When you build a code generator you have two basic options:
1. Generate code to be read, complemented and possibly modified by humans
2. Generate code purely as a convenient intermediate step toward bytecode/machine code compilation
The first approach seems to be the most common. It is the most pragmatic option. “Hey, I keep writing the same code over and over, can’t I simply generate part of it and make minor adjustments by hand?” Yes you can. But then you end up with a maintenance issue: it turns out that the code you generated initially was not quite right, and now what do you do, regenerate the code and lose all the modifications you made? As the kids say these days: FAIL!
An improved version of this naive approach is using the [generation gap pattern](http://www.research.ibm.com/designpatterns/pubs/gg.html). The idea here is to generate abstract classes, which you extend from custom code and override the parts that you need to override. The result is that you keep generated and manually written code separate, which is a good thing, because you can make changes to the code generator and simply regenerate code without your changes being lost. Usually. Not always, because if you make invasive changes to you code generator, you may generate completely different code altogether; different classes, different methods and so on. Although you do not lose your manually written code, this manually written code now no longer has any apparent relationship to the code that is generated and it needs to be rewritten to fit the new style of generated code. Again: FAIL.
These problems led us, in the [MoDSE research project](http://swerl.tudelft.nl/bin/view/MoDSE) (pronounced “modes”), to choose approach 2: generate code _only_ as an intermediate step. This also means we have to do 100% code generation, we hardly mix custom and generated code. And in rare edge cases that we _have_ to, we do only through well-defined fixed interfaces. One code generator we built using this approach is [WebDSL](http://www.webdsl.org). After you invoke the compiler on your WebDSL program _you do not look at the generated code_.
Now, let’s say you started your own software company and you got your first big customer. Congratulations. You’re going to build the website of a large international corporation. And because you want to be productive and cool ‘n stuff you’re using code generation techniques. Since you were so impressed with the arguments you just read against mixing custom and generated code, you decide to generate 100% of your code, and therefore no longer have to focus on extension and modification techniques. Thou shalt not read generated code. Good for you! You may even choose using WebDSL. Even better.
But what about your customer? What if you deliver your product? Either you deliver a perfect product that is done and will never have to be changed again — good luck with that. Or, as part of your delivery, you deliver the source code. What source code? Well, not the generated code, because it’s essentially worthless as it’s not intended for human consumption (and in the case of WebDSL, believe me, it’s not). So you deliver the model that was the input of your code generator (e.g. the WebDSL source code). Fantastic. However, your customer is worried. In the future they may need developers familiar with the input language of your code generator to continue work on the product. Where are they going to find such developers? Well, in your company. That’s great for _you_, but not great for your customer, because you essentially locked them in.
This is a problem that is not specific to programs written in domain-specific languages like WebDSL, it’s true for other languages and even frameworks too. [Yahoo rewrote its web store application](http://discuss.fogcreek.com/joelonsoftware/default.asp?cmd=show&ixPost=31402), after buying it, from Lisp to C++ and Perl, because Yahoo engineers were not familiar enough with Lisp. Java web applications written using obscure Java frameworks have a similar problems, as did Ruby on Rails when it just got started.
**If you produce software for a customer using languages and frameworks that very few other developers “speak”, you’re locking your customer in.**
So what’s the solution?
Last year, Jos Warmer of [mod4j](http://mod4j.org) gave a talk as part of our model-driven software development course. Mod4J is a set of DSLs for developing administrative enterprise applications in Java. Interesting twist is that they aim to generate code in _the same style that it would have been written by hand by a developer_. Consequently, when you deliver to your customer they do not _need_ mod4j developers to continue development. They may not even care that you used mod4j to develop the product and simply continue maintaining the generated code. Of course this is not the ideal case, but it’s a fallback option that make customers feel safer. I have no experience with Mod4J and do not know if it really works that way, but I like idea. But is it always feasible to do this?
I wanted to try to take this approach for [mobl](http://github.com/zefhemel/mobl), my DSL for mobile web applications, but it did not work out well. It turns out I missed an essential requirement for this approach to work.
But then I took a step back. To make this approach successful, what’s the plan of action?
1. Develop a framework that developers would find useful and usable even without mobl.
2. Promote that framework by itself, building a community around it, making sure that this was _the_ way to build mobile web applications. Solving the “nobody knows this framework” problem.
3. Build a nice DSL wrapper around the framework.
4. Promote the DSL, build a community around it.
Yeah, you’ll agree that this was a rather pointless mission to begin with. Why not simply build a community around _mobl_ immediately, making that the best way to build mobile web applications? Then I could drop the whole framework idea altogether.
What I did not realize earlier is that to make the Mod4J “generate human code” approach work there already has to be an established style that developers write their code in. There needs to be an established framework you can target. If there isn’t one, the approach is pointless.