## Monday, April 26, 2010

### DSL Interoperability and Language Cacophony

Many times I hear people say that DSL based development often leads to a situation where you have to manage a mix of code written in various languages, which are barely interoperable and often need to be integrated using glues like ScriptEngine that makes your life more difficult than easier. Well, if you are in this situation, you are in the world of language cacophony. You have somehow managed to pitchfork yourself into this situation through immature decisions of either selecting an improper language of implementation for your DSL or the means of integrating them with your core application.

Internal DSLs are nothing but a disciplined way of designing APis which speak the Ubiquitous Language of the domain you are modeling. Hence when you design an internal DSL, stick to the idioms and best practices that your host language offers. For domain rules that you think could be better modeled using a more expressive language, select one that has *good* interoperability with your host language. If you cannot find one, work within the limitations of your host language It's better to be slightly limiting in syntax and flexibility with your DSL than trying to force integration with a language that does not interoperate seamlessly with your host language. Otherwise you will end up having problems not only developing the interfaces between the DSL and the host language, you will have other endless problems of interoperability in the face of exceptions as well.

Don't let the language cacophony invade the horizons of your DSL implementation!

Consider the following example where we have a DSL in Groovy that models an Order that a client places to a securities broker firm for trading of stocks or bonds. The DSL is used to populate orders into the database when a client calls up the broker and asks for buy/sell transactions.

I am not going into the details of implementation of this DSL. But consider that the above script on execution returns an instance of Order, which is an abstraction that you developed in Groovy. Now you have your main application written in Java where you have the complete domain model of the order processing component. Your core application Order abstraction may be different from what you have in the DSL. Your DSL only constructs the abstraction that you need to populate the order details that the user receives from their clients.

It's extremely important that the Order abstraction that you pass from executing the Groovy script be available within your Java code. You can use the following mechanism to execute your Groovy code ..

This runs the script but does not have any integration with your Java application. Another option may be using the Java 6 ScriptEngine for talking back to your Java application. Something like the following snippet which you can have wiothin your main application to execute the Groovy script using ScriptEngine ..

Here you have some form of integration, but it's not ideal. You get back some objects into your Java code, can iterate over the collection .. still the objects that you get back from Groovy are opaque at best. Also since the script executes in the sandbox of the ScriptEngine, in case of any exception, the line numbers mentioned in the stack trace will not match the line number of the source file. This can lead to difficulty in debugging exceptions thrown from the DSL script.

Groovy has excellent interoperability with Java even at the scripting level. When integrating DSLs with your main application always prefer the language specific integration features over any generic script engine based support. Have a look at the following that does the same job of executing the Groovy script from within a Java application. But this time we use GroovyClassLoader, a ClassLoader that extends URLClassLoader and can load Groovy classes from within your Java application ..

You will have to make your DSL script return a Closure that then gets called within the Java application. Note that within Java we can now get complete handle over the Order classes which we defined in Groovy.

This is an example to demonstrate the usage of proper integration techniques while designing your DSL. In my upcoming book DSLs In Action I discuss many other techniques of integration for DSLs developed on the JVM. The above example is also an adaptation from what I discuss in the book in the context of integrating Groovy DSLs with Java application.

Thanks to Guillame Laforge and John Wilson for many suggestions on improving the Groovy DSL and it's interoperability with Java.

JodaStephen said...

The DSL example still shows how restricted writing a DSL in a language like Groovy is. The DSL isn't _truly_ natural - its the best possible within the confines of Groovy syntax.

The Fantom language takes a different approach, allowing any piece of text to be treated as a DSL, with no restrictions at all. More work for the author of the DSL, but a totally pure output.

http://fantom.org/doc/docLang/DSLs.html

Debasish said...

You are correct. In the example I have shown it's an internal DSL for which you need to be confined within the restrictions of the host language. We can be more flexible with an external DSL. And developing external DSLs have become much easier with parser combinator like stuff gaining momentum. The approach that Fantom takes is (I think) similar to what Converge or MetaOCaml takes. It would be interesting to see how it gains steam going forward.

Debasish said...

On further studying it seems that the Fantom approach is not like Converge or MetaOCaml. The latter uses some form of meta-programming through macros. Fantom allows writing DSL plugins using the compiler APIs underneath. One of the major drawbacks of this approach is that your code becomes coupled to real implementation artifacts that are subject to change. It's very difficult to evolve a DSL written using compiler APIs of a language. Possibly this is where code-as-data shines with Lisp.