementNS('http://bricolage.sourceforge.net/assets.xsd', 'assets');
$dom->addChild($assets);

my $story = $dom->createElement('story');
$assets->addChild($story);
$story->addChild( $dom->createAttribute( id => 1234));
$story->addChild( $dom->createAttribute( type => 'story'));
my $name = $dom->createElement('name');
$story->addChild($name);
$name->addChild($dom->createTextNode('Catch as Catch Can'));

say $dom->toString;

Does anyone actually think that this is intuitive? Okay, if you’re used to dealing with the XHTML DOM in JavaScript it’s at least familiar, but that’s hardly an endorsement. XML::Genx isn’t much better:

use XML::Genx::Simple;

my $w = XML::Genx::Simple->new;
$w->StartDocString;

$w->StartElementLiteral( $w->DeclareNamespace( 'http://bricolage.sourceforge.net/assets.xsd', ''), 'assets' );
$w->StartElementLiteral( 'story' );
$w->AddAttributeLiteral( id => 1234);
$w->AddAttributeLiteral( type => 'story');
$w->Element( 'name' => 'Catch as Catch Can' );
$w->EndElement;
$w->EndElement;
$w->EndDocument;

say $w->GetDocString;

It’s not like messing with the DOM, but it’s essentially the same: Use a bunch of camelCase methods to declare each thing one-at-a-time. And you have to count the number of open elements you have yourself, to know how many times to call EndElement() to close elements. Can’t we get the computer to do this for us?

Feeling a bit frustrated, I went back to XML::Writer, which is what Bricolage uses internally to generate the XML exported by its SOAP interface. It looks like this:

use XML::Writer;

my $output = '';
my $writer = XML::Writer->new(
    OUTPUT=> \$output,
    ENCODING => 'utf8',
);

$writer->xmlDecl('UTF-8');
#$writer->startTag(['http://bricolage.sourceforge.net/assets.xsd', 'stories']);
$writer->startTag('assets', xmlns => 'http://bricolage.sourceforge.net/assets.xsd');
$writer->startTag('story', id => 1234, type => 'story');
$writer->dataElement(name => 'Catch as Catch Can');
$writer->endTag('story');

$writer->endTag('assets');

say $output;

That’s a bit better, in that you can specify the attributes and value of an element all in one method call. I still have to count opened elements and figure out where to close them, though. The thing that’s missing, as with the other approaches, is an API that reflects the hierarchical nature of XML itself. I’m outputting a tree-like document; why should the API be so hideously object-oriented and flat?

With this insight, I remembered Jesse Vincent’s Template::Declare. It bills itself as a templating library, but really it provides an interface for declaratively and hierarchically generating XML. After a bit of hacking I came up with this:

package Template::Declare::TagSet::Bricolage;
BEGIN { $INC{'Template/Declare/TagSet/Bricolage.pm'} = __FILE__; }
use base 'Template::Declare::TagSet';

sub get_tag_list {
    return [qw( assets story name )];
}

package My::Template;
use Template::Declare::Tags 'Bricolage';
use base 'Template::Declare';

template bricolage => sub {
    xml_decl { 'xml', version => '1.0', encoding => 'utf-8' };
    assets {
        xmlns is 'http://bricolage.sourceforge.net/assets.xsd';
        story {
            attr { id => 1234, type => 'story' };
            name { 'Catch as Catch Can' }
        };
    };
};

package main;
use Template::Declare;
Template::Declare->init( roots => ['My::Template']);
say Template::Declare->show('bricolage');

Okay, to be fair I had to do a lot more work to set things up. But once I did, the core of the XML generation, there in the bricolage template, is quite simple and straight-forward. Furthermore, thanks to the hierarchical nature of Template::Declare, the tree structure of the resulting XML is apparent in the code. And it’s so concise!

Armed with this information, I whipped up a new module for CPAN: Template::Declare::Bricolage. This module subclasses Template::Declare to provide a dead-simple interface for generating XML for the Bricolage SOAP interface. Using this module to generate the same XML is quite simple:

use Template::Declare::Bricolage;

say bricolage {
    story {
        attr { id => 1234, type => 'story' };
        name { 'Catch as Catch Can' }
    };
};

Yeah. Really. That’s it. Because the Bricolage SOAP interface requires that all XML documents have the top-level <assets> tag, I just had the bricolage function handle that, as well as actually executing the template and returning the XML. More complex XML is just a simple, assuming that you use nice indentation to format your code. Here’s the code to generate XML for a Bricolage workflow object:

use Template::Declare::Bricolage;

say bricolage {
    workflow {
        attr        { id => 1027     };
        name        { 'Blogs'        }
        description { 'Blog Entries' }
        site        { 'Main Site'    }
        type        { 'Story'        }
        active      { 1              }
        desks  {
            desk { attr { start   => 1 }; 'Blog Edit'    }
            desk { attr { publish => 1 }; 'Blog Publish' }
        }
    }
};

Simple, huh? So the next time you need to generate XML, have a look at Template::Declare. It may not be the fastest XML generator around, but if you have a well-defined list of elements you need, it’s certainly the nicest to use.

Oh, and Bricolage users? Just make use of use Template::Declare::Bricolage to deaden the pain.

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