June 27, 2008

twill is mini-language done right

In my college career I found a book whose genre I can best describe as "computer science philosophy." The Pragmatic Programmer taught me several key philosophical ideals behind real-world code design, which I found to be quite useful in my every-day programming life. [*]

One of the aforementioned "Pragmatic Programming" practices that I had the most difficulty wrapping my head around dealt with the creation of mini-languages close to the problem domain (AKA domain specific languages). To me, this seemed like an unnecessary amount of additional work, when one could just specify an interface that was a close approximation of a mini-language in a semantically rich language. After looking at twill, I realize that it isn't too much work, specifically because you can create a close approximation in a semantically rich language.

Twill has a commands module that exports functions with simple names; for example, "go", "find", "back", etc. This is the more simple approximation of a mini-language that I had anticipated as a time savings. However, Twill also has a shell module which deals with the online execution of these functions and their arguments within a shell-like environment. This creates, as interpreted by the shell, a mini-language!

With Twill's elegant and maintainable (extensible) coding, the command loop is approximately 250 lines. It's not too hard to create a simple file reader to interpret commands line by line from a script and feed them into the command loop one by one, at which point you have a scriptable mini-language!

There's nothing extraordinarily amazing about any single component of this process, but the fact that a mini-language can be created with so little effort kind of stunned me — one of those "lightbulb going off in your head" situations. :)

Letting my mind wander a bit, I doubt that it's always possible to wrap an existing API with simple, domain-specific, script-like commands. If creating a domain-specific mini-language is a tangible possibility, it seems like a design decision you want to know early on.



My CS degree program had a tendency to emphasize provably correct programs, which I don't find great need for in my personal day-to-day work beyond some simple design-by-contract guidelines. This is the topic for another entry, however... perhaps several entries!