So it’s natural that I’d want to try using it with CAL, a functional language quite similar to Haskell which runs on the JVM.

I used a similar approach to marshalling Javabeans to CAL algebraic types as I used before, but this time I haven’t used any bytecode manipulation — as the Java classes are needed at compile time for the GWT client there isn’t any point in generating them at runtime (although generating them as a separate build step might be useful). I’ve also extended the previous work to include mapping a Java 5 enum to a CAL algebraic type which has constructors with zero parameters.

So in our GWT client we can write:

CaltestServiceAsync service = GWT.create(CaltestService.class);
((ServiceDefTarget) service).setServiceEntryPoint(
    GWT.getModuleBaseURL() + "CaltestService");
service.processPerson(
    new Person(Salutation.MR, "Jim", "Earl", "Jones"), new MyAsyncCallback(...));

and call the CAL function:

public processPerson p =
    let Person s f m l = p; 
    in Person s (toUpperCase f) (lift toUpperCase m) (toUpperCase l);

where the types are:

data Salutation = MR | MRS deriving Inputable, Outputable;
data Person =
    Person salutation :: Salutation 
              firstName :: String 
              middleName :: (Maybe String) 
              lastName :: String 
    deriving Inputable, Outputable;

All this is done via three different annotations, and a special subclass of the GWT RemoteServiceServlet.

The first annotation, @Cal is applied to the GWT service interface, and indicates the CAL module to map the functions on the interface to:

@Cal(workspace = "myworkspace.cws", module = "TDavies.GwtTest")
public interface CaltestService extends RemoteService {
    @Cal
    Person processPerson(Person p);
...

The CAL types Person and Salutation need to be mapped to Java classes:

Person is a simple Javabean with getters and setters for each attribute:

@CalBean(workspace = "myworkspace.cws", module = "TDavies.GwtTest",
    constructorName = "Person")
public class Person implements IsSerializable {
    private Salutation salutation;
    private String firstName;
    private String lastName;
    private String middleName;
...

Note that middleName has the type Maybe String in the CAL type. A value of null maps to Nothing while a value of "x" maps to Just "x".

Salutation is an enum:

@CalEnum(workspace = "myworkspace.cws", module = "TDavies.GwtTest",
    type = "Salutation")
public enum Salutation {
    MR, MRS
}

The names of the enum’s values must be identical to the names of the CAL constructors.

A subclass of RemoteServiceServlet checks for the annotations and transforms the values in both directions.

The source code for this experiment is available via anonymous svn from http://tgdavies.beanstalkapp.com/eddy/browse/trunk/cal. Please note that this repository contains various other half-baked and half-finished experiments! Look at the build.xml file to see how to set up an environment — you’ll need to supply OpenQuark, GWT and Jetty.

In my next post I’ll describe how to persist information on the server.