Seven signs YOU may have created a Gratuitous Domain Specific Language

Tom Moertel — Sat, 18 Aug 2007 13:01:00 -0400

Like chromatic, I have watched the recent irrational exuberance for domain-specific languages (DSLs) with bewilderment. In certain quarters of the programming universe, it seems that creating DSLs is nearly a rite of passage. The problem is, more and more of these DSLs appear to have been created mainly because, well, DSLs are cool these days, even if less “novel” solutions probably would have been more sensible.

Whereas chromatic unhesitatingly confronted the madness head-on, I have so far managed to avoid the fray. Sure, I’ve asked the occasional probing question of the DSL enthusiast, but mostly my reaction has been limited to standing back and staring in mute amazement at the runaway Domain-Specific Fun-Time Language Train, screaming down the tracks, destined for its inevitable high-speed derailment into what I can only expect will be a bridge abutment. But I’m starting to get the feeling that some of the train’s passengers are aboard because they think it’s the Right Thing To Do Train, so maybe it’s time to throw in my two cents.

To set the record straight, I don’t have anything against DSLs, embedded or otherwise. (I have created my fair share, some of which are actually useful.) No, my concern is limited strictly to the rise of the Gratuitous DSL. So let’s talk about it.

The reason – the right reason – for creating a DSL is because it ultimately lowers the cost of solving problems. If, then, you create a DSL and the cost of solving your problems does not go down, why did you create it? Think about it. Creating a DSL is an expensive proposition. Making people learn your DSL’s syntax, semantics, and underlying domain is a lot to ask – it’s costly. If you do ask, if you do make the imposition, you had better be sure your DSL pays its bills.

But what if your DSL turns out to be a deadbeat? What if using your DSL doesn’t lower the cost of solving problems? Well, guess what? You have created a Gratuitous Domain Specific Language.

Still unsure of whether you’re on the DSL Train for the wrong reason? No problem. Just take this simple, seven-step test:

Seven signs you may have created a Gratuitous Domain Specific Language (GDSL)

You can’t actually explain what a DSL is.
For your DSL, you can’t explain what the domain is.
You have a hard time explaining the DSL’s syntax and semantics.
You have a hard time explaining how the DSL interacts with the language it is embedded in. (For embedded DSLs only.)
A vanilla library API would have captured the domain’s semantics without awkwardness.
It’s easier to express complex domain concepts in general-purpose code than in your DSL.
Your colleagues have a hard time writing things in your DSL.

Did more than a few of the statements ring true? If so, take a bow. You are the proud creator of a Gratuitous DSL!¹

Even so, it’s not too late. You can always hop off the DSL Train at the next stop.

(Note for the humor-impaired: This post is meant to be interpreted in tongue-in-cheek fashion.)

1. Rationale for the Seven Signs. Signs 1–4 suggest that your DSL may not even be a DSL, so calling it a DSL is gratuitous. Signs 4–7 suggest that, though your DSL may be real, it may not be paying the bills; thus, creating it and foisting it upon the world was likely gratuitous.

Update: minor edit for clarity.

Update 2008-03-22: edits for clarity.

Unit testing is a tool, not a goal

Tom Moertel — Tue, 10 Oct 2006 15:23:00 -0400

Via the ever-entertaining programming.reddit.com, I discovered The (Misguided?) Faith in Unit Tests. I don’t agree with the article’s thesis, but the article did hit upon a grain of unfortunate truth: many unit-testing practitioners have a rather cultish aspect to their devotion. More and more, people push unit testing not because it’s an inexpensive way to establish a lot of confidence in our code’s correctness, but because it’s “good” or “a best practice” or “professional.”

When unit testing becomes an end in itself, something has gone haywire. Software-development practices, including unit testing, are just tools. No one tool is the solution to all problems. To the extent that we over-apply any particular tool, we cheat ourselves. We miss out on the opportunity to use other, more effective tools.

My favorite example of a common, important kind of problem that unit testing isn’t much help for is the “strings problem.” All those cross-site-scripting vulnerabilities and SQL-injection holes you have been reading about in recent days are prime examples. In short, the strings problem is having to keep track of and escape, as needed, the bazillion types of strings that flow as life-blood within the arteries of modern web applications.

Trying to detect string-escaping problems via unit testing is costly because it’s at least as difficult to write the tests correctly as it is to write the code the tests are testing. Further, a programmer who forgets that he needs to escape user-supplied strings before stuffing them into live web pages isn’t going to remember to write tests for his having forgotten to escape the strings.

If, then, you come to the programming party with the preconceived notion that unit testing is the only way to go, there’s a pretty good chance that your web applications are going to be sporting some XSS or SQL-injection holes. On the other hand, if you view unit testing as a tool, you’re likely to consider the possibility that other tools might be more effective for something like the strings problem. You might, for example, decide to use a type-system-based solution, which pretty much makes the strings problem trivial to solve. (I’ll write about solving the strings problem in Haskell in an upcoming article.)

I’m not saying that unit testing is “bad” or that it’s not “good.” What I’m saying is that unit testing is not the goal. The goal is having confidence in your code doing what it ought to do. Unit testing is often an inexpensive way to achieve much of that confidence, but for many situations, the strings problem being one of them, it pays to keep an open mind about supplementing unit testing with other tools.

Update: Minor edits for readability.

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Seven signs YOU may have created a Gratuitous Domain Specific Language

Seven signs you may have created a Gratuitous Domain Specific Language (GDSL)

Unit testing is a tool, not a goal