I’ve been working with Ruby for nearly a year now, which means I’m starting to feel the urge to tell people how awesome the language is. One of the most interesting aspects of Ruby to me is metaprogramming, which it seems to have quite a vocation for.
Since college I have a fondness for automata and formal languages theory. One of the topics I particularly like is text generation (if you haven’t already, check out the excellent SCIgen and the Dada engine), so I thought that building a Context-free grammar (CFG)-like text generator in Ruby would be a nice little exercise and an opportunity to use some of the language’s coolest features. Also I’ve implemented one of those using Java several years ago, and it was a mess, so I was curious as to how much of an improvement would Ruby offer.
Suppose the following script:
dictionary 'noun', 'dog', 'bus'
dictionary 'verb', 'barked', 'parked'
dictionary 'preposition', 'at'
rule 'phrase', 'noun', 'verb', 'preposition', 'noun'
dictionary to store some words according to their classes, and
rule to define a specific ordering of words. For now let’s not worry about
codex (it’s just a collection of rules).
At this point the seasoned programmer is mentally sketching some kind of text parser. It’s an okay solution, but isn’t there something nicer we can do? Well, there is: DSLs! In fact, Ruby is quite an excellent tool to build a DSL, and many famed Ruby-powered applications such as Rspec (and many others) define some kind of DSL.