Playing with Domain Specific Languages in Groovy and Grails

I’m just coming back from the Italian Java Day, where I talked about Domain Specific Languages on the Groovy - Grails platform. I’ve uploaded the reviewed version of the presentation on slideshare, with some extra notes, if you are interested.

The whole idea is related to the possibility of adopting multiple DSL simultaneously. I took some interesting hints from Guillaume Laforge, Graeme Rocher and Peter Backlund, for the DSL part, as well as some from Ninja Coding Monkey, when it came to integrate my DSL specific value Objects with Grails.

Groovy meta-programming offers tools for designing DSLs that were not possible in a static language such as Java. Operators overloading and coercition make it smooth to deal with numeric types including non-primitive types such as BigDecimal or BigInteger. Still, the most powerful tool is the possibility to re-define the reference operator ‘.’
allowing for more compact and explicit notation such as

100.EUR == 50.EUR * 2

for Money types.

However, this involves adding new behavior on the Number metaClass, which is not entirely correct in terms of object oriented responsibility assignment: it’s basically a new Money constructor that we’re attaching on the Number class instead. It works, it’s cool, but it’s definitely not the cleanest code on Earth.
Different DSLs have many points in common and their readability relies heavily on the same features, so a centralized coordination role turns out to be necessary. At the moment I am working on a prioritized DSL engine that allows pluggable fine-grained DSL to be activated separately in the same application, for example I can write something like:

1000000.EUR == 1000.k.EUR
1000.k.EUR == 1.M.EUR

but that’s only a small a part of the fun. More details in the next posts