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Monday, April 26, 2004

Let's boil the ocean: Simple Data Language

I'm having some fun today thinking about how to get the XML out of HiveMind.

I've long defended the use of XML inside Tapestry (and HiveMind), because it was a convienent, standard way to store hierarchical data. But it is hard to type, hard to read, hard to edit and has some challenges to parse. So I'm thinking about other options.

So ... I still want elements and attributes and nesting, though I don't actually care about nested character data. Nested character data is for documents, not hierarchical data files (despite the fact that Sun's DTDs never use attributes, just character data).

What I have so far (a couple of hours hacking around) is something small and simple. An element is a name. It may be followed by a list of attributes inside parens. An element terminates with a semicolon, or uses curly braces to denote nested elements. Attribute values don't have to be quoted if they look like identifiers, or are numeric literals. Looks something like:

module (id=foo.bar.baz version="1.0.0")
  service-point (id=Startup interface=java.lang.Runnable)
    create-object (class=foo.bar.baz.StartupImpl);

Of course, use of whitespace is totally at your discretion. For comparison, the equivalent XML document:

<?xml version="1.0">
<module id="foo.bar.baz" version="1.0.0">
  <service-point id="Startup" interface="java.lang.Runnable">
    <create-object class="foo.bar.baz.StartupImpl"/>

This is a chance for me to look at JavaCC, a common tool for writing language recognizers and parsers. Good experience in and of itself.

On the other hand, it may be smarter to follow others and simply implement the similarily themed YAML -- YAML Aint Markup Language. When I gave a HiveMind presentation a few month's back at a local Java user's group, I was asked about YAML then, and had a vague understanding of it. YAML has a few more features, better mimicing most (if not all) of XML.

I think the YAML version would be:

--- #YAML:1.0
  id:          foo.bar.baz
  version: 1.0.0
  - service-point:
      id:                Startup
      interface:     java.lang.Runnable
      - create-object:
         class:              foo.bar.baz.StartupImpl 

For either of these two formats, it would be straight-forward to create a filter that produces equivalent XML.

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