Declarative programming

If you have a problem and think "I'll describe everything as data in a nice declarative way (maybe even XML)", now you have two problems.

Seriously, unless you can prove your software will never be extended, don't program declaratively. (I'm speaking mostly to myself here but if this helps anyone else, you're welcome). The reason is obvious: declarative description is very pretty and concise as long as it's simple. But usually it becomes extremely complex to change things a bit if that bit isn't foreseen from the beginning. And the real world is always like that. I've written quite a few frameworks and declarative parsers and there hasn't been any one where I didn't regret that when adding new functionality.

Simple things should be simple, complex things should be possible (c). With declarative code they usually end up being impossible. Even if they're possible, the efforts you spend are huge, and it looks like a hack. Just look at Ant.

Alternatives? Write code that does something, not the one that creates some structures which are then stored, sorted, converted into other structures, stored and sorted again and then finally some completely different code does something based on these structures. Just do it as straightforwardly as possible. Execute the code instead of creating data objects. Code invocation may in fact look quite DSLish (even in Java), so the not-so-big-if-any lengthiness will definitely pay off in the long run when you have to change the foundations. And it's muuuch easier to debug this way.

The moral: when writing data instead of code, think again. And again. And again.