Calling methods on guest language objects from host language

[Jaroslav Tulach]

Hello Peter,
thanks for using TruffleVM!

### Sep 1, 2015, Peter Niederwieser ###
> I’m trying to make the following interaction between host language and my
> own guest language work: Call TruffleVM.eval(), which returns a guest
> language object, call a method on that object that returns another object,
> call a method on that object, etc. This is effectively traversing a guest
> language object graph that is computed lazily, which is a main way of
> interacting with my guest language (i.e. I’m going to have lots of such
> calls).

I assume host language is Java, right? So you'd like to make calls from Java 
into your TruffleObject. I am currently working on such kind of Java/Truffle 
interoperability.

> The problem I’m facing is that calling from the host language into the guest
> language, either by invoking a CallTarget or sending a TruffleObject
> message, doesn’t re-establish the VM context. The only way I found to do
> that is to invoke a Symbol obtained with TruffleVM.findGlobalSymbol().
> However, my guest language objects aren’t global symbols, but dynamically
> computed along the way.

You are right! Just directly calling TruffleObject will not set the context 
properly! I was not aware of this flaw. In spite of that I have a solution: 
as part of changes to introduce asynchronous execution I want to change the 
return value of eval to Symbol:
http://source.apidesign.org/hg/truffle/diff/1601491ae7c3/truffle/com.oracle.truffle.api/src/com/oracle/truffle/api/vm/TruffleVM.java
that would give you a way to invoke(...) operations with the correct TruffleVM 
context and (as invoke method now returns Symbol again), it would allow you to 
chain such invocations as many times as you want.

> For now, I hacked around this by manually creating a TruffleVM.Symbol using
> reflection (the constructor is non-public). Is there a better way, or can
> one be added? 

Your use-case is important, we have to address it somehow. I will try to merge 
my "TruffleVM.eval returns Symbol" changes today, we'll see if they work for 
you (or you can try revision 3c5aaa402a4e from 
http://source.apidesign.org/hg/truffle/ - that is the most recent one that is 
likely close to what will be merged).

> Am I using Truffle in an unanticipated way, or in a way that
> does not make much sense? 

The way you use it makes sense, but alas it was a bit unanticipated.

> Ideally, I’d like for calls from host language to
> guest language to be as lightweight as possible.

There is another work in progress in Java/Truffle interop:
http://source.apidesign.org/hg/truffle/file/256cd7fcd283/truffle/com.oracle.truffle.api.interop.java/src/com/oracle/truffle/api/interop/java/JavaInterop.java
at the end the Symbol should have a method:

public <T> T as(Class<T> interopInterface) {
  return JavaInterop.asJavaObject(interopInterface, get());
}

which should make it even more easier to call TruffleObjects from Java.

Thanks again for using Truffle!
-jt