Lately I’ve been developing a website. One issue that I’ll probably need to address in the near future is “persistent predicates”. By “persistent predicates” I mean the problem of treating predicates as data.

Consider the following situation: you are developing some big rss reader/aggregator and you want to allow users to specify handling rules. How would you keep these rules in memory, and how would you keep them on disk?

Obviously, this problem was solved before. Just consider email filters, or even packet filters in ethereal.

One way of approaching the problem is implementing simple predicate templates:

“%field contains %s” where field is subject, or body, etc.

Once that is accomplished, you can specify that a “filter” is some combination (for example logical and, or logical or) of multiple predicates. To store this, we’ll have an actual predicate table (or pickle) with their data, and a one-to-many mapping of filters to predicates.

Another option is allowing just some very simple predicates, and a filter will just “point” to (have an id/name of) the required predicate, and the required data. In this option, all data is stored with the filter.

A more complicated solution is to implement some logical serialize-able lanugage (such as the expression trees I used for diStorm or PyKoan). Using this language, the predicates can be very dynamic, and be combined and manipulated programmatically. This solution might be overkill for many projects though.

An interesting issue regarding handling of predicates, is their application to constraint solving. However, this is an issue for a future post. Suffice it to say, that when writing PyKoan I’m using a constraint solver. Since I’m representing predicates with expression trees, the ability to analyze and manipulate predicates is very handy.

Besides looking at existing solutions, I’m very curious to hear other peoples’ opinions. Feel free to write about your preferred solution in the comments.