Monday, August 17, 2009

Lambda, Linq, Dynamic Proxies

In the current project I've introduced some closure/function definitions: functions, actions. The usual stuff. (for reference look at the Scala API, or the google collections, or the Linq Func, or the functional java).

One problem with the current state of closures in java is that, even though objects are closures, and they can be realized with anonymous classes (that's how scala compiler does it), we still have an interface impedance: my function definitions (interfaces) don't match google one's which don't match scala's, which don't match functional javas. So we'll have to write adapters (scala reduces the adapter overhead).
The other problem is that writing closures with anon-classes is a pain: probably that's why Doug Lea is looking into scala. In my project's context we basically/mostly have operations on collections, a sort of
google collections dsl. The only 'ugly' part is the function definition: how the predicates/transformers are defined.

It remindes me of Scala Query & Formal Language Processing in Scala.

I thought of the idea of creating proxies in order to define a function: e.g.
Funcs.on(A.class).getB().create() will create a proxy, will record the method call, and will create a function, which could be called on A objects. I didn't implement that because it seemed such an overkill to create a proxy in order to have a closure.
But now I see that it has been implemented in lambdaj (query collections), and for jpa in liquidform.

What I've missed is that through dynamic proxies we might provide enough information to build queries a la LINQ. (i.e. the information which the c#/ compiler provides on parsing the query,
could be built, manually, by means of dynamic proxies, in java).
Update: Looking at Linq/query for java frameworks:
- one does byte code analysis in order to generate the query expression
- one does dynamic proxy
- one uses APT to generate query expressions from entities (something which people wished for jpa 2.0)

Overall I fear/see that the syntax used to build a query-expression-tree might get rather clunky. Maybe in this area Scala could shine.

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