That gets to the core of my puzzlement. In terms of dataflow if 'b' is an 'agent' then b.foo() is a message we send to it and then we use the result of b.foo()

That gets to the core of my puzzlement. In terms of dataflow if 'b' is an 'agent' then b.foo() is a message we send to it and then we use the result of b.foo() for further decision making. But in a typical dataflow-diagram data flows in one direction only, there is no link in the diagram from "b" to "result of b.foo()". A diagram could not (easily?) describe the fact that we get the 'b' from one component and then we send the message "foo" to that component 'b". The data-path from this component to "b" could hardly show in the diagram because "b" is the response to a previous request which could be one of many possible alternative responses. I'm not sure if I can express or even think about this very clearly but questions like this make me doubt the generality of dataflow as an alternative to current-day programming practice.