Implementing coroutines in Java

I would take a look at this: http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html, its pretty interesting and should provide a good place to start. But of course we are using Java so we can do better (or maybe worse because there are no macros :))

From my understanding with coroutines you usually have a producer and a consumer coroutine (or at least this is the most common pattern). But semantically you don’t want the producer to call the consumer or visa-versa because this introduces an asymmetry. But given the way stack based languages work we will need to have someone do the calling.

So here is a very simple type hierarchy:

public interface CoroutineProducer<T>
{
    public T Produce();
    public boolean isDone();
}

public interface CoroutineConsumer<T>
{
    public void Consume(T t);
}

public class CoroutineManager
{
    public static Execute<T>(CoroutineProducer<T> prod, CoroutineConsumer<T> con)
    {
        while(!prod.IsDone()) // really simple
        {
            T d = prod.Produce();
            con.Consume(d);
        }
    }
}

Now of course the hard part is implementing the interfaces, in particular it is difficult to break a computation into individual steps. For this you would probably want a whole other set of persistent control structures. The basic idea is that we want to simulate non-local transfer of control (in the end its kinda like we’re simulating a goto). We basically want to move away from using the stack and the pc (program-counter) by keeping the state of our current operations in the heap instead of on the stack. Therefore we are going to need a bunch of helper classes.

For example:

Let’s say that in an ideal world you wanted to write a consumer that looked like this (psuedocode):

boolean is_done;
int other_state;
while(!is_done)
{
    //read input
    //parse input
    //yield input to coroutine
    //update is_done and other_state;
}

we need to abstract the local variable like is_doneand other_state and we need to abstract the while loop itself because our yield like operation is not going to be using the stack. So let’s create a while loop abstraction and associated classes:

enum WhileState {BREAK, CONTINUE, YIELD}
abstract class WhileLoop<T>
{
    private boolean is_done;
    public boolean isDone() { return is_done;}
    private T rval;
    public T getReturnValue() {return rval;} 
    protected void setReturnValue(T val)
    {
        rval = val;
    }


    public T loop()
    {
        while(true)
        {
            WhileState state = execute();
            if(state == WhileState.YIELD)
                return getReturnValue();
            else if(state == WhileState.BREAK)
                    {
                       is_done = true;
                return null;
                    }
        }
    }
    protected abstract WhileState execute();
}

The Basic trick here is to move local variables to be class variables and turn scope blocks into classes which gives us the ability to ‘re-enter’ our ‘loop’ after yielding our return value.

Now to implement our producer

public class SampleProducer : CoroutineProducer<Object>
{
    private WhileLoop<Object> loop;//our control structures become state!!
    public SampleProducer()
    {
        loop = new WhileLoop()
        {
            private int other_state;//our local variables become state of the control structure
            protected WhileState execute() 
            {
                //this implements a single iteration of the loop
                if(is_done) return WhileState.BREAK;
                //read input
                //parse input
                Object calcluated_value = ...;
                //update is_done, figure out if we want to continue
                setReturnValue(calculated_value);
                return WhileState.YIELD;
            }
        };
    }
    public Object Produce()
    {
        Object val = loop.loop();
        return val;
    }
    public boolean isDone()
    {
        //we are done when the loop has exited
        return loop.isDone();
    }
}

Similar tricks could be done for other basic control flow structures. You would ideally build up a library of these helper classes and then use them to implement these simple interfaces which would ultimately give you the semantics of co-routines. I’m sure everything I’ve written here can be generalized and expanded upon greatly.