Monday, January 5, 2009

What Is All the Fuss About How You Can Write DSLs in Lisp?

I saw Chris’s recent post, and felt compelled to respond, despite the fact that I’m pretty sure I’m going to regret it. :)

Here’s what he says, abridged:

(when (and (< time 20:00)
      (trade (make-shares 100 x)))

IMO, that's not a DSL -- that's just a set of function calls in an existing language.

The short version of my response to this is: “Yes! Exactly! That’s a feature, not a bug!” On to the longer version.

This argument, as a question of semantics, is fundamentally doomed to remain unresolved, unless someone can magically provide an exact definition of DSL that a) has no imprecision, and b) everyone agrees with. I’ll just pass on waiting for that to happen, and make my own observation:

A set of function calls in an existing language is generally preferable to a completely separate language. Particularly if it’s the language that you’re using to write the rest of your system. Sure, there are cases where using a specialized language is the thing to do, but over the last few years I’ve come to have a lot more - what’s the word? respect? fear? caution? – anyway hesitance around introducing additional languages into the development process. It’s not rare to see C#, SQL, XSLT, XPath, NAnt XML, regular expressions, and four or five other grammars floating around in the development lifecycle of a single process, and it’s got a real price. If nothing else, it means that not everyone can work on every part of the system. Throw in the fact that some of those are hard to debug, or have conflicting metaphors, or different performance characteristics, and it adds up.

So yes, one of the reasons that I’m excited about Clojure is that it’s a Lisp, and as a Lisp it is flexible enough to adapt to the point where I often don’t need a DSL. Or, if you like, where my DSL is also a Lisp. That doesn’t mean “I never want to use a DSL” or “I always want to use Lisp” – all things in moderation. It certainly doesn’t mean “You shouldn’t use M” – how the hell would I know what you need? But for me, I think it’s nice to have a tool that gives me the choice.