The Parser as Commodity (Perl and XML)

4.3. The Parser as Commodity

You don't have to write an XML processing program that separates parser from handler, but doing so can be advantageous. By making your program modular, you make it easier to organize and test your code. The ideal way to modularize is with objects, communicating on sanctioned channels and otherwise leaving one another alone. Modularization makes swapping one part for another easier, which is very important in XML processing.

The XML stream, as we said before, is an abstraction, which makes the source of data irrelevant. It's like the spigot you have in the backyard, to which you can hook up a hose and water your lawn. It doesn't matter where you plug it in, you just want the water. There's nothing special about the hose either. As long as it doesn't leak and it reaches where you want to go, you don't care if it's made of rubber or bark. Similarly, XML parsers have become a commodity: something you can download, plug in, and see it work as expected. Plugging it in easily, however, is the tricky part.

The key is the screwhead on the end of the spigot. It's a standard gauge of pipe that uses a specific thread size, and any hose you buy should fit. With XML event streams, we also need a standard interface there. XML developers have settled on SAX, which has been in use for a few years now. Until recently, Perl XML parsers were not interchangeable. Each had its own interface, making it difficult to swap out one in favor of another. That's changing now, as developers adopt SAX and agree on conventions for hooking up handlers to parsers. We'll see some of the fruits of this effort in Chapter 5, "SAX".