I don't actually like the approach though. Ruby makes DSLs as easy as "normal" interfaces. Why not take advantage of that? Generating and parsing an AST is exactly the kind of extra complexity that Ruby lets you avoid.
Here's a use case: Imagine that you want a form validation library, something like active record's domain-specific language, but it is not attached to models, just to forms being submitted. But anyhow, you write your DSL. Now you have two options:
1. Execute it directly in the controller of your Ruby server
2. Convert it to sexps, export those as JSON, and write a Javascript validator that validates forms on the client-side.
That is indeed a nifty use case, but also kind of a special case in which a serialized AST is the required output of your program. For the general case where the DSL is just a sexy API, I wouldn't recommend building a parse tree.
EDIT: Actually, for your case I don't think I would use sexps. I would write some primitive validators in Ruby that can also generate equivalent JavaScript code, then use those to build higher order validators in Ruby alone. Compare that to maintaining complete validator engines in two different languages.
One advantage of this approach is that it lets you not care about the order of definitions if you don't want to. Doing things the usual way, ruby's execution model can have you tied in knots if you're not careful.
http://code.extension.ws/post/169602795/dal-rb
I don't actually like the approach though. Ruby makes DSLs as easy as "normal" interfaces. Why not take advantage of that? Generating and parsing an AST is exactly the kind of extra complexity that Ruby lets you avoid.