HTML is a derivative of SGML, just like XML is. Sure, they look pretty much the same for the most part, but there are a few key differences that prevent HTML from being parsed exactly like XML. Part of the reason why I like XHTML so much is that it’s more usable with more parsers, including many of simpler and full-featured XML parsers. Simplicity in parsing was one of the original motivations of the XML design, at least in comparison to a full SGML parser or even something like a full HTML parser.

But that’s all besides the point.

I’ve been in something of a backyard gardening kick lately. We bought our house only a few short months ago, and are only half way through the first summer growing season in my modest little garden. My plans for next year are much more expansive. I’ve finally talked my wife into letting me buy some cherry trees to plant. She was also pretty willing to get a few grape vines planted (especially when I sketched out the beautiful wooden arbor they would be growing on). She put her foot down when I started talking about blueberries, apples and pears, however. And another garden bed or two for more vegetables. For some reason she’s convinced that we need some measure of open space in our little plot so the kid has somewhere to run and play. Some people have weird priorities.

This is all sort of besides the point too.

Getting the things I need for all this gardening work I’ve talked myself into is not cheap. Cherry tree seeds actually do grow on trees so that’s not a big deal, but other things like fertilizers, soil amendments, tools, materials for building a grape trellis and raised garden beds, not to mention a longer hose to reach all the new things that are going to require regular watering all cost money. And maybe a sprinler, like one of those fancy ones on an electronic timer. I can avoid some of that cost by getting things used and at discount on sites like Craigslist. So I’ve been going there. Every day.

And it’s tedious. I have to sort through hundreds of listings for things I don’t want, in categories that seem far too course. Sometimes, because things often get incorrectly categorized, I have to look in other related categories too, sorting through things that are even less relevant on average to try and find the occasional gem. This is all on top of the hardware-related problems I have being unable to use the trackpad on my laptop so web navigation on sites without keyboard shortcuts is an extreme pain. I start to think to myself: I can do better, I’m a programmer! For some values of “better” and “programmer”.

Enter Rosella. Now with Parrot, Winxed and Rosella I can use the Net library to fetch the text of the HTML code of the page. After some hacking in the last few days, I can parse that code with my Xml library (set in a new lenient mode) and start to work with it in a meaningful way:

function main[main]() {
    var rosella = load_packfile("rosella/core.pbc");
    Rosella.initialize_rosella("xml", "net", "string");

    var ua = new Rosella.Net.UserAgent.SimpleHttp();
    var response = ua.get("");
    var doc = Rosella.Xml.read_string(response.content, false);

        .get_children_named("p", "row":[named("class")])
        .map(function(node) {
            return {
                "title": node.first_child("a").get_inner_xml(),
                "link":  node.first_child("a").attributes["href"],
                "price": node.first_child("span", "itempp":[named("class")]).get_inner_xml(),
                "has_pic": !Rosella.String.null_or_empty(
                    node.first_child("span", "itempx":[named("class")]).get_inner_xml()
        .filter(function(obj) {
            return indexof(obj["title"], "compost") >= 0;
        .map(function(obj) {
            return Rosella.String.format_obj("<a href='{link}'>{title} for {price}</a>", obj);
        .foreach(function(string s) { say(s); });

That second argument to Rosella.Xml.read_string tells the parser to go into “non-strict” mode, which is basically my attempt to fudge the XML parsing rules to allow for the SGML nonsense in HTML. Without that, the parser will blow up pretty early in the parse because of unbalanced tags. The XML parser by default does not handle tags which are not balanced and which do not have the trailing slash to indicate a standalone tag, and the Craigslist source is filled with those kinds of things.

All I need to do is set this scraper up on a timer, and have it send me results somehow. If I set up a small server with mod_parrot and some kind of tool for generating RSS feeds, I could have this output neatly delivered to me on a regular basis. Considering that mod_parrot is moving along so smoothly and RSS is just another XML format, I think this is a pretty reasonable idea.

So, I started working on that. As of last night, I’ve sketched out two small libraries, one for RSS feeds and one for the competing standard, Atom. These libraries are thin wrappers around the XML library to deal with the specifics of RSS and Atom. Here’s an example of consuming an RSS feed:

var rss = Rosella.Rss.read_url("");
    .foreach(function(i) {
        say(Rosella.String.format_obj("{title} (by {creator}) : {description}", i));

You can do almost exactly the same thing with an Atom feed too, if you’ve got one of those instead. Right now RSS and Atom are implemented in two separate libraries, but I may combine them together for simplicity and to avoid unnecessary code duplication.

I’m working on an interface to write and publish feeds as well, though that’s not quite ready yet. You can bet that when I’ve got that working, I’ll be setting up a copy of mod_parrot to use it with.

I’ve been sort of kicking around the idea of a specialized HTML parsing library, which would more or less be an SGML parser with some schema information. I’m not sure I want to get into that hassle because HTML is a pretty messy thing and it will take a huge amount of effort to get something that works most of the time. But, if you’re willing to put up with a little bit of oddity, the Xml library works well enough for many cases.